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JSBSim-Team / jsbsim

An open source flight dynamics & control software library
GNU Lesser General Public License v2.1
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Helicopters cannot be trimmed in flight #1139

Open bcoconni opened 4 weeks ago

bcoconni commented 4 weeks ago

I'm submitting a ...

Describe the issue As per the discussion #1132, helicopters can't be trimmed in flight.

What is the current behavior? Currently, helicopters can be trimmed on ground by setting simulation/do_simple_trim to 2 (i.e. trim on ground).

However, helicopters cannot be trimmed in flight because helicopters produce lift by their rotor while the trim algorithm assumes that lift is produced by wings and hence adjusts the angle of attack to maintain constant altitude.

https://github.com/JSBSim-Team/jsbsim/blob/b41fcddc505baf041a3082ac3be2312e392c7792/src/initialization/FGTrim.cpp#L808-L809

What is the expected behavior? JSBSim should be able to trim helicopters in flight just as it does for aircraft.

dershow commented 4 weeks ago

In general helicopters are inherently unstable while most airplanes are stable. So, trim algorithms have to handle them differently.

Adam

gallonmate commented 4 weeks ago

So helicopters do successfully trim on ground? I'm seeing trim failure for trim on ground when testing in UE5 and with helicopter engine running. With engine off, trim is successful. I have not debugged or looked further in it. Just thought I would clarify.

There are a few other JSBSim helicopters being used in Flightgear, so I'd be curious what they do for trim, if at all.

bcoconni commented 4 weeks ago

So helicopters do successfully trim on ground?

Well, this is what we inferred from the discussion mentioned in the PR description. I have to admit that I did not test helicopter trimming myself. But this statement seemed reasonable as, from an algorithmic point of view, trimming an helicopter on ground is not different than trimming an aircraft on ground: you have a mass and a list of contact points and the trim algorithm has to compute the location of the contact points such that the solid (helicopter or aircraft) is in a stable equilibrium.

https://github.com/JSBSim-Team/jsbsim/blob/0b99bf8b173fe21cd3a1481695ff199b5e4a4335/src/initialization/FGTrim.cpp#L349-L373

I'm seeing trim failure for trim on ground when testing in UE5 and with helicopter engine running. With engine off, trim is successful. I have not debugged or looked further in it. Just thought I would clarify.

Thanks for the clarification. It's possible that this limitation applies to aircraft as well.

seanmcleod commented 4 weeks ago

There are a few other JSBSim helicopters being used in Flightgear, so I'd be curious what they do for trim, if at all.

Are you sure that they're JSBSim based as opposed to YASim based? When I looked around for helicopter JSBSim FDMs a couple of years ago the majority I found at the time were YASim based.

gallonmate commented 4 weeks ago

I'm seeing trim failure for trim on ground when testing in UE5 and with helicopter engine running. With engine off, trim is successful. I have not debugged or looked further in it. Just thought I would clarify.

Thanks for the clarification. It's possible that this limitation applies to aircraft as well.

Most aircraft I've used so far, have trimmed successfully on ground with engines running. It was just something I noticed that was different with the helicopter.

There are a few other JSBSim helicopters being used in Flightgear, so I'd be curious what they do for trim, if at all.

Are you sure that they're JSBSim based as opposed to YASim based? When I looked around for helicopter JSBSim FDMs a couple of years ago the majority I found at the time were YASim based.

Correct there are only a few JSBSim helicopters among the many for Flightgear. There was maybe 4 made with JSBSim that I found. The Alouette-III and the R22 are both JSBSim helicopters currently available directly through flightgear. The other ones were available from third parties but I can't find them at the moment. Anyways, it was more a curiosity to see how/if they use jsbsim trim.

andgi commented 3 weeks ago

Hi all,

I have a few airships and boats that don't trim that well.. but I didn't expect that anyway. :)

See https://github.com/andgi/ for my examples. The repositories are called FlightGear-.

Cheers, Anders

On Fri, 16 Aug 2024 at 21:59, gallonmate @.***> wrote:

I'm seeing trim failure for trim on ground when testing in UE5 and with helicopter engine running. With engine off, trim is successful. I have not debugged or looked further in it. Just thought I would clarify.

Thanks for the clarification. It's possible that this limitation applies to aircraft as well.

Most aircraft I've used so far, have trimmed successfully on ground with engines running. It was just something I noticed that was different with the helicopter.

There are a few other JSBSim helicopters being used in Flightgear, so I'd be curious what they do for trim, if at all.

Are you sure that they're JSBSim based as opposed to YASim based? When I looked around for helicopter JSBSim FDMs a couple of years ago the majority I found at the time were YASim based.

Correct there are only a few JSBSim helicopters among the many for Flightgear. There was maybe 4 made with JSBSim that I found. The Alouette-III and the R22 are both JSBSim helicopters currently available directly through flightgear. The other ones were available from third parties but I can't find them at the moment. Anyways, it was more a curiosity to see how/if they use jsbsim trim.

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