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jcomm

The CESSNA AG WAGON pilot....

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JCOMM: Hi, good afternoon, allow me to ask you something about your agricultural flyer :-)

 

PILOT: Go ahead!

 

JCOMM: Now, there's this flight simulator, you know, one of those programs used in PCs for simulating flight, just like Microsoft Flight Simulator you probably heard about... And... the one I am using now has some rather peculiar tendency to make prop aircraft bank left when the prop rotates just like the one in your aircraft, but showing almost no yaw to the left after takeoff. Also, when you reduce throttle you'll notice a right bank tendency(*)... Since I am from the gliders team, I really don't know if it is ok?

 

PILOT: No, no bank, just some yaw and I use the rudder to counteract...

 

JCOMM: Ok, just as my fellow tow pilots say, but their Robin or Rallye is a lot less powerful than your plane.... Thank you for the info, have a nice flight!!!!

 

PILOT: You're welcome! Hey João, please fill it less on the left tank!

 

João (the arodrome fuel station operator): How much less?

 

PILOT: 60

 

JCOMM: Ah! curious! Why are you filling the left tank with less fuel?

 

PILOT: Oh, otherwise I have to keep pushing the column right all the way up, or when I really have to firewall the beast :-)

 

JCOMM: :-( Ok, thanks... Moooommmmmmmmyyy!!!!!!! I want to get my hands one one of these things, can I? Will I ever be able to tell for sure who is right? Explain?!.... Well, maybe X-Plane :-)

 

JCOMM came home that day and started MS FLIGHT ... Picked the Maule M7, went up to 6000', used Clear Skies (winds non significant) and throttled back... He starred at the AH and took a reference point in the far distance... Soon the Maule was banking right, at iddle and turning right as well.... Then he repeated with the RV6 (the same even if not that evident), with the Stearman, etc... They all did the same, while as usual, even if not as intense as in X-Plane, they all banked left on takeoff or high power climbs....

 

JCOMM knew previous versions of MSFS didn't do it... and... having a degree in mathematics, he started to investigate, not before sending Austin yet another email on the subject, which, as usual, soon got a reply back.... A week later, after never having dedicated the right amount of time to understanding the right banking tendency (**), he finally got it :-)

 

Now, if only X-Plane10.30 also included spiraling slipstream effects on the tail surfaces, enforcing the yaw to the left, that would really really be a great thing!

 

 

(*) JCOMM also knows ELITE does it as well since v7, with all aircraft models except the Seneca, which is counter-rotating, starting to bank and yaw right when throttle is reduced for descent...

(**) Even on an aircraft without any structural compensation for the prop effects, such as canted engine or tail surfaces, etc...

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I am not completely clear about your background but if your not an aeronautical engineer and are interested in more information concerning the aerodynamics of flight in conventional aircraft I would recommend "See How It Flies by J.Denker". It is an internet based book (easily Googled) by a physicist who also happens to be a GA flight instructor. IMHO it is simply the best explanation bar none available to the none specialist public.

 

Obviously if you are an aeronautical engineer....ignore the above.

 

TIM

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Obviously if you are an aeronautical engineer....ignore the above.

 

I am not... I'm just a glider pilot and someone very interested in flight simulation. Thanks for the Denker link, which I know for long ;-)

It's indeed a very good and didactic text!

 

Main idea with this true "story" is to point out how sometimes we tend to make wrong assumptions about reality, in many fields, and to support it for years, sometimes based on empirical observation subject to strong "noise" / "bias", leraning by being told, etc...

 

Without being able to experience things for real, and if that is possible, dedicate the right amount of attention to the details, and even preferably base our assumptions/observations/measures/etc... on some theory, it is very difficult to get it right....

 

It's also intended to show that sometimes we think that by asking who we think is the right source is far from good, either because the source himself never cared about thinking about it, or simply never noticed it, or couldn't care less about it... We have a few RL prop aircraft pilots at this and other forums, such as Geofa, LAdmson, some 3pds, Austin himself. They, as pilots and also flight simulation fans, are probably the best to ask to.

 

Until very recently I couldn't understand why a prop aircraft could bank right (with a clockwise rotating prop) when throttle is brought back, as for instance during a descent, something very evident on most prop aircraft in X-Plane, but also in ELITE and, in MS FLIGHT - the so called simulation game... :-)

 

It also took me years to understand that, contrarily to what I was taught when I got my license, as well as in highschool, the pressure differential between the "topside" and the "downside" of a wing is not the source but rather one of the consequences of the generation of lift.

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I think slipstream as well as propwash is already modelled, but to me prop aircraft in X-Plane yaw way too less when airborne. While on take off run you have to steer quite a lot to hold the plane on centerline.

Real aircraft like a Robin DR 400 with a 180hp Lyco have an engine side cant of 3 degrees! And you still have to hold right rudder in climb, but not so much on ground roll. 3 degrees side cant for that aircraft in X-Plane would be way to much.

I did some testing in XP and took the vert Stab away from may Robin, that I'm working on in Plane-Maker for my own purposes and guess what, the plane doesn't steer anymore(it still steers a little, but this time to the right and only just slightly), but of course is unflyable!

I did further testing and found out that you can design vert Stabs which neutralize steering on ground and yawing entirely.

 

So my conclusion is that there must be some kind of slipstream modelled in XP, may be not enough.

 

A few words to prop torque:

The banking due to prop torque is still way exaggerated imo, I never found something like that in RL.

I don't have a PPL, but I had the chance to take control of some small single engine piston aircraft like the DR's in flight.

I'm not saying that there is no torque, it has to go somewhere, but there may be some forces which are not present in XP, which eliminate prop torque on a broader level, because on the aircraft I flew with in RL I never felt the urge to aileron trim the plane.

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Marcus, you are absolutely correct. Bank due to torque is exaggerated in most add on aircraft.

Some developers, who will remain nameless, have actually blamed Austin and have openly said this is a flaw in the core flight model. Their solution is to set the aileron trim in the aircraft flight model to a default value in Planemaker to compensate for the bank at high/full power. This works on takeoff and high speeds, but it does not work when bringing the throttles back to idle when there is minimal torque. Because of the excessive trim to compensate for the torque at those high power levels, bringing the power back to idle results in the aircraft banking too far in the opposite direction. So, doing this kind of correction is incorrect.

The flight model can be adjusted to not bank so much from the prop torque without having to set a default position for the aileron trim to compensate for it. Again, it's a series of those default values that Planemaker cannot make a guess at. eg. tire friction, fuselage friction, nacelle friction that need adjusting.

I'll see if I can make a video to show its effects in the near future.

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I have started flying a 160HP Airplane weighting Less than 800KG, it requires quite a lot of Right Rudder During the Takeoff Roll and ClimbOut, I only notice the Tendency of the Aircraft to Bank with power when doing Slow Flights and Power On Stalls, but maybe that's just the Strong Yaw that makes the Airplane to Bank also, but never during normal flight. Flying on a day of Smooth Weather, it's incredible how stable that airplane feels, it would make a lot of people that says FS feels on Rail to be surprised!

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I have started flying a 160HP Airplane weighting Less than 800KG, it requires quite a lot of Right Rudder During the Takeoff Roll and ClimbOut, I only notice the Tendency of the Aircraft to Bank with power when doing Slow Flights and Power On Stalls, but maybe that's just the Strong Yaw that makes the Airplane to Bank also, but never during normal flight. Flying on a day of Smooth Weather, it's incredible how stable that airplane feels, it would make a lot of people that says FS feels on Rail to be surprised!

 

First off, I like the part I hi-lighted. It's what I keep telling everybody! Smooth means "real smooth". As to banking from torque..........my Van's 180 HP constant speed RV6 would push down of the left tire, if I gave it power quickly, and especially on a touch and go. A bit of right aileron takes care of the problem, and it doesn't remain for the climbout. At least not to the point of being noticeable. The short tailed 6 takes a lot of right rudder on the takeoff roll & initial climbout. Once airspeed picks up (which it does very quickly), the need for right rudder is much less. Engine is canted to the right, and rudder has a fixed tab.....for right rudder.

 

L.Adamson

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Well, I do not find X-Plane aircraft unstable. I am using only the default Baron 58, C 90, and the LES Sundowner, LES dc-3 and Carenado Seneca.

Quite on the contrary I find them all very stable (even before starting to use the augmented stability sliders!).

I strongly believe that only 2 factors contributing to the rather twitchy rides we sometimes experience in X-Plane10, and that should really get addressed:

 

1) The way X-Plane translates variable winds to it's internal weather model which only uses "shear"

2) The way X-Plane models turbulence

 

Provided I manually disable any turbulence after reading RW weather, my rides are perfect. Of course my experience ( RL ) comes from flying 15 to 18 meter (54ft) wingspan gliders, and believe me it is quite a ride, with the stick being allover, when on "bombastic" thermal days, or under ridge lift, or even worse, when entering wave or by mistake hitting rotors bellow... Usually gliders are very sensible to any air currents, but I have flown as a passenger on many GA aircraft, and also as a prospective ULM pilot on a couple of models, including the only RV-9 presently operating in Portugal (from LPSC).

Another very positive aspect I came across when flying X-Plane aircraft is what I find to be a much better:

- power / pitch / VS relationship

- braking effect of CS props when throttle is reduced/idled

compared to even the best FSX aircraft models.

Sideslips/forward slips, ate least in the default K21 feel also very good to me, the same not being true about many FSX aircraft, including some well acclaimed add-ons...

 

Regarding that left / right bank due to torque, I am trying to get deeper into the why's and the how's on that, with help of one of this forum members :-) I also find it way too "strong", but at the same time I know that X-Plane needs to address spiraling slipstream effects (assymetric hitt of the tail surfaces generating yaw...). Maybe when this get's finally addressed by Austin he will also take the chance to make things easier for aircraft designers...

 

Yet, even if too intense, these effects make sense, and one of my objectives with the CESSNA AG WAGON pilot story, a real one, was to point out how sometimes we (pilots) don't even pay attention to some types of corrections we "automatically" use in RL. If you look at the yoke/manche on X-Plane models, the amount of deflection requiered to counteract these effects is minimal, and in RL one would probably not even notice it sometimes... The yaw, that's another story... :-)

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Another very positive aspect I came across when flying X-Plane aircraft is what I find to be a much better:

- power / pitch / VS relationship

- braking effect of CS props when throttle is reduced/idled

compared to even the best FSX aircraft models.

Sideslips/forward slips, ate least in the default K21 feel also very good to me, the same not being true about many FSX aircraft, including some well acclaimed add-ons...

 

I can not wholefully go along with those statements. I do use both sims, and there are a variety of aircraft for the other sim, that do quite well..... including power to pitch. As to slips...........there are some acclaimed addons for FSX/FS9 that do extremely well. In regards to braking effects of the C/S...........it's hit & miss with both sims, and none impress me completely. BTW--- a recent pilot report for a lightweight three blade composite constant speed prop on an RV.............say's that this prop doesn't have near the speed reducing effect, that their Hartzell C/S's do.

 

L.Adamson

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I can not wholefully go along with those statements. I do use both sims, and there are a variety of aircraft for the other sim, that do quite well..... including power to pitch. As to slips...........there are some acclaimed addons for FSX/FS9 that do extremely well. In regards to braking effects of the C/S...........it's hit & miss with both sims, and none impress me completely. BTW--- a recent pilot report for a lightweight three blade composite constant speed prop on an RV.............say's that this prop doesn't have near the speed reducing effect, that their Hartzell C/S's do.

 

L.Adamson

 

The big difference is, these flight model attributes that jcomm is speaking of, are present in the X Plane core flight model. Not coded in as plug ins.

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