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Error: Hurricane pitches up when flaps are lowered


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The Hurricane has curtain flaps behind the centre-of-lift, therefore it should pitch down as the flaps are deployed.

The historical pilots' notes say the same.

 

I seem to remember the same problem, initially, with one of the Spitfire models.

 

Otherwise, it seems really great. The cockpit is a delight.

 

56RAF_phoenix56

Edited by 56RAF_phoenix56
typo
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  • 4 weeks later...

(not talking about specific planes, simply generally):

 

Even the most simplified model for flight dynamics like the one from a starters university course would usually place the generatation of lift from flaps centered half-way down the chord at a "most forward point". Never in front of that. And the centre of pressures of the airfoil will always be in front of this point, in subsonic flight anyway. 

 

So, as a result, with any trailing-edge flaps, there should be a nose down moment around the aerodynamic wing's centre, even when flying a most primitive sort of simulator. 

 

Talking about the entire plane, if no nose-down is observed it means the plane's neutral point is so far behind the wing's aerodynamic centre, that the addition of flap lift is creating a bigger nose-up moment around the neutral point that it counters and exceeds the nose down induced moment from earlier. This can only happen if the wing is so far in front of the neutral point that the lift generates sufficient leverage. Can anybody think of an airplane in BOX that follows this type of configuration?

Edited by ACG_Cule
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43 minutes ago, ACG_Cule said:

So, as a result, with any trailing-edge flaps, there should be a nose down moment around the aerodynamic wing's centre, even when flying a most primitive sort of simulator. 

 

There actually are aircraft, where throwing out flaps will lead to a pitch-up.

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Hawker Hurricane PN:

 

Page 26:

 

11. GENERAL FLYING

 

(i) Stability:

 

The aircraft are normally just stable longitudinally, but when carrying 90-gallon drop tanks, or R.P. and one 90-gallon drop tank, they become unstable longitudinally and, in the first case, 190 M.P.H.-I.A.S. is the minimum comfortable flying speed. In conditions of absolute calm this can be re-duced to 180 M.P.H.-I.A.S. When carrying bombs, R.P., or con-tainers, longitudinal stability is unaffected.

 

(ii) Change of Trim:

 

Undercarriage Down - Nose slightly down

Flaps Down - Nose down

 

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Isn't it also speed dependent? Can't the shift in the position of the centre of lift and the shift in the position of the centre of drag differ depending on the speed? (something similar in principle to aileron reversal)?

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At speeds you should deploy the flaps, nose wants to go down. Other than that, I'd say it is more AoA dependent (AoA does move the center of lift on the wing) than speed dependent how trim changes are. Lots more speed just makes flaps go away.

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If you look into the NACA test of the Hurricane, and compare the gliding (up/up) and the landing (down/down) condition, you'll see that

at 130mph IAS the elevator:

needs to be pulled ~3°

at 100mph IAS the elevator:

needs to be pulled ~5°

 

with flaps deployed, based on the differences between the two curves that give elevator angle over speed for both conditions. Means the aircraft wants to put the nose down, as flaps (and gear) are lowered.

 

There are also curves for elevator forces, but they are not as easily comparable as the angles, because they are for different trim settings. But given that the landing configuration is trimmed fully tail heavy and the gliding configuration trimmed nose heavy, one can epxect higher pull forces than noted for the landing configuration if it was also trimmed nose heavy. But, even as it is, the landing configuration requires significantly more pull force than the gliding configuration at the same speed. Also means the aircraft wants to put the nose down, as flaps (and gear) are lowered.

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3 hours ago, Bremspropeller said:

 

There actually are aircraft, where throwing out flaps will lead to a pitch-up.

 

Please differentiate in my statement the difference between the pitching moment around the wing's aerodynamic centre (which you quoted)  and the pitching moment acting on the whole plane (last paragraph, which you should have quoted). The hypothesis for aircraft configuration which pitch up with flaps was stated, and consequently I asked whether there are any planes in the game which correspond to this configuration, because I do not know if there are, but the hurricane should definitely not be one of them.

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 I guess someone needs to go fly every aircraft with the flaps at various detentes and with various airspeeds?

Btw. There is an old development update where they used real data from a Yak-52 to better inform their approach to this.

 

10 minutes ago, JtD said:

[...] (and gear) are lowered.

 

Hmm... that could complicate things? :)

 

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1 hour ago, Avimimus said:

Isn't it also speed dependent? Can't the shift in the position of the centre of lift and the shift in the position of the centre of drag differ depending on the speed? (something similar in principle to aileron reversal)?

centre of pressures indeed shifts forward with AoA which has an implication on the trim for different speeds. With higher Mach number with compressibility/shockwaves  it moves back. But at speeds at which flaps are able to deploy this is not something to consider.

 

I wonder if they implemented aileron reversal, as it requires an aeroelasticity model and custom parameters per plane... I think they simply implemented control lock with speed and change in maneuvrability of the different surfaces with speed which may take into account (artificially) some of the aeroelastic effects.

Edited by ACG_Cule
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Just now, Avimimus said:

Hmm... that could complicate things?

 

Yes, but as you can see from the manual which ZachariasX quoted, the gear has the same effect as the flaps, just to a lesser degree. So the trend is the same, but the numbers may be smaller.

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1 minute ago, ACG_Cule said:

centre of pressures indeed shifts forward with AoA which has an implication on the trim for different speeds. With higher Mach number with compressibility/shockwaves  it moves back. But at speeds at which flaps are able to deploy this is not considerable.

 

I wonder if they implemented aileron reversal, as it requires an aeroelasticity model and custom parameters per plane... I think they simply implemented control lock with speed and change in maneuvrability of the different surfaces with speed which may take into account (artificially) some of the aeroelastic effects.

 

Sorry! My apologies... I was thinking about adverse yaw... (which has similarities, but isn't the same) :)

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52 minutes ago, von_Tom said:

I'd just go with what a pilot says:

[...] Expect a BIG nose-down pitch trim change with flap extension. After all, the split flaps extend 85 degrees for landing! Oh, and it normal to run out of aft elevator trim on approach. [...]

 

It does what the manual says it does.

 

But I don't think thers is too much dama into this. They fixed it with the Spit and they can fix this. And I mean, honestly, when flying the Hurricane I am so happy that I get to the point where I need flaps again, I don't mind the issue too much.

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21 hours ago, ZachariasX said:

[...] Expect a BIG nose-down pitch trim change with flap extension. After all, the split flaps extend 85 degrees for landing! Oh, and it normal to run out of aft elevator trim on approach. [...]

 

It does what the manual says it does.

 

But I don't think thers is too much dama into this. They fixed it with the Spit and they can fix this. And I mean, honestly, when flying the Hurricane I am so happy that I get to the point where I need flaps again, I don't mind the issue too much.


First step of fixing an issue is admitting there is an issue ;)

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On 12/2/2020 at 4:19 PM, ACG_Cule said:

Please differentiate in my statement the difference between the pitching moment around the wing's aerodynamic centre (which you quoted)  and the pitching moment acting on the whole plane (last paragraph, which you should have quoted). The hypothesis for aircraft configuration which pitch up with flaps was stated, and consequently I asked whether there are any planes in the game which correspond to this configuration, because I do not know if there are, but the hurricane should definitely not be one of them.

 

All fair and square.

Since the thread is talking about the Hurricane, which is the complete airplane, I was referring to complete airplanes.

It's more than just the center of pressure moving about. It's also the increased downwash impinging on the stabilizer, which makes things more complicated than just taking a look at the wing and comparing resulting pitch moments.

 

 

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As I recall it the developers motivated the pitch up when flaps were deployed based on testing in a Yak trainer. They posted some pictures from this trial and wrote some text about it and the conclusion from this trial IIRC was that if you hold the stick fixed there is a pitch-up and if you let the stick move due to the change in elevator hinge moment due to the changed downwash on the tail, then the nose would pitch-down. Based on this the argument was that the modeling with a pitch-up is “correct” since the pilot is not moving the stick.

 

In my opinion this is a convoluted argument because the elevator hinge moment is not only affected by downwash but also by a trim tab and it’s inconsistent to model effects due to a change in elevator hinge moment in one case but not the other.

 

So if the argument is that the modeling is “correct” since the pilot is “holding” the stick, then by the same argument you should get no change in trim when you move the trim tab because the pilot is holding the stick in that case as well and why would he in one instance be affected by a change in the elevator hinge moment but not in the other?

 

So IMHO there is an inconsistency in the flight modeling here and I’m sure we all agree that a change in the trim tab angle should move the stick in-game and by the same token then so should the stick move when the flaps are deployed because both actions change the elevator hinge moment.

 

A change in elevator hinge moment results in a new stick position, be it from a change of trim tab angle or as a result of a changed downwash angle over the tail due to flaps.

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On 12/4/2020 at 3:54 PM, jcomm-in-il2 said:

Looks more or less evident from the following video that the pilot has to pull when gear and then flaps are lowered during approach...

 

https://youtu.be/h6m7z2HPW34?t=464

Edit, I was an Ass, I mis-read this entirely.

Edited by Melonfish
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56 minutes ago, Melonfish said:

With the greatest of respect I suggest you watch the video carefully.

I'm not sure why people are arguing over this issue, it's clear from pilots who CURRENTLY fly this plane that the flaps cause the nose the lift and downward/forward trim is required on landing. I can't see why this needs to be argued over when we have actual sources for it?

Thx for your comment Melonfish !

 

Could you help me find those evidences in the net? I searched for a while but all I found was this video. Would really like to read / ear that the way it's modelled in IL2 is the right one. I remember the Spitfires were initially released with that pitching up tendency too on flap deployment and latter fixed to agree with reports from true pilots of the type.

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54 minutes ago, jcomm-in-il2 said:

Thx for your comment Melonfish !

 

Could you help me find those evidences in the net? I searched for a while but all I found was this video. Would really like to read / ear that the way it's modelled in IL2 is the right one. I remember the Spitfires were initially released with that pitching up tendency too on flap deployment and latter fixed to agree with reports from true pilots of the type.

Actually I apologise entirely, I completely mis-read this and from what i can see the dropping of the flaps does appear to cause a nose down characteristic WITHOUT the requirement for trimming.
I'm now camp Nose down.

"I normally lower the flap about one third of the way around the finals turn. This produces a strong nose down trim change which is easily corrected."
source:

https://haa-uk.aero/document/the-shuttleworth-collection-hawker-sea-hurricane/

 

 

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  • 2 weeks later...

Hi

 

If the wings are above the cockpit there is nose up effect when flaps deployed, and if wings are installed below the cockpit there is nose down effect when flaps down irl.

 

thats why germans invented to put level stabilizer and flaps to same instalment so when you roll flaps down you can at same time trim up to neutralize nose down effect. Very well invented by Germans...

 

-Veccu-

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10 minutes ago, LLv24_Veccu said:

If the wings are above the cockpit there is nose up effect when flaps deployed, and if wings are installed below the cockpit there is nose down effect when flaps down irl.

 

Wing layout has very little to do with trim changes as flaps go down. It is much more about change of centre of lift on the wing due to changed camber and change of angle of attack of the stabilizer.

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  • 1 month later...
21 hours ago, ACG_Vietkong said:

We have everything right )

Unfortunately in this instance they do not.

everything else about the plane seems fine but as the thread has demonstrated the plane should nose down, not up when flaps are engaged.

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On 12/19/2020 at 1:26 PM, LLv24_Veccu said:

If the wings are above the cockpit there is nose up effect when flaps deployed, and if wings are installed below the cockpit there is nose down effect when flaps down irl.

 

No.

 

 

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4 hours ago, Melonfish said:

Unfortunately in this instance they do not.

everything else about the plane seems fine but as the thread has demonstrated the plane should nose down, not up when flaps are engaged.

Let me translate what I quoted from a Dev/tester (or something) into plain english. It means they know it´s not  right, but they aren´t going to do anything about it, at least not now. Il 2 is not competing with the big daddy sim for now for FM quality. Only the little "sim". 

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10 minutes ago, ACG_Vietkong said:

Let me translate what I quoted from a Dev/tester (or something) into plain english. It means they know it´s not  right, but they aren´t going to do anything about it, at least not now. Il 2 is not competing with the big daddy sim for now for FM quality. Only the little "sim". 

where on earth did you get that from?

 

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