# Pitts / Fixed Wing Flight Model Review

• Thanks! The airbrake works pretty well. Slows down cruise slightly which I can adjust. Just requires some thinking about parameters to prevent each change from having unintended consequences on something else.

W.r.t. props, I tested lowering the governor minimum pitch to 0 which made the plane sink like a stone at idle albeit with the engine maintaining cruise RPM. So the negative alpha of the prop blades w.r.t incoming air in this case is properly working to slow the plane down rapidly. At least thats what i'm assuming is happening. I also tried increasing the lift coefficient of the blades thinking this would also increase negative lift in this negative alpha scenario. That also seemed to work. But, it would be useful if the lift coefficients for airfoils had optional separate values for positive and negative alpha. What I did also was influencing the lift in positive alpha cases.

My sense is part of the issue is related to what you mentioned -- the RPM is staying too high even at relatively low manifold pressure settings. I.e., the engine in real life has more of a braking effect than what is currently being modeled. I am going to see what RPM the actual plane drops to at 11 inches MP and 110 mph -- the conditions from abeam the numbers and base leg.

2. Related to my second question, I found that lowering the tailwheel spring constant to ~200 makes the ground handing fairly realistic at ~30-45mph, a little too squirrely below that, and a little too stable above that. So I thought it would be useful to have the spring constant a function of speed.

• Official Post

Airfoils have a linear lift increase from the negative stall all the way to the positive stall, we can't deviate from the experimental results here, that would be the wrong direction.

ClAlpha is the gradient of the lift, that is lift coefficient per radiant of angle ot attack. 2 pi is the theoretical value for a thin airfoil, so around 6.28 is the correct physical value. Only for very short props you would see a decrease and only for insanely thick airfoils (e.g. towards the spinner) you would have a slightly higher value.

Cl0 is the lift at zero angle of attack.

Please check out the wiki for more details on the airfoil parameters: https://www.aerofly.com/dokuwiki/doku.…aft:tmd:airfoil

What you probably want to do is have the blades stall earlier in the negative region, for this you will need to change the AttachedCenter value, I think towards negative, so sth. like -0.07 for 4 degrees.

Regards,

Jan

• An airflow driven flater pitch blade acts like an upside down thin cambered wing so the airflow on the rear/reverse side will be flowing against unstable, increasing then decreasing pressure unless the rpm and airspeed dependent coarse relative flow direction totally stalls the blade. The rotating blade will be generating significant ‘lift’ in the reverse direction unless the engine stops when the simple turbulent flow past the blades will produce less rearwards force.

• My sense is part of the issue is related to what you mentioned -- the RPM is staying too high even at relatively low manifold pressure settings. I.e., the engine in real life has more of a braking effect than what is currently being modeled. I am going to see what RPM the actual plane drops to at 11 inches MP and 110 mph -- the conditions from abeam the numbers and base leg.

And the anti rotation drag from the ‘flicked angle’ airflow encountering what resembles the wing of the FS2 Sopwith Camel flying inverted.

• Hi Jan (Jet-Pack) ! Thanks a lot for your forthcoming informations regarding the Pitts flight-model --> it matches to some observations I made on my rig at that time when I had both Aerofly versions installed: Aerofly FS (released 2012) and of course AFS2 from now.

Which leads to my question: May it be true, that the flight-model of the Pitts in Aerofly FS 2 has been "defused" for PC - Simmers (less rudder input) compared to the Pitts in Aerofly FS (2012) ?

Because I find the Pitts in version one more realistic regarding its flight-physics/model, more depending on correct rudder inputs. Especially during take-off it acts - from my perspective - more aggressively and realistically! ---> annotation: I am a strikt rudder-pedals "flyer".

Then as I tried the Pitts an AFS2 I even became some kind of disappointed (reg. too easy, less challenging to "fly").

Are these observations only the result of my imagination, or do I have a point there?

All the very best,

sincerely Alex, from good old Germany

• Official Post

Short answer: yes it was changed, not particularly to "defuse" it. I know especially the knife edge suffered a bit from the changes I made to get it to tumble...

Regards,

Jan

• I got my tailwheel endorsement and basic aerobatic course in the Super Decathlon as well. It's the only TW airplane I've flown, but boy the most fun plane as well!

Forward slips are very impressive in it, compared to doing them in the Cessnas and Pipers I've flown. A lot of fun.

Took a little while to nail the wheel landings though!

"I did a lot of them in the real Super Decathlon and the behavior was very similar to doing them in the DCS Mustang. Was actually easier in the real plane for me which again is why is see so much value in simulation even for stick and rudder training."

Yep, definitely easier in the real plane than the DCS Mustang! But very challenging at first. Had a few PIO moments, in which I promptly powered up and went around.

Man, I'd love to fly a Pitts or Extra one day!

Thanks.

Redtail

KFRG, KTEB, KEWR, KLGA

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Edited 2 times, last by Redtail (January 23, 2019 at 1:43 PM).

• Thanks! The airbrake works pretty well. Slows down cruise slightly which I can adjust. Just requires some thinking about parameters to prevent each change from having unintended consequences on something else.

(...)

When you get a satisfying improved behaviour, please make sure to share your tmd's as a package on https://flight-sim.org/

Thanks for your inputs

Cheers

Antoine

Config : i7 6900K - 20MB currently set at 3.20GHz, Cooling Noctua NH-U14S, Motherboard ASUS Rampage V Extreme U3.1, RAM HyperX Savage Black Edition 16GB DDR4 3000 MHz, Graphic Card Gigabyte GeForce GTX 1080 8GB, Power supply Corsair RM Series 850W, Windows 10 64 bit.

• Short answer: yes it was changed, not particularly to "defuse" it. I know especially the knife edge suffered a bit from the changes I made to get it to tumble...

Well, which lines do I have to edit to get back the "old" flying behaviour? Is it an possible or even easy task?

• Official Post

Well, which lines do I have to edit to get back the "old" flying behaviour? Is it an possible or even easy task?

Pretty much all of them

It can be done but I'd rather not put back old faulty values, rather fix the current pitts

Regards,

Jan

• The NASA Foilsim program has unfortunately died with Java but a simplified version is working off their web site. The pressure distribution across the chord isn't available but lift and drag for various aerofoils can be demonstrated. I tried a 3% thickness 2% camber foil to represent a propellor blade. The relative airflow direction hitting the blade depends on the airspeed and propellor rotation speed, imagine the airflow coming from say a nominal 45 degrees.

The pics represent the lift and drag of a prop in cruise, feathered and windmilling at low rpm with the relative airflow approaching the blade stall. The numbers are arbitrary and the relative wind is simplified but it gives an idea of how disasterous it is to have a failed feather in the event of an engine failure. The windmilling prop has an enormous rearwards thrust and the drag keeps the rotation speed low maintaining the coarse negative angle of attack on the blade.

• I got my tailwheel endorsement and basic aerobatic course in the Super Decathlon as well. It's the only TW airplane I've flown, but boy the most fun plane as well!

My first hands on aerobatics was in the earlier Citabria version, ‘airbatic’ spelt backwards. It was my first time starting a fuel injected engine, I had to, the instructor in the back seat didn’t have the full engine controls so I had to fuel pump prime her and start her initially in idle cut off. It seemed so strange, sophisticated and high tech’ at the time. We did nearly all the flying inverted, out of Moorabbin in Melbourne, Australia

There was talk of FS2 getting a Ju 52/3m, possibly from outside.

• Official Post

The NASA Foilsim program has unfortunately died with Java ...

I've experimented with a lot of different tools like this, but it doesn't help much. Sure, if I know the airfoil and have the coordinates then I can plot the lift , drag and momentum vs angle of attack and that helps for the normal flying behavior but Aerofly also needs to simulate the angle of attack region beyond 20 degrees and below -10 degrees, where these tools start to fall apart. We need a full 360 degree chart and we need one for the fuselage as well, good luck with that

That's why these tools are good for a first guess, which I have already done with the new pitts. Anything beyond that has to be checked manually.

Regards,

Jan

• Thanks for the comments everyone. Foilsim looks pretty neat too. So, I found in the real plane the rpm doesn't drop much when power is reduced to 11 inches. More the deceleration that is noticeable I guess. Flying a pattern I have 21 inches on downwind, ~120mph, 2,700 rpm. Abeam the numbers, 11 inches and RPM is still at 2,700 but plane is decelerating fast at this point. RPM stays pretty much between 2,500-2,700 while pitching for 105-110 until power is reduced to idle in which case rpm drops to 2,100 at ~100 mph. This is in the S-2C which has the 3-bladed Hartzell Claw. I believe the Extra 330 has the same prop.

Regarding using the airbrake to fine tune the prop, seems like it would be useful if the drag from the airbrake could be set proportional to the magnitude of propeller alpha for negative alphas but then zero for positive alphas. I think it would help dial in the deceleration without affecting normal flight and also be more true to what is actually happening, albeit still a hack.

I should have time this weekend to play around a lot with the sim parameters and will report back with any questions / findings.

• Had a couple initial questions regarding yaw in the Aerofly model:

1. There seems to be excess stability in the yaw axis in the air. The plane will react to a change in rudder position but then decay back to neutral rudder even if rudder is still pressed. I tested moving the y position of some things (like airbrake and prop) to create yaw. Then tried correcting for this yaw with the rudder. The two things that seemed apparent is that it is sort of hard to create sustained yaw and it is also sort of hard to correct for any sustained yaw with the rudder. Is there some sort of yaw stability augmentation or auto-trim currently in place? If so, is there a way to reduce?
2. On a related note, is there a way to significantly increase adverse yaw? The documentation says induced drag is modeled entirely inside the aerowing class. So was thinking there isn't an easy way to increase it. As an alternative, I had considered putting two airbrakes (one out on each wing) and having their input tie to aileron.output. Is this doable and can you have more than one airbrake per TMD file? Also, I see there is an AeroDrag class but it isn't documented. Curious if that could be used.

Thank you!

• Official Post

1. What you can do in the sim that you can't do in real life is adding an infinite force to move the rudder. IRL you can't instantly deflect the rudder which you sort of can in the sims. This creates side effects that seem unrealistic but actually come from unrealistic inputs. But what you are probably experiencing is another effect, yaw inertia. It might still be too high, because, as mentioned earlier, I just guessed the masses of the wings. And the heavier the wings the more inertia you have.

2. I'm not actually sure how we could increase it. Yes you can add as many airbrakes as you want, I have at least 7 in the airbus (for each gear and for ground and flight spoilers on each side). But this is also not the correct way of doing it unless you know for sure that the ailerons cause an asymmetric drag due to flow separation, e.g. the nose of the ailerons deflects downwards and causes significant drag.

AeroDrag would be used to model drag of wires or hot air ballons, etc. We usually account for this in the Cd0 of the fuselage and therefor never used it in FS 2. Same code as the airbrake just without any inputs.

What I plan to do once I get time to change the pitts is to add aerodynamics for the vertical wing struts and gear. Both of these would destabilize the yaw and help to maintain a constant slip. Also the aerofuselage Cm could be higher to significantly destabilize any yaw and pitch.

Last but not least there are parameters for the prop (drag and force coefficients for when the incoming airflow for the propeller is off the rotation axis)

Regards,

Jan

• I mentioned about a year ago that the AFS2 Pitts will not hold knife-edge. IPACS contacted a Pitt's pilot who said that's the way it really is. What has been your experience? I've seen Pitt's doing knife-edges in air shows. What are they doing to make that happen?

• Official Post

I've never seen an actual knife edge of a Pitts in any video, most of them were high bank angle side slips, none were really 90°. All of them cheated by tilting the wing slightly.

Regards,

Jan

• So I've made some good progress and have learned a lot about the physics model after experimenting with many settings. The ground handling in my opinion is pretty much as good as you could ask for and that was simply done by turning the tailwheel spring constant down. I increased adverse yaw by reducing the lift coefficient of the vertical stabilizer. This generally destabilizes the yaw axis which also feels a bit more real. Has the additional benefit of making the high speed landing and takeoff roll slightly more realistic. I also slightly reduced the lift coefficient of the rudder (flap) to balance with the decreased vertical stabilizer effectiveness. I increased propwashrotation slightly on the vertical stabilizer which gives a slight left turning tendency on the ground. However, nothing I have done is really giving a sustained yaw in the air. Even a fairly massive increase to propWashRotation will make ground handling uncontrollable but once in the air everything flies pretty much the same. Same goes for crosswind.

I also found this in the aerowing documentation:

"... P-Factor which can also be modeled with the AeroPropeller class using its `.LateralDragCoefficient`, `.LateralForceCoefficient`, `.SideThrust` and `.DownThrust` attributes."

Would be curious as to who wrote this and if its accurate, if an example of modeling p-factor could be provided? I do not see a way to do it if I understand these settings correctly but hoping I am wrong ; )

Other things I am trying to figure out are: 1) getting prop speeds and airspeeds closer. 2) forward slip. 3) getting the bounce on landing slightly more realistic. If you touch down slightly on the main, the oscillation is too high a frequency. No questions on these latter parts yet as I haven't experimented enough.

RoyPettit I have done transition training into the Pitts but not anything other than basic aerobatics so don't want to comment on the knifeedge as it would be just speculation. That's why I have limited my Aerofly efforts to basic ground handling, adverse yaw, etc. Having said that, most of the low and slow knifeedges you see (Skip Stewart) are in planes that have much more horsepower and also not at 90 degrees as JetPack mentioned. Four point rolls at speed seem reasonable in Aerofly. But again, don't take my word for it!

• In proper knife edge, the fuselage is providing virtually all of the lift. Flat sided fuselage is best. The Pitts has a tiny rounded fuse.

…in theory, with sufficient fuselage side area (fuselage is now the lift device), lots of thrust, and a powerful rudder (to control the fuselage angle of attack), you could fly all day long in knife edge.

Edited once, last by Mr. Bean (February 2, 2019 at 6:52 PM).