power / vs relationship...

[Anatoli-Kagari9]

I am really finding this bird weird, I must confess.... All I have learned about basic prop aircraft operation, namely the well know - power to control vs, trim for speed - apparently fail in this p-51d.

 

I have to assume it's realistic the way it is, but it is still very strange to me...

 

In level flight, after triming for a given speed, say, 200knot, I pull the throttle, and, V/S instead of starting going down, rises. There is actually a mild climb sometimes. The way around if I increase throttle (the aircraft pitches down and VS decreases).

 

With time, an apparently huge inertia (not very conform with the so sensible pitch response to stick inputs) the aircfat starts descending / climbing according to my throttle inputs.

 

Is this correct? I mean - should a power reduction be immediately accompanied by a V/S reduction and a power increase by a V/S increase???

 

I am also finding the stall characteristics very weird, I must confess... I was waiting for it after months reading this forums, but I am using the accelerometer to perform my loopings / tight turns and make sure I am between 2 - 4 G, and even so it's very very difficult not to depart...

 

Tell me this is real, I will believe, but at the same time think those WW2 p-51 fighter pilots were very tallented people...

Edited November 21, 2012 by jcomm

[sobek]

In level flight, after triming for a given speed, say, 200knot, I pull the throttle, and, V/S instead of starting going down, rises. There is actually a mild climb sometimes. The way around if I increase throttle (the aircraft pitches down and VS decreases).

 

My first guess would be that you are seeing yaw induced pitching due to gyro forces, second would be changes in slipstream affecting control surfaces.

 

Also, the flight characteristics depend very much on fuel load, with full center tank, the pitch axis has reversed stability characteristics.

[Anatoli-Kagari9]

My first guess would be that you are seeing yaw induced pitching due to gyro forces, second would be changes in slipstream affecting control surfaces.

 

Also, the flight characteristics depend very much on fuel load, with full center tank, the pitch axis has reversed stability characteristics.

 

Thx for the quick answer sobek... I am using the default "Instant Action Flight over Poti", where the central Fuel Tank is empty.

 

I trim for level flight at 45MP, 2500RPM, about 285 knots IAS, and for instance if I reduce throttle drastically, then the V/S slowly goes down as well as pitch. I then add, say, full power, the descent continues with not even the slightest hint of a reduction in V/S (?).

 

Under the same scenario but say at 50MP, 2500RPM, if i suddenly cut the throttle ( I know it's not the correct procedure ) the aircraft actualy starts to climb (V/S indicator rises clearly).

 

In terms of pitch, at least, the aircraft appears to have a neutral pitch stability (center tank empty).

Edited November 21, 2012 by jcomm

[sobek]

Are you sure that your ball is centered at all times during the maneuver?

[Anatoli-Kagari9]

Are you sure that your ball is centered at all times during the maneuver?

 

Yes.

 

I use the Instant Action scenario - free flight over Poti - level off and trim the airplane, using MP and RPM as indicated above, and allways between 280 and 300 knot IAS.

 

Then i reduce throttle, or increase, depending on the test, and of course, retrim due to roll caused by torque (reducing causes right bank, increasin left bank, and I am using the aileron trim to compensate...) The ball stays perfectly centered all the time.

 

I would record a movie if I could... I know tracks can be saved, but I still have to find my way around the various menus and key options :-)

Edited November 21, 2012 by jcomm

[EtherealN]

I would record a movie if I could... I know tracks can be saved, but I still have to find my way around the various menus and key options :-)

 

Not sure what menus and key options you're looking for?

 

After flight, on the debrief screen, press "Save Track". Done.

[Anatoli-Kagari9]

Not sure what menus and key options you're looking for?

 

After flight, on the debrief screen, press "Save Track". Done.

 

See? I told you - I am still learning a completely different World - DCS :-)

 

Well, I'm a former Flaming Cliffs user, from 2005... At least I found the best way to configure my Saitek X52 and Saitek rudder system...

[Yo-Yo]

Thx for the quick answer sobek... I am using the default "Instant Action Flight over Poti", where the central Fuel Tank is empty.

 

I trim for level flight at 45MP, 2500RPM, about 285 knots IAS, and for instance if I reduce throttle drastically, then the V/S slowly goes down as well as pitch. I then add, say, full power, the descent continues with not even the slightest hint of a reduction in V/S (?).

 

Under the same scenario but say at 50MP, 2500RPM, if i suddenly cut the throttle ( I know it's not the correct procedure ) the aircraft actualy starts to climb (V/S indicator rises clearly).

 

In terms of pitch, at least, the aircraft appears to have a neutral pitch stability (center tank empty).

 

The aircraft has positive stability otherwise you would apply constant control inputs to fly at certain IAS. It is close to neutral only if the fuselage tank is full.

 

The reason of power/VS relationship is your IAS. Imagine that you fly level at certain power and at the speed drag = thrust at. If you add power EVEN IF IT DOES NOT CHANGE YOUR TRIM (and it really changes though) you IAS begins to rise. The plane is rimmed for certain speed level flight, i/e/ for certain AoA, so the lift increases with the IAS. Now you have lift more than the weight and the airplane begins to bend its flightpath up - VS goes up.

 

The main point is that VS begins to change not after power is changed but after your speed is changed.

[Anatoli-Kagari9]

The aircraft has positive stability otherwise you would apply constant control inputs to fly at certain IAS. It is close to neutral only if the fuselage tank is full.

 

The reason of power/VS relationship is your IAS. Imagine that you fly level at certain power and at the speed drag = thrust at. If you add power EVEN IF IT DOES NOT CHANGE YOUR TRIM (and it really changes though) you IAS begins to rise. The plane is rimmed for certain speed level flight, i/e/ for certain AoA, so the lift increases with the IAS. Now you have lift more than the weight and the airplane begins to bend its flightpath up - VS goes up.

 

The main point is that VS begins to change not after power is changed but after your speed is changed.

 

Yes yo-yo, my problem is that when in straight & level flight (difficult to achieve with the p-51...) I increase throttle, the v/s dacays (aircraft descends initially). On the way around I have situation under which decreasing throttle makes v/s rise (?) Both were unexpected to me...

 

On such tests, run based on the default instant action situation "flight over Poti", the fuselage tank is empty!

 

I also find the phugoids very very long... The aircraft sometimes behaves like it had some sort of stability augmentation system (?). In level flight I deflect momentarily the yoke (up or down) and, instead of the aircraft showing positive static stability, it assumes the new pitch attitude and mantains it. If I instantly pull the yoke and release it the nose pitches up and stays there, and the airplane continues to climb, without any hint of trying to return to the original trimmed situation. The same if I instantly push the yoke forward and release it - aircraft pitches down and starts to descend, not showing any prompt return to the original trimmed situation.

 

On the stall characteristics, I find it rather strange that it stalls in turn at 2G or just a bit above it, again starting from level flight and initiating a turn, around 280/300knots IAS. Fuselage tank empty, clean wings (no ammunition)...

[Yo-Yo]

Yes yo-yo, my problem is that when in straight & level flight (difficult to achieve with the p-51...) I increase throttle, the v/s dacays (aircraft descends initially). On the way around I have situation under which decreasing throttle makes v/s rise (?) Both were unexpected to me...

 

On such tests, run based on the default instant action situation "flight over Poti", the fuselage tank is empty!

 

I also find the phugoids very very long... The aircraft sometimes behaves like it had some sort of stability augmentation system (?). In level flight I deflect momentarily the yoke (up or down) and, instead of the aircraft showing positive static stability, it assumes the new pitch attitude and mantains it. If I instantly pull the yoke and release it the nose pitches up and stays there, and the airplane continues to climb, without any hint of trying to return to the original trimmed situation. The same if I instantly push the yoke forward and release it - aircraft pitches down and starts to descend, not showing any prompt return to the original trimmed situation.

 

On the stall characteristics, I find it rather strange that it stalls in turn at 2G or just a bit above it, again starting from level flight and initiating a turn, around 280/300knots IAS. Fuselage tank empty, clean wings (no ammunition)...

 

Possibly you mix PITCH and AoA terms. Short-period or acceleration stable plane keeps not the pitch but AoA. If you perform doublet you must leave the plane at the same pitch it has before. If your doublet was not accurate the plane will maintain the pitch you leave it at (in short-period terms) and it will begin to change it only when its IAS begins to change due to new power balance.

It concerns the case if the plane was in steady level fligh - no acceleration/climb (power balance).

If you change pitch when you have some excessive power the plane begins to accelerate and climbv in he same time.

 

Anyway, I found the tests you are providing very tricky because when you change power the yaw changes and it causes significant pitch trim changes due to gyroscopic effect. Moreover, if you leave yaw the non-axial prop inflow causes pitch moments that can distort your experience.

For example, as you add power the plane yaws left. As it reaches significant left yaw it causes nose heavy pitch moment decreasing AoA - it's obvious that before the plane begins to move phugoid up it can start going down.

Edited November 22, 2012 by Yo-Yo

[Anatoli-Kagari9]

Yo-Yo, I also believe my tests have some problem in the way they are performed.

 

I notice mainly the bank (to the left) due to torque when adding power than the yaw. I used to complain about to much bank instead of yaw on another simulator I use. I believe on most prop aircraft, certainly not of the order of power of the p-51, it's mainly yaw and not bank you get when adding power, mostly due to asymmetric slipstream hiting various regions of the left fuselage... On some GA aircraft, take a Robin DR-400 or a Rallye Commodore, both lowing glider tugs used at my airclub, power, mainly during climb, cause a noticeable yaw to the left, with the ball slipping right and requiring right rudder, but no hint of left bank.

 

I'll keep testing and maybe even save some tracks and post it her, so that you can help identifying what I did wrong.

 

Enjoying the p-51 anyway :-) ... A lot to learn!!!!

 

Possibly you mix PITCH and AoA terms. Short-period or acceleration stable plane keeps not the pitch but AoA. If you perform doublet you must leave the plane at the same pitch it has before. If your doublet was not accurate the plane will maintain the pitch you leave it at (in short-period terms) and it will begin to change it only when its IAS begins to change due to new power balance.

It concerns the case if the plane was in steady level fligh - no acceleration/climb (power balance).

If you change pitch when you have some excessive power the plane begins to accelerate and climbv in he same time.

 

Anyway, I found the tests you are providing very tricky because when you change power the yaw changes and it causes significant pitch trim changes due to gyroscopic effect. Moreover, if you leave yaw the non-axial prop inflow causes pitch moments that can distort your experience.

For example, as you add power the plane yaws left. As it reaches significant left yaw it causes nose heavy pitch moment decreasing AoA - it's obvious that before the plane begins to move phugoid up it can start going down.

Edited November 22, 2012 by jcomm

[Anatoli-Kagari9]

Here goes a "track". It's very poor, I know, and was recorded in a hurry, but I try to show what I mean - decrease power V/S increases, Increase power V/S decreases...

power_vs.trk

[Yo-Yo]

Yo-Yo, I also believe my tests have some problem in the way they are performed.

 

I notice mainly the bank (to the left) due to torque when adding power than the yaw. I used to complain about to much bank instead of yaw on another simulator I use. I believe on most prop aircraft, certainly not of the order of power of the p-51, it's mainly yaw and not bank you get when adding power, mostly due to asymmetric slipstream hiting various regions of the left fuselage... On some GA aircraft, take a Robin DR-400 or a Rallye Commodore, both lowing glider tugs used at my airclub, power, mainly during climb, cause a noticeable yaw to the left, with the ball slipping right and requiring right rudder, but no hint of left bank.

 

I'll keep testing and maybe even save some tracks and post it her, so that you can help identifying what I did wrong.

 

Enjoying the p-51 anyway :-) ... A lot to learn!!!!

 

Do you take in account that yawing causes bank if you do not remove it?

Anyway, the yaw/bank reaction is approved by the P-51 pilots.

[Anatoli-Kagari9]

Do you take in account that yawing causes bank if you do not remove it?

Anyway, the yaw/bank reaction is approved by the P-51 pilots.

 

Of course I do :-)

 

I fly gliders in RL, so, I know it into an even better extent :-)

 

Please don't get me wrong. I bought DCS p-51 after months reading this forum, and exchanging oppinions with users. I am sure this is the most detailled p-51 simulation available... I am just trying to understand some aspects of the simulation, and trying to figure out how they match my sometimes certainly wrong expectations about how I thought a P-51 would perform :-) Also, in the above attached track, I am everywhere in terms of yaw and bank, because it was recorded in a hurry....

[Yo-Yo]

Of course I do :-)

 

I fly gliders in RL, so, I know it into an even better extent :-)

 

Please don't get me wrong. I bought DCS p-51 after months reading this forum, and exchanging oppinions with users. I am sure this is the most detailled p-51 simulation available... I am just trying to understand some aspects of the simulation, and trying to figure out how they match my sometimes certainly wrong expectations about how I thought a P-51 would perform :-) Also, in the above attached track, I am everywhere in terms of yaw and bank, because it was recorded in a hurry....

 

I do understand. :) But discussing the P-51 and any of high-wingloaded extraordinary-powered prop planes we always must remeber that they are slightly different from the GA planes. The main mechanics is, surely, the same but the amount of the certain effect can be different.

[aaron886]

I believe on most prop aircraft, certainly not of the order of power of the p-51, it's mainly yaw and not bank you get when adding power

 

No.

[Anatoli-Kagari9]

Yo-Yo and aaron886,

 

thank you for your answers :-) - aaron886, short/concise one :-)

 

One of the best things about having embraced this simulator, rather reluctantly because I do not like the military aspects of flight... is that:

 

1) I feel back at basic school, avidly reading / learning the manual and trying to master the aircraft in the sim, which implies properly configuring the joystick, rudders, etc...

 

2) The challenge is there, and the detaill, the immersion of both the DCS World and the p-51 are so good that I am now fully concentrated, at last, in a single simulation platform :-)

 

Going online and trying a few missions will take at least a couple of months of learning, I am sure :-)

Edited November 22, 2012 by jcomm

[ttaylor0024]

Either it is a bug, or the Mustang is one weird bird in real life.

 

If you add power, you are going to get a left turning tendency, not a decent. Opposite is true if you decrease power, your plane will have less of a left turn and will descend. Gyroscopic procession is ruled out in this case.

 

If you trim in normal cruse at say, 180kts, your plane will want to stay at that same airspeed. Pull power, your airspeed will decrease, thus pitching over due to your drag overcoming your thrust, which pitches down until you pass 180kts, in which lift will take back over, etc until you reach 180kts. Again, the same is true when you climb but backwards, power increase, lift, climb, decrease in airspeed, evens out at 180kts. This is now ruled out.

 

Spiraling slipstream has to due with left turning tendency also, pretty much a non-factor in pitch.

 

Not too sure why it's doing that, I'll look at the track in a little bit.