FM question

[Kula66]

Ok, question for any real jet pilots out there ...

If I get a 27/29/15 up to 40k' at 350IAS then pull into a dive straight down it seems to accelerate very slowly (I haven't timed it ... perhaps I will later)

 

I would have thought that with low air resistance, 30,000+lbs thrust and gravity it would have accelerated very rapidly indeed ... am I missing something or is this a FM problem?

 

Thanks,

James

 

PS> Please don't turn this into a 77 v 120 thread ...

[GGTharos]

Ah, but here's the thing: Up there, your thrust gets cut to shreds. Jet engine thrust dimishes significantly with altitude.

 

Ok, question for any real jet pilots out there ...

If I get a 27/29/15 up to 40k' at 350IAS then pull into a dive straight down it seems to accelerate very slowly (I haven't timed it ... perhaps I will later)

 

I would have thought that with low air resistance, 30,000+lbs thrust and gravity it would have accelerated very rapidly indeed ... am I missing something or is this a FM problem?

 

Thanks,

James

 

PS> Please don't turn this into a 77 v 120 thread ...

[JaBoG32_Dirty]

Hi,...

 

Speed results from an equilibrium between drag and thrust. When your speed increases during the dive, the resulting Drag will increase by the 2nd power. Therefore, twice the thrust (thrust + weight) will give you only 1.4 times the Speed at best.

 

-Additionally air density increases in the Dive --> more Drag --> Less Speed (TAS)

 

-Additionally supersonic flow effects cause additional Drag --> Less Speed

 

You might see a TAS decrease (external view) and IAS increase (HUD).

 

CU, Dirty

[Kula66]

Ok, so even forgetting thrust and being subsonic (350kts IAS) ... in a dive I should accelerate at 9.81m/s/s? With only drag to slow me down ... the faster I get the draggier I get ... still seems very sluggish in a dive ...

 

James

[GGTharos]

Ok, so even forgetting thrust and being subsonic (350kts IAS) ... in a dive I should accelerate at 9.81m/s/s? With only drag to slow me down ... the faster I get the draggier I get ... still seems very sluggish in a dive ...

 

James

 

The thing is, your maneuver also creates a lot of turbulence which increses drag ... there are very many factor, and drag is also very significant (ie. it's not a small quantity) ... I'll also point out that at 350IAS at 40k you're supersonic - it's the TAS that matters, and it's typically about 600+ at that point. Although you have a lot of air going over the contorl surfaces to control the plane swiftly, you may encounter 'compression' which reduces their effectiveness, and the reduced thrust compounds the sluggishness. Maneuvering at high alt takes some finesse.

[KILSEK]

Hi all.

 

I noticed that too, the seemingly slow decent even though you are full throttle and at -85* angle. I found the best way to get speed in a decent, without losing all my altitude, is just like about the same way I gain altitude. I point the nose at -15* angle and go full throttle.

 

I think GGTharos said it best: "Maneuvering at high alt takes some finesse". It sure does. Try going 500mph at 30K and pulling up to do 1/2 of a loop and then rolling over so that you are upright, yet have reverse heading, I end up real close to stall alot, but I have gained alt and reversed heading in a shorter amount of time than turning normally. I like taking an Eagle with three tanks and seeing what I can get done at high altitude. -KILSEK

[169th_Cobra]

i think you may be right Kula in that drag seems overmodelled. try a glide descent from altitude and you fall out of the skywhich doesnt seem right.

i'll ask an F18 pilot what sort of glide ratio he reckons they would get at idle thrust.

[GGTharos]

i think you may be right Kula in that drag seems overmodelled. try a glide descent from altitude and you fall out of the skywhich doesnt seem right.

i'll ask an F18 pilot what sort of glide ratio he reckons they would get at idle thrust.

 

F16 flame-out aproaches are done at 11 degrees angle ... probably not much different for most other aircraft (except an F-104 ;) )

[Mako]

I'll also point out that at 350IAS at 40k you're supersonic - it's the TAS that matters, and it's typically about 600+ at that point. Although you have a lot of air going over the contorl surfaces to control the plane swiftly, you may encounter 'compression' which reduces their effectiveness, and the reduced thrust compounds the sluggishness. Maneuvering at high alt takes some finesse.

 

Actually using TAS is not very useful for anything other than navigation. For aerodynamic considerations IAS should be used, it really is telling you how much air is flowing around your aircraft. As an example, by using IAS for your "stall speed" you will use the same value independant of altitude. Whereas by using TAS you will have to adjust for altitude.

 

At 40k there is not "a lot of air going over your control surfaces". While the air molecules may be moving faster over your control surfaces, there are not very many of them. The fewer molecules there are, the less force on the surfaces. "Compression" does have an effect but it is not the main reason for reduced control at high altitudes.

 

Regarding the seemingly low rate of acceleration: Drag is the culprit. Compared to the force of drag (at the speeds we're talking about) the force of gravity is really pretty insignificant. It is, after all, only one 1G.

[Kula66]

i think you may be right Kula in that drag seems overmodelled. try a glide descent from altitude and you fall out of the skywhich doesnt seem right.

i'll ask an F18 pilot what sort of glide ratio he reckons they would get at idle thrust.

 

Cheers Cobra ... I would expect an a/c to accelerate fairly quickly to terminal velocity, but I'm no aerodynamics expert :)

[169th_Cobra]

F16 flame-out aproaches are done at 11 degrees angle ... probably not much different for most other aircraft (except an F-104 ;) )

 

Thanks GGTharos, this is the sort of info we need.

 

When you say 11 degrees do you mean flight path angle or nose attitude?

 

I might try a few idle descents and see what sort of fpa (and distance) i get maintaining something like a .85M/300kt descent from FL350.

[Kula66]

Thanks GGTharos, this is the sort of info we need.

 

When you say 11 degrees do you mean flight path angle or nose attitude?

 

I might try a few idle descents and see what sort of fpa (and distance) i get maintaining something like a .85M/300kt descent from FL350.

 

Cobra, get hold of copy of F-16v5.pdf ... its FULL of this sort of stuff!!

 

James

[169th_Cobra]

cool, where can i get one of these.

[GGTharos]

It's 11 deg flight path. In LOMAC you can get about 1nm to 1000' using that glideslope in an f-15, provided you start at a high speed to begin with, engines idle or flamed out all the way in. If you start at 250kts, you'll lose a couple nm to speed up. Gotta be careful though, you need to lose all that speed for touchdown.

[Mako]

I just ran a simple test in the F15.

 

I found a source which gives the best climb speed (corresponds to maximum L/D speed) at about 170 kts. Payload wasn't specified so I just took 50% fuel.

Under these conditions I obtain an L/D of about 9. I got a similar number at 1000m and 9000m.

 

There are possible problems with my experiment.

 

1) Wrong speed. If 170 were too fast or too slow the true max L/D would be higher still.

 

2) Wrong payload. This would only affect the speed at which max L/D is obtained.

 

3) I used the distance measuring utility on the map. There may be accuracy problems with that which would introduce error.

 

 

Though I haven't been able to find a L/Dmax number for the F15, this seems a bit high but not unreasonably so. So it seems that if anything, drag may be "undermodelled"

[JaBoG32_Dirty]

Hi,...

 

A Glideratio of 1/9 seems about right. Aircraft with a low aspect ratio (F15) might have one as low as 1/5 depending on Loadout (not weight!). There's plenty of relatively simple formulars out there to calculate those ratios.

As for LockOn, I have to say, it's aerodynamics are far more realistic than the average simmer is able to appreciate. The realism of LO is definitly "player-limited" :-) ,... and I like it like that!!!! :-)

 

Dirty

[Kula66]

As for LockOn, I have to say, it's aerodynamics are far more realistic than the average simmer is able to appreciate. The realism of LO is definitly "player-limited" :-) ,... and I like it like that!!!! :-)

 

Dirty

 

Agreed, it certainly is a great piece of code! Best £30 I've ever spent!

 

James

[KILSEK]

Hi all.

 

edited:

Hi,...

As for LockOn, I have to say, it's aerodynamics are far more realistic than the average simmer is able to appreciate. The realism of LO is definitly "player-limited" :-) ,... and I like it like that!!!! :-)

 

Dirty

 

 

That is absolutely true. I think I have spent half my time flying an eagle and seeing what I can do at high altitudes(30K+), and the only limit was my imagination to try something. -KILSEK

[169th_Cobra]

I just did a few quick test flights with the F-15 and here is what i got.

 

Config: F-15 with full internal fuel and no external stores.

Cruizing at FL350 300KIAS at the start of both tests then an idle thrust descent.

 

300KIAS = 66nm

250KIAS = 78nm (flew level @350 till speed bled to 250KIAS then commenced descent)

 

To give you something to compare it too:

 

B737-800 from FL350 flying:

.78/280/250kt = 125nm or

.78/250 = 130nm

but what is most interesting is that flying a descent with the GEAR DOWN they reckon you will get 77nm!!

 

The F15 must be one draggy beast even without any external stores!

[SUBS17]

F-16 Flame out Landing

 

1st Jettison all stores

2/ Fly at 6deg AOA (210kts)

3/ Find the runway and fly towards it and note your glide path(must be between 11 and 17deg)

4/ once everything is under control contact tower and Declare an emergency.

5/Keep flight path marker at threshold until you reach 500ft, make sure landing gear is down and locked. Shift aim point down the runway and start to flare.

6/ On touch down raise nose to 10deg (gun cross) and aero brake.

 

(this is just a cut down version from the Falcon 4 manual and it works but I don't know how far you can glide but I do know its not that as far as the distances mentioned above considering that the F-16 is a flying brick.)

[Yo-Yo]

Actually using TAS is not very useful for anything other than navigation. For aerodynamic considerations IAS should be used, it really is telling you how much air is flowing around your aircraft. As an example, by using IAS for your "stall speed" you will use the same value independant of altitude. Whereas by using TAS you will have to adjust for altitude.

 

At 40k there is not "a lot of air going over your control surfaces". While the air molecules may be moving faster over your control surfaces, there are not very many of them. The fewer molecules there are, the less force on the surfaces. "Compression" does have an effect but it is not the main reason for reduced control at high altitudes.

 

Regarding the seemingly low rate of acceleration: Drag is the culprit. Compared to the force of drag (at the speeds we're talking about) the force of gravity is really pretty insignificant. It is, after all, only one 1G.

 

 

You are very far from the truth: Mach number is calculated using just TAS. That's why the high altitude flight has relatively LOW IAS values but high TAS and M values.

The main equation for the aerodynamic force is

 

F=.5*S*C*q, where q is the dynamic pressure proportional to IAS.

 

The C coefficient (drag or lift) is a function of M and it significantly depends on it.

 

The maximum of the CD (drag coeff) is around the M=1. It raises about 3 times regarding to the low M value.

 

Diving at 350 KTAS from 40' (12 km) you dive at M=1.13 where unfortunately the drag coeff is very close to its maximum value. The full AB thrust there is about 63 kN that is about 0.5 of the MiG-29 weight for example.

The drag at this point is about 43 kN. The clean force that accelerates the plane is about Weight+20 kN or 1.16*Weight and the overall acceleration is 1.16g only.

 

 

So, it's not a bug - it's just pure physics...

[Kula66]

Ok Yo-Yo, I can believe the formulas you posted ... now if you put numbers into them ... start diving at 350kts and after 10secs how fast should you be going? My physics is fairly basic ...v=u+at?

 

James

[bflagg]

Try going 500mph at 30K and pulling up to do 1/2 of a loop and then rolling over so that you are upright, yet have reverse heading

 

correct me if I'm wrong, but isn't this called the Immelman manuever?

[Yo-Yo]

Ok Yo-Yo, I can believe the formulas you posted ... now if you put numbers into them ... start diving at 350kts and after 10secs how fast should you be going? My physics is fairly basic ...v=u+at?

 

James

 

 

Though 10 secs is too much to use this formula for precise calculation and the FM does it more frequently because "a" is not a constant it depends on V and altitude...

OK, let's imagine that you start to dive from 12000 m with the speed of 350 KIAS (1.2 M and about 360 m/s) and you are gaining about 11.5 m/s each second you dive. After 10 s of diving it seems to be 360+11.5*10 ~ 475 m/s and the altitude seems to be 12000-(360+475)*10/2~7800 m.

 

We assumed that the acceleration from the Thrust-Drag was constant. If we calulate it once again @7800 m we can see that now it's about 12.2 m/s that is within the engineering accuracy and our assuming was correct.

 

So the IAS must be about 1150kph or 625 kts.

 

I have performed the test with the MiG-29 (13 000 kg) and here they are:

 

at 12080 m (the plane passes this point diving vertically, wings unloaded, AB already on and has full thrust) - 1291 kph (359 m/s)TAS

ten secs later (time was slowed down for x4 and it was 40 secs on the external chronograph I like to use):

altitude - 7909, and 1738 kph (483 m/s) TAS.

 

What's wrong?

 

Should I mention that it's a good idea to record the flight and then analize the record? :)

[Kula66]

Yo-Yo, Thanks for taking the time to explain it ... I'm off to do some testing!

 

James