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Taildraggers

Postby dilbert » Sat Jul 31, 2021 4:23 pm

I flew a lot of taildragger light planes in the early 1950s: J3, J5, PA11, PA12, PA18, Tailorcraft, Aeronca Champion, and Cessna 140.

None of them had noticeable ground looping tendencies. In fact, it was something I never thought or worried about. The Cessna 195 (which I never flew) did have a reputation of being touchy on landing.

Despite the fact that their CGs were behind their wheels, I attribute the longitudinal stability on the runway of those planes to aerodynamic lateral resistance to rotation being greater than the turning moment caused by their modest weight being behind their CGs.

Kind Regards

P.S. found these comments interesting: https://www.supercub.org/forum/showthre ... In-Toe-out
Last edited by dilbert on Sun Aug 01, 2021 9:43 am, edited 1 time in total.
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Re: Taildraggers

Postby wlbragg » Sat Jul 31, 2021 4:57 pm

to aerodynamic lateral resistance to rotation

How? What force counteracted this and made it more stable? If this is FDM related then this would be a good subject to bring up with the JSBSim folks and see if you can get to the bottom of the forces in play. It would be great if pilot experience match simulator experience. There needs to be some logic thrown at it.
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Re: Taildraggers

Postby dilbert » Sat Jul 31, 2021 5:50 pm

Quite simple. The sides of the fuselage resist turning because they are impacted by the airstream, if the weight moment tries to swing the plane horizontally. Stick your vertical hand out the car window at 60 mph, and rotate it horizontally. You'll get the idea real quick. However, the resistance to ground looping diminishes with wing loading; as there is greater weight moment relative to the aerodynamic resistance of the fuselage to rotation. The planes I mentioned all had light wing loadings. If JSBSim only takes into account the position of the CG, it won't necessarily produce an accurate result for aircraft with light wing loadings.
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Re: Taildraggers

Postby S&J » Sat Jul 31, 2021 7:14 pm

I think it's not a single thing. Jsbsim provides a mass of data that could be looked at to provide the justification needed to make changes.

Aerodynamics I don't think is a major contributor at low speeds.

If you were to apply a pushing force without the engine working, there's no turning force and the plane would just go straight down the runway.

So we know it's derived from the engine prop combination... Moments and p value, are these too great ?

The turning rotation is opposed by the planes mass inertia and the resistance of the undercarriage, are these too low ?
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Re: Taildraggers

Postby dilbert » Sat Jul 31, 2021 8:27 pm

"Aerodynamics I don't think is a major contributor at low speeds."

Nor is weight moment, as both vary with the square of the velocity.

The initiation of the ground loop does not occur with the plane traveling straight down the runway. However, if the weight moment exceeds the plane's aerodynamic resistance to rotation,
the plane may lack dynamic directional stability, such that even small rudder applications, or anything else deflecting it from a straight ahead course can initiate a ground loop.

Not sure about reducing engine and prop effect , as they are normal factors to overcome, either by pilot input or horizontal trim. I do know in the case of the SNJ-5,
that if you failed to lock the tailwheel and attempted to maintain lateral control with rudder during runout, the probability of ground looping was high. I also know that its engine and prop effect were
such that learning to taxi with the tailwheel unlocked was a challenge: you stood hard on the right toe brake and added power to turn right; you relaxed, but maintained a little right toe brake, while reducing power to turn left. If you didn't maintain some degree of right toe brake, the plane would continue turning left. Don't recall anything like that in the light planes mentioned.

As I recall, the light planes were easily steered by rudder, though a blast of the prop helped if a sharp turn at very low speed was desired. :)
Last edited by dilbert on Sat Jul 31, 2021 9:14 pm, edited 2 times in total.
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Re: Taildraggers

Postby Johan G » Sat Jul 31, 2021 8:33 pm

S&J wrote in Sat Jul 31, 2021 7:14 pm:Aerodynamics I don't think is a major contributor at low speeds.

I agree.


I think this topic from earlier this year should be of interest: landing gear toe-in. :wink:

The tl;dr is that tail draggers apparently rarely if ever had parallel main landing gears, but rather main landing gears pointing out a few degrees (toe-out, see illustration in first post). This supposedly reduced the tendency to ground loop quite a lot. There seem to be ways to do this in both YASim and JSBSim, and it seem to help a lot in FlightGear as well.

For a more in-depth explanation of how this very likely work out, see the article linked in this post in that topic.[1]
____
[1] Or this link: Hoppenworth, Marvin V., Landing Gears: Toe In Or Toe Out? (0.7 MB PDF file), Sport Aviation, August 1963, pages 24-25.
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Re: Taildraggers

Postby dilbert » Sat Jul 31, 2021 9:49 pm

Thanks Johan.
Toeing in does make sense, as the outer wheel in the turn would resist. Be interesting to know which planes used it. :)
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Re: Taildraggers

Postby wlbragg » Sat Jul 31, 2021 11:41 pm

though a blast of the prop helped if a sharp turn at very low speed was desired


Watching video on the subject, maybe that's something missing in jsbsim. Propeller wash over the fuselage, that's got to be a big factor and if that's not modeled that could be a contributing cause?
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Re: Taildraggers

Postby Johan G » Sun Aug 01, 2021 7:10 am

dilbert wrote in Sat Jul 31, 2021 9:49 pm:Toeing in does make sense [...]

No, no, no; toe-out (with the main landing gears pointing slightly out).

Consider a cross section through the fuselage and the wheels and consider how high up the the center of gravity CoG) is from the wheels. Now imagine if that aircraft would swerve to the right. As the CoG is quite high above the wheels there would be a momentum rolling the aircraft slightly to the left also moving the CoG to the left, both causing the left main landing gear to dig in a bit more and the right main landing gear to lift a bit. If the main landing gears would be pointing inward (toe-in) the left landing gear would now steer the aircraft more to the right exacerbating the swerve, making it even more severe. On the other hand, if the main landing gears would point a slight bit out (toe-out) the left landing gear would now steer the aircraft more to the left, countering the swerve.
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Re: Taildraggers

Postby dilbert » Sun Aug 01, 2021 9:28 am

Sorry for the mislabel, and thanks for the correction.

Understand the process. Toeing out makes the outer wheel to want to keep going straight, even though the plane is turning, which causes the outer wheel to drag.

The tendency to spin on the runway-ground loop-seems to me more prevalent in FlightGear now, than it used to be. I don't know the reason.

Still don't recall the light tail draggers I flew having a ground looping tendency, for which there must be a logical explanation.

Kind Regards

P. S. Found these comments interesting: https://www.supercub.org/forum/showthre ... In-Toe-out
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Re: Taildraggers

Postby S&J » Sun Aug 01, 2021 12:04 pm

I understand toe in or out has an effect, I'm just not convinced that this is the main culprit.

I have to wonder about gear compression and damping and would be interested in seeing the effects of completely rigid gear over the normal gear behaviour.

Roll inertia cannot be ignored, as the frequency of oscillation of 'weight' from one sides gear to the other has to be an issue.

As for the planes overall inertia values, I've yet to see any proof of working out to confirm that the values used are correct. And this applies to the props inertia values and the engines p value. It's all guesswork in FG as far as I can see.

I would therefore as stated above look into proving the forces and moments being applied are first correct before hanging my hat on toe in or out.
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Re: Taildraggers

Postby dilbert » Wed Aug 04, 2021 1:13 pm

Aah

Think I may have found a generally workable method for taming these birds: I move the CG some % forward by altering "x" for the entire aircraft in the set file.

The addon C18 was pretty unflyable. Its main wheels are at "x"=2.92109. I reduced "x" by 2.5 for the entire aircraft, effectively moving the CG forward that amount, by adding the following to the c18-set.xml file :

<cgadjustment>

<adjustment>x=x-"2.5"</adjustment>

</cgadjustment>

This seems to have tamed the beast without undo damage to weight & balance or flight dynamics. So far, so good.

The C18 is YASIM. Will see how method works on the C140, which is JSBSim.

Kind Regards :)
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Re: Taildraggers

Postby wlbragg » Wed Aug 04, 2021 5:21 pm

But is that fix just a work around or the correct fix. Shouldn't the COG be readily available in the POH for that particular aircraft. If so does the new one match. It just seems like arbitrarily moving COG isn't the "best" fix. Also on the Cub for example adding fuel or passengers or baggage changes the COG. Are you verifying that it is still a POH valid COG position with different loads?
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Re: Taildraggers

Postby S&J » Wed Aug 04, 2021 5:39 pm

It won't work on the jsbsim flight model.
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Re: Taildraggers

Postby dilbert » Wed Aug 04, 2021 6:32 pm

As wlbragg pointed out, the change doesn't address the underlying problem. The plane steers a little better, but when it gets to about 40 Knots, it starts the same wild and nearly uncontrollable rotation.
Nearly, because countering it with a lot aileron and rudder at just the right moment, sometimes keeps the plane on the runway.

Have tried keeping the tail down until achievement of sufficient speed for the rudders to take effect (standard technique for the real plane); also tried letting tail rise when ready. Neither technique
makes any difference.

Tried similar modification in the C140, but it lacked effect, just as S&J indicated.

Being paranoid, went so far as to compare FG 2018.3.2 with 2020.3.11, directory by directory; but found nothing to indicate the cause to be from changes to the main program.

Back to square one. :cry:
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