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jcomm

Torque, Roll, Slip, Yaw, Spiraling Slipstream... Freeeze!

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Guys,

 

I'm talking about the prop effects, and how they affect prop aircraft in X-Plane 10, bringing up some frequent discussions at the forums.

 

I believe that Austin is always commited to turn X-Plane into an even better simulation. The subject of prop effects brought to so many posts, not only here at AVSIM, is just another one that will find it's slot in the complex TODO list that LR manages...

 

In a video Austin recorded a few days ago, and in other experiments he is running using his own aircaft, he his gathering important information to fine tune prop effects for X-Plane.

 

I'm sure whatever will result will only bring us an even better flight simulator!

 

Honestly, when I think of the amount of work that will probably be required, and I know LR is involved already in quite a big deal of work to bring up 10.30 and following versions, and because, given the nature of X-Plane's approach to flight modellling it will not be done with some thing as simple as a scalar to tune p-factor, torque, etc, but rather as an updated model of the prop effects as a whole ( probably including the interaction of the spiraling slipstream with different aircraft surfaces at various flight attitudes, etc...), I don't really care if it's going to be available for 10.40, 50, 60... or even only for X-Plane 11. What matters is that this particular subject, so many times discussed at this forum, has found it's time slot ( among the many other features other users ask for... like me who also keep asking for proper Moon phases, while I can perfectly recognize the importance of the subject is NILL in terms of flight dynamics accuracy...). We all want something, not all want the same, so, I guess it's impossible to please all at the same time :-)

 

This is what makes X-Plane different - it's Alive and Kicking! ;-), and can only get better!

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Probably by adrressing the fact that something needs tuning when it comes to torque, more than other factors, being the main factor in X-Plane 10 causing the rolling tendencies, and thus creating a non-coordinated flight situation if we use only rudder to overcome it, as we are supposed to in RL...

 

When in X-Plane's props we counter the prop effect using rudder, right rudder on a CW rotating prop aircraft, then, when the wings are level, the ball will be well to the left.

 

In the real thing the ball will be centered, and sometimes left aileron can even be required because the aircraft might have rolled a bit to the right.

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Ok, with Austin's permission, I will reproduce here the message exchange between him and Andy Goldstein, following the publication of Austin's video on his C400 - the one I had posted a few days ago.

 

I believe this will help us to better understand what is really this all about :-)

 

Also, Austin confirmed yesterday Andy's observation during another test flight in his C400 and has stated that he will start looking at the subject and investigating to further enhance X-Plane's FDM!

 

Austin's availability is an example, Andy's contribution fundamental, just as that from Alec, Larry, Geof, Murmur and others who have discussed this subject on many posts. Alec was probably the trigger when he sent a private message to Austin where he described with very concise words his own flight experience. Austin decided to carefully investigate his claims, and... here's the result :-)

 

 

 

Transcript follows:

===============

 

Hi Austin -

Good experiment! I've done the equivalent in the Cherokee 180 I fly, only in the opposite direction - from stable 75% power cruise, pull the engine to idle and watch it roll to the right.

However, there's an added wrinkle that makes the situation more complicated than you think: Look at the slip indicator in your video, after you've applied full power. It's going to the right, indicating a need for right rudder, obviously to counter the increased spiral propwash. So what's happening (to some extent) is you're getting a right slip from the propwash, which is in turn picking up the right wing. (Towards the end of your video, BTW, the slip indicator is now going the other way, because you're slipping to the left due to the extreme bank angle. That actually tends to stabilize the roll.)

I got exactly the same thing, in the opposite direction because I was cutting power. What's important is that the slip precedes the roll - i.e., when I tried it, first the ball went left, then the airplane started to roll right. I did the experiment again, hands still off the yoke, using trim to hold the nose up, but using pedals to keep the ball centered. The result: the airplane did not roll to speak of. What that tells me is the primary cause of the roll is the change in spiral propwash, translated through yaw-roll coupling.

In truth, the engine torque in most light planes is small potatoes. Yes, obviously it matters, but the roll torque that results from the spiral propwash change is much greater. So I'm suspecting that when others try this experiment (or the equivalent) the engine torque is getting lost in the noise. Here's another benchmark that I'm not done working out: The Cherokee, like other low wing airplanes, draws fuel from one side or the other, never both. The fuel tank is about 6 feet off the centerline, so a 10 gallon fuel imbalance gives me a 360 ft-lb roll torque on the airplane. That's about the same as the full power torque of the engine. It requires a modest amount of pressure on the yoke to hold the airplane level - or I can crank in some rudder trim to fix it. And the point is, when I use the rudder trim to hold up the heavy wing, there isn't a lot of ball deflection. (The power off experiment gives me a good half ball deflection.) What I still need to do is to go up on a really quiet day (rare in New England), trim the airplane for level with the tanks full, then fly an hour on one tank, adjusting rudder trim as needed, and note how much the ball deflection changes. My impression so far is it's not much, but most days the ball wanders around enough due to turbulence to mask any real change.

At last July's Oshkosh, I was able to corner Barnaby Wainfan for about 15 minutes to discuss this whole issue. (Barnaby Wainfan is one of the leading aero experts in the home builder community and the industry in general.) What I got out of the conversation was
(1) I'm not missing any important aero effects in terms of what I've thought of so far.
(2) Barnaby thinks X-Plane's roll damping is understated. This is something I intend to research.
(3) He acknowledges that there is an important psychological component to roll stability - i.e., the pilot
     subconsciously holds the airplane level, just like we normally stand up straight without thinking about it.

Point (3) is really important, and it's supported by my observation that when I'm flying under the hood, the airplane always seems to be less stable than otherwise. Likewise, in a sim you don't have the same level of feedback (peripheral vision and seat of the pants) that you have in a real airplane that makes you keep it upright subconsciously.

There's another important point regarding sims: Many people put in a null zone in their control setup. Worse, the Saitek yoke has a null zone built into the controller hardware! The null zone makes the control response completely unrealistic. Where in a real plane a tiny bit of yoke pressure will correct a small roll, with a null zone you get no effect from small control movements - you need a substantial amount of yoke movement before you get any response, and then it's usually an overcorrection. Null zones are fundamentally evil - they expose small perturbations in the airplane's behavior that you would otherwise unconsciously correct. No airplane flies dead straight with exactly zero yoke input, and that's what a null zone forces you to do.

Anyway, from having played around with this stuff for a long time, I've found the following things I need to look into:
(1) X-Plane's yaw-roll coupling seems weak. This is especially true for high wing airplanes, where a slip causes
     a pressure buildup on the side of the fuselage that substantially increases the effective dihedral of the wing.
(2) I want to review the whole spiral propwash model. I'm suspecting that it's understated, and its in flight behavior
     seems really wonky. Single engine planes don't seem to require any right rudder during climbout, which
     suggests to me that the propwash is going in the wrong direction or something. I also suspect that it won't
     be possible to get spiral propwash right without modeling the airflow over the fuselage.

All stuff that promises to keep me busy for a long time...

- Andy

 

End of Transcript:

==============

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Excellent stuff. The last number 2 is something I have been observing for quite a while when I said rudder input needs work. Perhaps finally sometime in the future a proper Chandelle will be able to be simulated! Cheers to progress!

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(3) He acknowledges that there is an important psychological component to roll stability - i.e., the pilot

subconsciously holds the airplane level, just like we normally stand up straight without thinking about it.

 

I'm sure that's true. But............. I have taken a lot of climb and cruise photos of the RV6 left and right wings in climb and cruise. The RV has much shorter wings than a Cessna 172. The engine is the same 180 HP Lycoming, as the Cessna. The RV is a two place. Much lighter, climbs three times as fast, cruises around 60 knots faster, and rolls a lot faster. It's just not as stable as a Cessna 172.

 

 

Reviewed a bunch of these pics again. When I built the plane, I put the auto-pilot servo in the right wing tip, and the aileron trim tab/servo on the right wing...........to offset pilot weight. I see, that in most of my cruise photos with a passenger, that the aileron trim tab is slightly up. This pushes the right aileron slightly down, which is in effect ..... left roll.

 

 

For the climb photos, the ailerons either look in perfect trail, or just that bit of right aileron "down again". This depends on whether there is a passenger or not. Fuel would usually be even, at take off.

 

In the meantime, the RV was sensitive, but not "twitchy". The air loads on the flight surfaces, keep the stick in place, as long as it's trimmed. A non-pilot could fly it for hours. Just would have to make slight corrections for heading and altitude...........which is actually easier, than I find with any desk top flight simulation.

 

 

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Prop effects :-)

I hope he adds spiraling slip stream to the Prop effects ! I just took the default Cessna for a spin and it requires no right rudder on climb. A single engine prop in real life requires a lot of right rudder to counter act the slip stream when climbing.

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I hope he adds spiraling slip stream to the Prop effects ! I just took the default Cessna for a spin and it requires no right rudder on climb. A single engine prop in real life requires a lot of right rudder to counter act the slip stream when climbing.

 

Yes, that's exactly one of the points on Andy's observations:

 

(2) I want to review the whole spiral propwash model. I'm suspecting that it's understated, and its in flight behavior seems really wonky. Single engine planes don't seem to require any right rudder during climbout, which suggests to me that the propwash is going in the wrong direction or something. I also suspect that it won't be possible to get spiral propwash right without modeling the airflow over the fuselage.

 

This is, after all, the same problem we have in FSX, where there's no spiraling slipstream modelling but rather p-factor ( and torque ) being modelled, plus gyroscopic precession, as the only prop effects available ( inertia from the turning prop too, of course... )

 

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So nice after pointing this funky rudder behavior two years ago that it is finally being taken seriously.Glad Andy has the power to be listened to instead of being labelled an xplane hater/ fsxer. Truth although it can take time usually triumphs...

 

I hope the funky scenery in my area of the world will finally also be addressed-it needs similar attention.

All good-let's make this sim the best!

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 (2) I want to review the whole spiral propwash model. I'm suspecting that it's understated, and its in flight behavior

     seems really wonky. Single engine planes don't seem to require any right rudder during climbout, which

     suggests to me that the propwash is going in the wrong direction or something. I also suspect that it won't

     be possible to get spiral propwash right without modeling the airflow over the fuselage.

 

 

 

This is correct it is too weak now, I have made the same observations on my Piper Archer a long time ago.

HOWEVER, there is one problem.  Austin should not fix this until AFTER he fixes the groundmodel - here is why.

 

If you want to test how the real prop effect is, open my Piper in PlaneMaker, set the engine sideways cant

to 3 deg right (the real aircraft has this).

Now try to take off.... Most likely you will end up in the ditch to the right.

This is because of the sideload friction model in XP is too weak.  This is not really a flightmodel problem, but a computation thing.  Ben S explained it once, don't remember exactly what it was.

 

So basically, if he fixes the prop, then most aircraft will become useless on the ground.... doh..

 

M

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Morten, you're absolutely right about the problems with ground behavior.

 

I wasn't yet able to glimpse if it's really a friction problem or some other factor, or a mix though...

I only remember how much I loved X-Plane's ground handling in v8, and during the first stages of v9. It was far better than what we had in MSFS ( and still is IMO ), but people kept complaining about how their airliners were so difficult to land under strong x-winds, probably because they never thought a 737 on a crosswind landing (after touchdown ) can have the crew pushing the yoke all the way ahead and upwind, while the feet are doing opposite to keep the centerline... Apparently Austin changed it to please those... and we were left with what we still have now :-(

 

As far as engine cant goes, if a good prop slipstream / wash / p-factor / torque / gyroscopic... model results from his work than, like in RL, even with a realistically canted engine to the right on a CW rotating prop single, we will still have to use right rudder to counter the prop effects and their "left tendencies".

 

Above all we will not be forced to use art stab for what it wasn't meant to, or set trim tabs opposite what they are IRL. He has to start from some point, and yes, some models will be compromissed and require an update, but, that's certainly a lot better than what we have now.

 

I have been using either Art Stab to overcome the ground handling problems and the torque roll, or a mix of art stab and the "Torque fix", but each of this solutions has it's problems too. I sometimes understand those authors who keep designing their models as close to real specs as possible (and being "accused" of how irrealistically they roll due to prop effect ), in the hope that some day thsese problems with the modelling of prop effects are solved, and that's what I really believe will start with this move from Austin :-)

 

In the end, we will all be better, for sure :-)

 

P.S.: How's the 737 project going ?  We want that beauty!!!!

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737 is doing well, we are getting closer each day :) 

 

Actually at high speeds (on the ground) real aircraft are very slippery sideways due to low weight on wheels (wing lift)

which reduces friction significantly.  So in XP most of the problem is below Vmcg.  E.g. doing a single engine takeoff with a twin engine airliner

like a 737 is almost impossible.  (We offcourse can on our 737 though)

 

M

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