Did this actually happen or is it just propaganda nonsense?

[Exorcet]

From my limited knowledge of design for cruise, engines are usually fitted such that the thrust vector points upward at an angle to the horizontal such that the tangent of that angle = Drag/Lift at that design cruise speed and altitude.

This will only work for one cruise speed for a given altitude though.

 

The problem with TV is that it acts at the tail of the a/c not the centre of mass, so visualising exactly how you would trim with it in order to improve efficiency is not easy.

I don't understand, this isn't a problem. Traditional trim surfaces are not at the center of mass either. You can't trim at the cg. You need a moment arm relative to the cg. This makes engine thrust a natural candidate.

 

Move the nozzles and thrust no longer points forwards, which doesn't sound efficient for high speed or cruising

Cosine at angles near zero approximates to one. You don't need huge deflections to trim the plane. 5 degrees of nozzle deflection yields 99.6 % of thrust propelling the plane forward and 8.7% generating the moment required for trim. At 10 degrees, it's 98.4% forward thrust, 17.4% trim.

 

and you change the AOA, which will in fact induce more drag than trimming, that's the whole point of trimming.

For a given speed, there is only one steady AoA you can achieve. Whether you use TVC or horizontal stabs won't change that AoA. However, the stabs will produce induced drag unless you are flying in such a condition that trimmed flight occurs when they (the stabs) are at zero AoA.

 

You've also got the weight of a TV system increasing fuel consumption (and maintenance costs).

Both issue. The latter does not change performance though. The former is simply balancing, calculate how much drag the TVC will cause by increasing the needed wing lift and then compare to trim drag from the stabs.

 

Stall is related to speed

It only depends on flow separation. At low speed, this is caused by exceeding critical AoA. At high speed, it is caused by shockwaves or exceeding critical AoA. You can calculate a stall speed, but all that really is, is the speed where the plane experiences flow separation.

[marcos]

I don't understand, this isn't a problem. Traditional trim surfaces are not at the center of mass either. You can't trim at the cg. You need a moment arm relative to the cg. This makes engine thrust a natural candidate.

Okay but if you divert the thrust, less of it is pointing forward and it isn't quite the same as having the engine installed at a different angle as per my example for optimum cruise at a given speed. That said, the majority of the weight, is at the back for modern fighters.

 

Cosine at angles near zero approximates to one. You don't need huge deflections to trim the plane. 5 degrees of nozzle deflection yields 99.6 % of thrust propelling the plane forward and 8.7% generating the moment required for trim. At 10 degrees, it's 98.4% forward thrust, 17.4% trim.

True, I'm well aware of the maths, but the gains in aerodynamic efficiency are also likely to be small, so the marginally reduced forward thrust is a factor. I'm not saying it isn't better, I'm just not sure how much better. % trim doesn't directly equate to % improvement, whereas % reduced forward thrust clearly does. I've seen figures suggesting that the F-15 ACTIVE gained an equivalent of an extra Mach 0.1 at 30,000ft and Mach 1.3 but that was a lot to do with efficiency software:

 

http://www.nasa.gov/centers/dryden/pdf/120302main_FS-048-DFRC.pdf

 

I'd be interested in like-for-like figures.

 

 

For a given speed, there is only one steady AoA you can achieve. Whether you use TVC or horizontal stabs won't change that AoA. However, the stabs will produce induced drag unless you are flying in such a condition that trimmed flight occurs when they (the stabs) are at zero AoA.

Not necessarily true. That's the point of trim and flaps for landing, they fundamentally change the shape of the wing seen by the airflow and provide differing lift at the same AOA.

 

Both issue. The latter does not change performance though. The former is simply balancing, calculate how much drag the TVC will cause by increasing the needed wing lift and then compare to trim drag from the stabs.

No argument there. There's a trade-off to be made. The heavier the aircraft, the less the impact on weight I suspect.

 

It only depends on flow separation. At low speed, this is caused by exceeding critical AoA. At high speed, it is caused by shockwaves or exceeding critical AoA. You can calculate a stall speed, but all that really is, is the speed where the plane experiences flow separation.

Yes but flow separation depends on speed whether it's slow speed separation or transonic separation.

[Exorcet]

Okay but if you divert the thrust, less of it is pointing forward and it isn't quite the same as having the engine installed at a different angle as per my example for optimum cruise at a given speed. That said, the majority of the weight, is at the back for modern fighters.

There isn't a difference between TVC and angled thrust when you're off cruise though. They both lose forward thrust.

 

The cg isn't "at the back", it's behind the aerodynamic center (that's what causes instability) and ahead of the main gear. This can be extremely far forward, though usually, it's around the middle of the plane.

 

 

True, I'm well aware of the maths, but the gains in aerodynamic efficiency are also likely to be small

It depends, though at high speed, any aerodynamic force can be huge. It grows with V^2, and above Mach 1 you add to the V^2 shock losses.

 

I'm just not sure how much better.

Right, it's hard to tell just in general.

 

% trim doesn't directly equate to % improvement

By % trim, I meant the amount of thrust available to act as the force that generates the trim moment. Although, you can think of that moment as a kind of "trim authority", so it is a pretty good indicator of how much ability you have to trim the plane. The ability to trim grows faster than the loss in thrust for angles near zero.

 

I've seen figures suggesting that the F-15 ACTIVE gained an equivalent of an extra Mach 0.1 at 30,000ft and Mach 1.3 but that was a lot to do with efficiency software:

 

http://www.nasa.gov/centers/dryden/pdf/120302main_FS-048-DFRC.pdf

What the ACTIVE did is probably the same thing that the F-22 does.

 

"An adaptive performance software program was developed and successfully tested. The performance-optimization program installed in the aircraft’s flight control computer automatically determines the optimal setting or trim for the thrust-vectoring nozzles and aerodynamic controls to minimize aircraft drag. On the last flight of 1996, the F-15 ACTIVE demonstrated the software’s effectiveness by gaining a speed increase of Mach 0.1 with no increase in engine power while in level flight at 30,000 ft altitude and a speed of approximately Mach 1.3."

 

 

 

Not necessarily true. That's the point of trim and flaps for landing, they fundamentally change the shape of the wing seen by the airflow and provide differing lift at the same AOA.

Well, deploying flaps is a change in aircraft configuration. While you're right that with flaps, you can have two steady AoA's, there is usually only one cruise configuration on a given flight. Whether TVC or the tail is used to trim, the AoA remains the same.

 

 

 

Yes but flow separation depends on speed whether it's slow speed separation or transonic separation.

It depends more on the Reynolds number and pressure gradient. You can pretty much make a wing stall free at any speed you want, but the AoA is a hard limitation that doesn't go away with modifying your wing design.

 

There isn't really a slow speed separation. Separation at low speed is caused by exceeding critical AoA. If you stay below critical AoA, your wing will never stall.

 

One way to look at it is like this, you can find a critical AoA for a wing, by itself, but not a stall speed. To find a stall speed, you need a plane. The stall speed would be the speed where the AoA required to generate the plane's weight in lift is above the critical AoA.

[aaron886]

Just to add a credibility-vote... Exorcet appears to know what he's talking about.

 

Marcos- you suggestion is that thrust vectoring can be used to trim the aircraft at high subsonic and supersonic velocities, for the purpose of reducing "trim drag." A good thought, but by inclining the thrust line you are in effect producing the same amount of down-force in nearly the same place as you would with the stabilators. The resultant inefficiency is still there.

[marcos]

There isn't a difference between TVC and angled thrust when you're off cruise though. They both lose forward thrust.

Hmmm... but visualizing it it just isn't the same. The nozzle is diverting the thrust and the nozzle is at the back, therefore the 3rd law reaction force normal to motion is at the back with TV. That isn't the same as the whole engine pointing up slightly at a location far closer to the cog. On a see-saw it depends where you sit if you pardon the analogy. I want to pull the whole plane up slightly, not just the rear end.

 

It depends, though at high speed, any aerodynamic force can be huge. It grows with V^2, and above Mach 1 you add to the V^2 shock losses.

I've no doubt the forces are huge but how much are they being changed relative to the added weight changing them. I can't help but feel, maybe wrongly, that the absence of TV on airliners, where fuel economy is paramount, and the added weight would be negligible, hints that the chief gain is definitely manoeuvrability

 

By % trim, I meant the amount of thrust available to act as the force that generates the trim moment. Although, you can think of that moment as a kind of "trim authority", so it is a pretty good indicator of how much ability you have to trim the plane. The ability to trim grows faster than the loss in thrust for angles near zero.

I realised that.:thumbup: But how much is that force helping to reduce drag as a % was what I was trying to say?

 

 

What the ACTIVE did is probably the same thing that the F-22 does.

 

"An adaptive performance software program was developed and successfully tested. The performance-optimization program installed in the aircraft’s flight control computer automatically determines the optimal setting or trim for the thrust-vectoring nozzles and aerodynamic controls to minimize aircraft drag. On the last flight of 1996, the F-15 ACTIVE demonstrated the software’s effectiveness by gaining a speed increase of Mach 0.1 with no increase in engine power while in level flight at 30,000 ft altitude and a speed of approximately Mach 1.3."

It does sound significant. It equates to a drag reduction of almost 20% but perhaps some of that was better trim adjustments too.

 

Well, deploying flaps is a change in aircraft configuration. While you're right that with flaps, you can have two steady AoA's, there is usually only one cruise configuration on a given flight. Whether TVC or the tail is used to trim, the AoA remains the same.

Think you may be right on that. Not an expert on trim but it seems to just affect the stuff you move anyway during normal flight.

 

It depends more on the Reynolds number and pressure gradient. You can pretty much make a wing stall free at any speed you want, but the AoA is a hard limitation that doesn't go away with modifying your wing design.

Well no, the Reynolds number determines whether the means of reducing flow-separation and boundary layer velocity gradient is achieved via favouring laminar or turbulent flow. For long high speed objects, laminar flow is preferable but for slow blunt objects, like golf balls, turbulent flow is better, hence why they have dimples to facilitate better velocity mixing in the boundary layer and prevent localised flow reversal.

 

Re (Density*v*L/u) must be kept the same in all scale-model wind-tunnel testing. For any given design outcome, density and velocity are the only variables that will change during flight and density is obviously linked to altitude.

 

There isn't really a slow speed separation. Separation at low speed is caused by exceeding critical AoA. If you stay below critical AoA, your wing will never stall.

If the plane is supported in a wind-tunnel then theoretically yes, but if it is falling out of the sky due to lack of lift then no. I won't argue the semantics.:)

[marcos]

Just to add a credibility-vote... Exorcet appears to know what he's talking about.

 

Marcos- you suggestion is that thrust vectoring can be used to trim the aircraft at high subsonic and supersonic velocities, for the purpose of reducing "trim drag." A good thought, but by inclining the thrust line you are in effect producing the same amount of down-force in nearly the same place as you would with the stabilators. The resultant inefficiency is still there.

Actually that is Exorcet's suggestion. My point is similar to the one you've just made but there is some test evidence to the contrary. You have our arguments reversed.:D

[aaron886]

Actually that is Exorcet's suggestion. My point is similar to the one you've just made but there is some test evidence to the contrary. You have our arguments reversed.:D

 

Interesting, must have read you wrong, although you're completely off-base when it comes to the characteristics of a stall. "Stall is related to speed?" No, not really. Anyway, maybe I made a bad sub-conscious association? :D

[marcos]

Interesting, must have read you wrong, although you're completely off-base when it comes to the characteristics of a stall. "Stall is related to speed?" No, not really. Anyway, maybe I made a bad sub-conscious association? :D

In aerodynamics it's tough to find anything not related to speed in some way.

[wilky510]

The F-35 is invincible until proven otherwise! End of discussion :thumbup:

Sarcasm? If so, noone said it was invincible here :music_whistling:.

[Exorcet]

Hmmm... but visualizing it it just isn't the same. The nozzle is diverting the thrust and the nozzle is at the back, therefore the 3rd law reaction force normal to motion is at the back with TV. That isn't the same as the whole engine pointing up slightly at a location far closer to the cog. On a see-saw it depends where you sit if you pardon the analogy. I want to pull the whole plane up slightly, not just the rear end.

The center of mass reacts to net forces, always.

 

Draw a free body diagram, maybe that will make things clearer. The thrust force is in the same place, TVC or not.

 

 

I've no doubt the forces are huge but how much are they being changed relative to the added weight changing them. I can't help but feel, maybe wrongly, that the absence of TV on airliners, where fuel economy is paramount, and the added weight would be negligible, hints that the chief gain is definitely manoeuvrability

Airliners have one cruise speed. The F-22 has two (or more). Subsonic cruise and Supersonic cruise. Subsonic cruise would be the true cruise, as that is where the plane is most efficient. If the horizontal stab was designed to be most efficient in the subsonic cruise condition, it will be less efficient in the supercruise condition. To mitigate this, the F-22 uses TVC for supercruise.

 

The induced drag coefficient is proportional to the lift coefficient squared. 2% of weight equates to 4.04% of induced drag.

 

 

I realised that. But how much is that force helping to reduce drag as a % was what I was trying to say?

That depends on how much trim you need. There is however, one point where the horizontal stabilizer will naturally be set to minimum drag. Airliners can take advantage of this because they only fly at one speed. A fighter that has to cruise and supercruise needs to choose.

 

 

It does sound significant. It equates to a drag reduction of almost 20% but perhaps some of that was better trim adjustments too.

From the article, it sounds like it was all the same. In other words, the combined effect of minimum drag trim and TVC trim.

 

 

The blue line is a plane with TVC. The green lacks TVC. Without TVC, the drag coefficient from the horizontal stab must increase to maintain trim whenever the plane is required to fly off cruise speed. For the TVC craft, only the induced drag coef of the wing changes.

 

Note that the graphs intersect at exactly one point. This means the planes are the same weight and the one without TVC is designed to fly while neutrally stable.

 

The graph isn't 100% correct, but it shows the general trend.

 

 

Well no, the Reynolds number determines whether the means of reducing flow-separation and boundary layer velocity gradient is achieved via favouring laminar or turbulent flow. For long high speed objects, laminar flow is preferable but for slow blunt objects, like golf balls, turbulent flow is better, hence why they have dimples to facilitate better velocity mixing in the boundary layer and prevent localised flow reversal.

 

Re (Density*v*L/u) must be kept the same in all scale-model wind-tunnel testing. For any given design outcome, density and velocity are the only variables that will change during flight and density is obviously linked to altitude.

Re doesn't determine whether separation is reduced by laminar or turbulent flow. Turbulent always resists separation more. Laminar is however, less draggy when it does not separate. Laminar flow is always preferred (barring cases where you want to slow down) unless there is a risk of separation.

 

Despite the second paragraph, stall speed doesn't really make sense though. The Re number applies to stall because the critical AoA is a function of Re. Re is a function of velocity, but velocity alone won't tell you when you're going to stall. You need more information if you base stall around V. With Re, you just need to know the AoA. With V, you need weight and density, L, and mu. But that just brings you back to the Reynolds number and allows you to drop weight.

 

If the plane is supported in a wind-tunnel then theoretically yes, but if it is falling out of the sky due to lack of lift then no. I won't argue the semantics.

It's not a semantics argument though. If you stay below critical AoA, the wing never stalls. The problem is, sub critical AoA may mean there isn't enough lift to stay in the air. There is a difference.

 

You can take the Su-25T in DCSW and set it to autolevel at some speed. Then kill the engines. The autopilot will prevent the wing from stalling until the plane's attitude exceeds the AP's limits. On the other hand, if you manually held altitude you would stall at a higher speed than with the AP on. The AP chooses to give up lift required for level flight to prevent a stall.

 

You will stall at the same critical AoA* regardless however.

 

*This doesn't contradict my statement that crit AoA is a function of Re. The crit AoA can be approximated as a constant over certain ranges of Re. While in reality, since Re changes between the two cases, the crit AoA could be off by something like .000001%, but this is for all intents and purposes the same. The difference in the speed at which you stall can be large.

[leafer]

Oh, boy...

[Slayer]

This Dr. Kopp is obviously full of it an you guys got reeled in hook line and sinker, the man is obviously biased as he has a pic of a Russian jet behind him as he speaks.

 

Video from 2008 saying the F-35 was double inferior and it's not even in full service 4 years later in 2012. So how pray tell did it get owned in this PACAF exercise when there were none to participate? They only had a few prototypes in 2008 an I daresay they didn't gamble them on some fictional PACAF exercise...

 

Other inconsistencies showing the BERKUT SU-47 in the video? huh? Another aircraft proptotype not even in service yet.

 

That whole video is pretty much abject garbage. No I'm not an F-35 apologist I think it's really not that great of an aircraft, way over budget, way overpriced. Whenever I look at its single engine and 1 single cockpit display I think the designers aught to be shot. There is no redundancy at all. (Wondering what happens when they get the software bug that the F-22 had that shut down the computers...lol)

[aaron886]

Just to make sure we nip this one in the bud again...

http://www.rand.org/news/press/2008/09/25.html

 

:music_whistling:

 

 

In aerodynamics it's tough to find anything not related to speed in some way.

 

How about... stalling? You've found one, really. It's related to airflow, but not dependent upon airspeed.

[marcos]

The center of mass reacts to net forces, always.

 

Draw a free body diagram, maybe that will make things clearer. The thrust force is in the same place, TVC or not.

I don't see it that way. Applying force to the end of a see-saw is not the same as applying it to the middle. That's why aircraft rotate when using TVC for turning.

 

 

Airliners have one cruise speed.

True, I forgot that.

 

The induced drag coefficient is proportional to the lift coefficient squared. 2% of weight equates to 4.04% of induced drag.

Yes but kCl^2 is < Cdo during level flight. So 4.04% induced drag is less that 2% overall drag, combined with the fact that TVC adds weight as well as producing lift.

 

 

That depends on how much trim you need. There is however, one point where the horizontal stabilizer will naturally be set to minimum drag. Airliners can take advantage of this because they only fly at one speed. A fighter that has to cruise and supercruise needs to choose.

Exactly, so the overall benefit is unclear.

 

 

From the article, it sounds like it was all the same. In other words, the combined effect of minimum drag trim and TVC trim.

 

The theory is there for some aircraft in level flight certainly but what impact does the extra weight have when the aircraft is performing combat turns at 9g? The impact of the extra weight is multiplied by a factor of 9 and extra drag is induced whilst turning, not only affecting fuel economy but also the ability to maintain speed/altitude (energy) in hard turns.

 

Re doesn't determine whether separation is reduced by laminar or turbulent flow.

Errr.... Yes it does. Check your facts on that one. It also determines whether the flow is likely to be turbulent of laminar. Above 500,000 and it's most likely turbulent.

 

It's not a semantics argument though. If you stay below critical AoA, the wing never stalls. The problem is, sub critical AoA may mean there isn't enough lift to stay in the air. There is a difference.

Only semantically. When you fall, a change in AoA is inextricably linked to the reduction of speed causing the fall. The speed also determines the required AoA for level flight, as you've already discussed. So, one way or another, stall and AoA are inextricably linked to speed. It's like arguing whether death is caused by a fall or the sudden stop. They're inextricably linked. Speed also determines when shock waves will become a factor.

 

You can take the Su-25T in DCSW and set it to autolevel at some speed. Then kill the engines. The autopilot will prevent the wing from stalling until the plane's attitude exceeds the AP's limits. On the other hand, if you manually held altitude you would stall at a higher speed than with the AP on. The AP chooses to give up lift required for level flight to prevent a stall.

 

You will stall at the same critical AoA* regardless however.

And the speed will determine the AoA for any flight regime.

 

*This doesn't contradict my statement that crit AoA is a function of Re. The crit AoA can be approximated as a constant over certain ranges of Re. While in reality, since Re changes between the two cases, the crit AoA could be off by something like .000001%, but this is for all intents and purposes the same. The difference in the speed at which you stall can be large.

Speed affects Re too.

[marcos]

How about... stalling? You've found one, really. It's related to airflow, but not dependent upon airspeed.

And since when is airflow not related to speed for any given aerodynamic problem, really.

 

Show me any equation in aerodynamics involving stall but not involving a V anywhere. The equation for Re has V for instance.

Edited September 4, 2012 by marcos

[Exorcet]

I don't see it that way. Applying force to the end of a see-saw is not the same as applying it to the middle. That's why aircraft rotate when using TVC for turning.

TVC causes rotation because it's a control surface. The nozzle is where the thrust force is felt in either case. TVC just allows you to move the nozzle and control the plane.

 

Where do you think the thrust force is coming from in the non TVC engine?

 

 

 

Yes but kCl^2 is < Cdo during level flight. So 4.04% induced drag is less that 2% overall drag, combined with the fact that TVC adds weight as well as producing lift.

I'm confused here, the 4% drag is the induced drag added by the extra 2% weight. You're right that the impact on overall drag is smaller than the impact on induced drag, but that is a good thing.

 

Exactly, so the overall benefit is unclear.

The amount of benefit is unclear, but that there is a benefit is clear.

 

 

The theory is there for some aircraft in level flight certainly but what impact does the extra weight have when the aircraft is performing combat turns at 9g? The impact of the extra weight is multiplied by a factor of 9 and extra drag is induced whilst turning, not only affecting fuel economy but also the ability to maintain speed/altitude (energy) in hard turns.

The impact of the weight doesn't go up by nine, if it's 2% weight, it's still 4.04% Cdi. As for the rest, design is always a trade off. The TVC will improve dash speed, but might hurt turning (although, with TVC you can avoid having to deflect your aero control surfaces, which reduces drag during transient maneuvers).

 

Errr.... Yes it does. Check your facts on that one. It also determines whether the flow is likely to be turbulent of laminar. Above 500,000 and it's most likely turbulent.

Around 500,000 there is transition, agreed. However, I can't agree that laminar is ever less likely to separate than turbulent. Turbulent flow consists of macroscopic eddies which serve as a device to transport momentum across the boundary layer. This momentum diffusion energizes the near wall boundary layer and is what prevents separation. With laminar flow, the near wall boundary layer just continually loses momentum and then separates when subject to a severe enough adverse pressure gradient.

 

Only semantically. When you fall, a change in AoA is inextricably linked to the reduction of speed causing the fall. The speed also determines the required AoA for level flight, as you've already discussed. So, one way or another, stall and AoA are inextricably linked to speed. It's like arguing whether death is caused by a fall or the sudden stop. They're inextricably linked. Speed also determines when shock waves will become a factor.

But you don't have to fall. You can fly, without stalling at any speed, though you might not be able to maintain altitude.

 

Let's see if we agree on what the definition of stall is: loss of lift from exceeding the maximum cl of the wing.

 

Yes or no?

 

And the speed will determine the AoA for any flight regime.

For a given flight condition (ie, level). AoA has no conditions, exceed the AoA, and you're done.

 

Speed affects Re too.

Yes, but speed effects can be opposed by other changes, such as density. The Re is a "more complete" number.

[marcos]

TVC causes rotation because it's a control surface. The nozzle is where the thrust force is felt in either case. TVC just allows you to move the nozzle and control the plane.

 

Where do you think the thrust force is coming from in the non TVC engine?

In a regular engine the force is felt in the place where momentum is added to the airflow, which occurs over a length, not at one point at the end of the nozzle but it's not as simple as that because force struts act to transmit the force back to aircraft. Those struts do not act in the same way if thrust is turned through a large angle at the nozzle. At small angles though, it may not make a difference.

 

If there was no difference between a regular engine being re-orientated and a TVC nozzle being re-orientated then the F-35 STOVL would not require a lift fan, since tilting the rear nozzle would have the same effect by your argument. During steady flight however, it may be that the angle is so small, that the effect on rotation is negligible. Turn the nozzle through 90deg however and you will see there is a huge difference.

 

I'm confused here, the 4% drag is the induced drag added by the extra 2% weight. You're right that the impact on overall drag is smaller than the impact on induced drag, but that is a good thing.

Not necessarily because the same argument applies to the reduction of induced drag thanks to TVC. The thing you're working to reduce with TVC is also something you've increased by adding TVC, compounded by the fact that you're reducing forward thrust marginally to do it. Then there's the problem of balancing the drag reduction of not having to use trim against the drag increase of having nozzles pointing slightly down. One way or another work has to be done to create X amount of lift. Doing it directly with TVC is a 1:1 trade and reduces work in the forward direction which is bad. Doing it with lift causes a sub-1:1 trade because an X % rise in Cl cause an overall rise in Cd of < X% because kCl^2 < Cdo. AND Cl is increases by TVC weight anyway.

 

So doing the lift traditionally causes Cl to rise by 2% that causes Cd to rise by <2%, demanding an increase in thrust of less than 2%.

 

Doing the lift with TVC causes Cl not to require the 2% increase but increases the weight by Y%. If Y is > 2, then the benefit is counteracted already but you also have the reduction in forward thrust and extra non-lift producing drag caused by TVC. And over the life of an aircraft you also have cost of maintenance vs cost of fuel saved, plus you have to consider the possible reduction in conventional turn performance and air-combat fuel economy caused by the weight of TVC multiplied by 9+. It's not a simple equation at all. My guess is that TVC is there mainly to save planes not for fuel economy.

[marcos]

The impact of the weight doesn't go up by nine, if it's 2% weight, it's still 4.04% Cdi. As for the rest, design is always a trade off. The TVC will improve dash speed, but might hurt turning (although, with TVC you can avoid having to deflect your aero control surfaces, which reduces drag during transient maneuvers).

But you'd need to generate 9 times as much lift with TVC as you would in level flight plus the extra for the extra weigh of the TVC times 9. That would cause a substantial reduction in forward thrust tangential to the turn and definitely increase the drag on the rear of the plane while doing so. I don't see that one playing out beneficially whatever the possible advantages in level flight.

 

Around 500,000 there is transition, agreed. However, I can't agree that laminar is ever less likely to separate than turbulent. Turbulent flow consists of macroscopic eddies which serve as a device to transport momentum across the boundary layer. This momentum diffusion energizes the near wall boundary layer and is what prevents separation. With laminar flow, the near wall boundary layer just continually loses momentum and then separates when subject to a severe enough adverse pressure gradient.

In high speed flow on a slender body, why would you want to transport momentum across the boundary layer? Don't you think that turbulence in high speed flow presents a problem wrt separation? Think about momentum as being about direction as well as speed.

 

Turbulent flow is more able to resist separation at higher values of Re but the ability of laminar flow to resist separation is independant of Re and therefore better at low values of Re. As well as considering pressure gradient you must consider the affect of different flow regimes on the pressure gradient.

 

But you don't have to fall. You can fly, without stalling at any speed, though you might not be able to maintain altitude.

So now we're arguing about the semantics of falling and not being able to maintain altitude.:lol:

 

Let's see if we agree on what the definition of stall is: loss of lift from exceeding the maximum cl of the wing.

 

Yes or no?

 

 

For a given flight condition (ie, level). AoA has no conditions, exceed the AoA, and you're done.

 

 

Yes, but speed effects can be opposed by other changes, such as density. The Re is a "more complete" number.

Yes but I've already mentioned way back that stall speed varies with altitude but for a given plane at a given altitude, the pilot can only control one variable in the equation for Re and that variable is V. Density he can't control. Kinematic viscosity he can't control. The characteristic length of his aircraft he can't control (yet). The original point which I countered (way back) was that stall isn't related to V, and clearly it is.

[aaron886]

And since when is airflow not related to speed for any given aerodynamic problem, really.

 

Show me any equation in aerodynamics involving stall but not involving a V anywhere. The equation for Re has V for instance.

 

Stalling angle of attack varies with Reynolds Number, but it has nothing to do with stalling airspeed. An airfoil with high Re will generally stall at a higher angle of attack. (Think: STOL bush-plane.) However, the association of Re to stalling angle of attack is a small factor. Again, there is no such thing as a stalling airspeed... it's one of the most brutally primary concepts of aerodynamics.

 

In level flight, you would need to associate a given airfoil, weight, cg, sideslip angle, etc etc in order to derive a "stall speed." Aircraft are given rated stalling speeds to simplify things for the pilot, and because most aircraft don't feature an angle of attack indicator.

 

Let's look at an example that puts a dent in your theory. We'll use a Cessna in a static condition for a simple example. A Cessna at 30 knots attempting to maintain level flight would be stalled. A Cessna at 30 knots pointed straight down at the ground would most likely not be stalled. Why? Airflow separation from the wing is dependent upon angle of attack, not airspeed. If the airflow is aligned with the wing such that the critical angle of attack is not exceeded, the wing is not stalled. Period.

 

So now we're arguing about the semantics of falling and not being able to maintain altitude

You're displaying a bit of a logical flaw here. Flight has nothing to do with maintaining altitude... nor is it semantics.

 

Consider a standard straight-wing Cl vs. a diagram. The right-most blue line indicates the critical angle of attack. That is the point at which this airfoil is considered to be stalled. Nowhere on this graph do you see velocity labeled.

 

Can you believe that you're wrong now? Please? :D

Edited September 4, 2012 by aaron886

[marcos]

Stalling angle of attack varies with Reynolds Number, but it has nothing to do with stalling airspeed.

Please write down the equation for Re and then repeat that sentence and see if it makes sense.

 

For a given aircraft, at a given altitude, speed is the only thing a pilot can change that affects Re.

 

 

You're displaying a bit of a logical flaw here. Flight has nothing to do with maintaining altitude

Well done, you've pointed out my error, I thought it was. You've also made my signature.

 

Falling, flying, whatever, the critical AoA is determined by the Re, which, for any given aircraft, at any given altitude is determined by V. The aircraft designer can influence Re by other means, the pilot cannot. Of course data is issued to the pilot as a stalling speed because it's all they can control. What do you expect, an AFRC stating that the pilot needs to change Re by thickening the local freestream below a given speed, or changing the dimensions of the aircraft mid-flight, or perhaps changing what the air is made of??? Or maybe just go for a -90deg AoA to avoid stall.

Edited September 4, 2012 by marcos

[marcos]

Consider a standard straight-wing Cl vs. a diagram. The right-most blue line indicates the critical angle of attack. That is the point at which this airfoil is considered to be stalled. Nowhere on this graph do you see velocity labeled.

That's because the critical AoA is determined by speed, and the required value of Cl is also determined by speed. Next up, a graph of weight, that is independent of mass?

[aaron886]

Please write down the equation for Re and then repeat that sentence and see if it makes sense.

 

For a given aircraft, at a given altitude, speed is the only thing a pilot can change that affects Re.

 

 

 

Well done, you've pointed out my error, I thought it was. You've also made my signature.

 

Sweet. Glad to see you're taking your lessons to heart. I've got no problem with you quoting that, so long as you punctuate it properly. Even better, include it in its entirety. After all, if flying an airplane were about maintaining altitude, you'd never leave the ground.

 

I could write down the equation for Re, but anyone here can go look that up on Wikipedia. What I'd like you to do is explain to me why the notion of a critical angle of attack exists if we can just predict the occurance of a stalling phenomena using an airspeed? If you know what you're talking about, you know that the static Reynolds number for an airfoil is a measurement that can help describe where it will reach critical angle of attack, but that Reynolds number is a dimensionless description of the condition of the airflow at a set condition for a specific airfoil. It's useless to say, "well at higher Re, the airfoil will stall!" And here's why: the airfoil is at higher Re because it is at a higher angle of attack. You've essentially found a way to incorrectly solve a "chicken and the egg" problem.

 

If you understand the nature and purpose of the Reynolds number, you would understand this.

 

That's because the critical AoA is determined by speed, and the required value of Cl is also determined by speed. Next up, a graph of weight, that is independent of mass?

You've got to be kidding me now. Where are you getting this stuff?

 

Shall we keep going? Do we need to wait until more people who know what they're talking about can arrive to help you out?

Edited September 4, 2012 by aaron886

[Exorcet]

In a regular engine the force is felt in the place where momentum is added to the airflow, which occurs over a length, not at one point at the end of the nozzle but it's not as simple as that because force struts act to transmit the force back to aircraft. Those struts do not act in the same way if thrust is turned through a large angle at the nozzle. At small angles though, it may not make a difference.

The engine mounts work practically the same with or without TVC. Yes, the angle makes a difference, but it's the same difference. You just don't see engines mounted at 90 degrees, while you may see nozzles positioned at such angles.

 

If there was no difference between a regular engine being re-orientated and a TVC nozzle being re-orientated then the F-35 STOVL would not require a lift fan, since tilting the rear nozzle would have the same effect by your argument. During steady flight however, it may be that the angle is so small, that the effect on rotation is negligible. Turn the nozzle through 90deg however and you will see there is a huge difference.

The F-35 would need a lift fan even if the engine was rotated 90 degrees, unless it was also moved so that the thrust line was perfectly matched with the cg.

 

 

Not necessarily because the same argument applies to the reduction of induced drag thanks to TVC. The thing you're working to reduce with TVC is also something you've increased by adding TVC, compounded by the fact that you're reducing forward thrust marginally to do it. Then there's the problem of balancing the drag reduction of not having to use trim against the drag increase of having nozzles pointing slightly down. One way or another work has to be done to create X amount of lift. Doing it directly with TVC is a 1:1 trade and reduces work in the forward direction which is bad. Doing it with lift causes a sub-1:1 trade because an X % rise in Cl cause an overall rise in Cd of < X% because kCl^2 < Cdo. AND Cl is increases by TVC weight anyway.

The loss of forward thrust though is essentially negligible. It's not small and marginal, it's basically zero. You don't trade 1 unit of forward thrust for 1 unit of thrust lift. It's more like 1:20 or possibly greater.

 

The nozzles don't create additional drag directly. They only add weight directly. That weight is responsible for increased wing induced drag. The wing is often more efficient than the horizontal stabilizer though.

 

Really though, the ACTIVE sort of proves that TVC works.

 

 

Doing the lift with TVC causes Cl not to require the 2% increase but increases the weight by Y%. If Y is > 2, then the benefit is counteracted already but you also have the reduction in forward thrust and extra non-lift producing drag caused by TVC. And over the life of an aircraft you also have cost of maintenance vs cost of fuel saved, plus you have to consider the possible reduction in conventional turn performance and air-combat fuel economy caused by the weight of TVC multiplied by 9+. It's not a simple equation at all. My guess is that TVC is there mainly to save planes not for fuel economy.

 

There is no meaningful loss in forward thrust or TVC produced drag though. I'll get to the 9 g thing next.

 

But you'd need to generate 9 times as much lift with TVC as you would in level flight plus the extra for the extra weigh of the TVC times 9. That would cause a substantial reduction in forward thrust tangential to the turn and definitely increase the drag on the rear of the plane while doing so. I don't see that one playing out beneficially whatever the possible advantages in level flight.

Yes, the TVC weighs 9 times as much at 9 g, but it's still only X% of weight. That percent is all that matters. The TVC is only ever 9% of weight, so 1 g, 9 g, -50 g, 1000 g, the TVC has the same effect.

 

 

In high speed flow on a slender body, why would you want to transport momentum across the boundary layer? Don't you think that turbulence in high speed flow presents a problem wrt separation? Think about momentum as being about direction as well as speed.

You only want to transport momentum to combat flow separation. If there is no threat of separation, you want laminar flow because of reduced drag.

 

The turbulence on a high speed body is not threat to separation, but it does cause additional drag.

 

Maybe you're confusing turbulent flow with turbulent air. The former produces disturbances and instabilities which are for the most part invisible to the plane. The latter is the kind of stuff that makes airliners rock around at altitude. The latter can causes separation, but the former doesn't really.

 

Turbulent flow is more able to resist separation at higher values of Re but the ability of laminar flow to resist separation is independant of Re and therefore better at low values of Re. As well as considering pressure gradient you must consider the affect of different flow regimes on the pressure gradient.

Turbulent flow resists separation at all Re. This is how Vortex generators work, trip the flow to turbulent to prevent separation.

 

So now we're arguing about the semantics of falling and not being able to maintain altitude.

Gliders don't maintain altitude, but they aren't stalled. Stall comes from AoA, not speed or altitude.

 

 

Yes but I've already mentioned way back that stall speed varies with altitude but for a given plane at a given altitude, the pilot can only control one variable in the equation for Re and that variable is V. Density he can't control. Kinematic viscosity he can't control. The characteristic length of his aircraft he can't control (yet). The original point which I countered (way back) was that stall isn't related to V, and clearly it is.

But it's not. You can find a stall speed, but that's just calculating the speed at which the critical AoA will be exceeded in your current flight condition.

 

However, once you hit that stall speed, if you simply push the flight stick forward enough, you will never stall. However, no matter how fast you fly, pull the stick back while right on AoA crit, you stall.

[marcos]

Sweet. Glad to see you're taking your lessons to heart. I've got no problem with you quoting that, so long as you punctuate it properly. Even better, include it in its entirety. After all, if flying an airplane were about maintaining altitude, you'd never leave the ground.

If it wasn't about maintaining altitude you'd always return to the ground unexpectedly at inconvenient times.

 

I could write down the equation for Re, but anyone here can go look that up on Wikipedia.

Maybe you should, it'd be a step in the right direction.

 

What I'd like you to do is explain to me why the notion of a critical angle of attack exists if we can just predict the occurance of a stalling phenomena using an airspeed?

The critical AoA is determined by the speed for any given aircraft at any given altitude. Speed is the fundamental input, critical AoA is just an output. Why are stall warnings based on speed? The original statement I corrected said that stall wasn't related to speed. It is. It's fundamentally inextricable. Every damn piece of crap you write to try and prove it isn't mathematically links back to speed somehow.

 

If you know what you're talking about, you know that the static Reynolds number for an airfoil is a measurement that can help describe where it will reach critical angle of attack, but that Reynolds number is a dimensionless description of the condition of the airflow at a set condition for a specific airfoil. It's useless to say, "well at higher Re, the airfoil will stall!" And here's why: the airfoil is at higher Re because it is at a higher angle of attack. You've essentially found a way to incorrectly solve a "chicken and the egg" problem.

 

If you understand the nature and purpose of the Reynolds number, you would understand this.

 

 

You've got to be kidding me now. Where are you getting this stuff?

 

Shall we keep going? Do we need to wait until more people who know what they're talking about can arrive to help you out?

Not only do you not understand aerodynamics, you don't understand maths. I may as well try teach a monkey to write poetry.

 

I've done an MEng in Engineering with multiple aerodynamics modules, a PhD based on UAV control algorithms, and worked in a wind tunnel at a major defence contractor, but feel free to continue talking absolute shit. What you're doing is trying to de-couple the fundamental equations of lift from speed.

Edited September 4, 2012 by marcos

[Exorcet]

The critical AoA is determined by the speed for any given aircraft at any given altitude. Speed is the fundamental input, critical AoA is just an output. Why are stall warnings based on speed? The original statement I corrected said that stall wasn't related to speed. It is. It's fundamentally inextricable. Every damn piece of crap you write to try and prove it isn't mathematically links back to speed somehow.

Stall warning are based on AoA. Basing them on speed would do no good. If your plane was banked but flying level, a speed based stall warning system set up for level flight would warn you after you stalled and started falling, which wouldn't be helpful.

 

Consider a case where there is no gravity, you will be able to avoid stall at any speed, but AoA still applies.