Beagle Pup experiment

[bomber]

Richard and anyone else...

I do have a question..

I calculate the speed that each section is rotating at, using the rotation speed and the sections distance from the CoG.

Code: Select all

    <pure_gain name="T4T/velocities/wing/_1_p_rotational-fps">   
      <input>velocities/p-aero-rad_sec</input>
      <gain>T4T/structure/wing/_1_y</gain>
    </pure_gain>

As you see I use velocities/p-aero-rad_sec...

the question is should I being using

velocities/p-rad_sec

what is the difference ?

Half the problem with using JSBsim is understanding just what all it's properties are defining.

Simon

[Richard]

Sitting on the runway with 15kts of wind, engine at idle (12fps induced veloctiry) it will pitch up to the maximum. Turn the engine off and there is still enough to lift the nosewheel slightly.

velocities/p-aero-rad_sec is in the aerodynamic (wind) axes. velocities/p-rad_sec is in the body axes. I suspect you should be using the body axes for this but it depends on the rest. Axis confusion is a great source of.

The pitching moment for the tail will be something like (this is something I already had worked out in 1991 for the Hawk) - so it is just to give you an idea.

MomentX = -(TailLift * COS(TailIncidence) + TailDrag * SIN(TailIncidence)) * XTailArm;
MomentZ = (-TailLift * SIN(TailIncidence) + TailDrag * COS(TailIncidence)) * ZTailArm
TailMoment = (MomentX + MomentZ) / Chord

[sanhozay]

I had a little time to do some testing of Richard's Beta 3 FDM this afternoon. I haven't yet tried Bomber's but I'll try and get to that next week.

Some initial observations ...

The erratic ground behaviour that plagues my vanilla Aeromatic++ FDM is gone, which is a good thing.

There is too much power. Given that the engine is the Continental 0-200 and the propeller is the McCauley, this looks like a Pup 100 model. Most pilot accounts suggest this is an underpowered aircraft, requiring an 800m runway and producing climb rates < 500fpm even lightly loaded. It's effectively a single seater.

I usually use the 830m tarmac runway at EGCJ for testing the Pups and with this FDM I get up to 65kts and I am airborne before the touchdown zone at the near end. After retracting flaps and with your default two-person payload, I can climb out at 2200fpm. At 0.75 throttle I was whizzing along at 135kt at 1500ft. Vno is 120kt. With idle throttle, I also got up to 40kt after releasing the parking brake on a long runway at Heathrow, which seems high for an aircraft that can fly at not much more than that.

Up at 7000ft, things seem to be nearer where they should be. Maybe the speed is a little high but less pronounced than at low level. I looked at the fdm/jsbsim/propulsion tree and found 170hp at full throttle. That doesn't sound right.

I sometimes get slow pitch and roll oscillations during cruise, which seem to get worse with any degree of flaps and speeds below 100kt (which is where this aircraft should be most of the time). I found it hard to get a controlled initial climb, having to fight an initial nose-down and pitch oscillation in particular. It's very easy to create a weird stall with full flaps at approach speeds.

I can't really make any worthwhile judgement about handling because I'm not a pilot and I've never been in a Beagle Pup but it is a light sport aircraft used for simple aerobatics and handling should probably be quite responsive. Comparing it to other FG aircraft of a similar size, I'd say this FDM feels quite sluggish. Take that with a pinch of salt really because I'm not speaking from a position of knowledge and joystick settings make a difference.

If this FDM works out are you planning to relicense it as GPL2?

PS. I am aware of a bug with autostart failing when not using live weather. Until I fix it, the workaround is to run the autostart with live weather and then switch to a scenario.

[bomber]

Well that's 15 minutes of my life I wont get back trying to start the engine on Richards plane...

Richard I make it as simple as can be to start flying with my fdm... can I suggest for this testing period you do the same.

[IAHM-COL]

@bomber
LMAO

He is already implementing complex start-ups ?

[IAHM-COL]

I am testing only Bomber's version (and R. Senior off course)

I dont use proprietary software

[sanhozay]

Does autostart not work? I was using it this afternoon and it was OK.

[KL-666]

I had the chance to shortly fly Richards model this afternoon (yes, took a while to start). This model finally flies, just a bit of left torque which can be expected. Good testable now.

The main thing i noticed is that the inertia that Bombers model completely lacks, is way too much in this plane. In roll it is reasonable, good controllable, might just be a tad less. But in pitch it is really too much and hard to control, to put it mildly.

Just a tiny pitch up and immediate neutral after that, results in the plane continuing way too long pitching up more. To adjust a rather big counter elevator action is needed, resulting in an overreaction. Necessary to stop the continuing pitch up, but immediately causing a too strong pitch down when "over the top". So i have been flying a sort of roller coaster. I think this is the sluggishness Sanhozay talked about.

To Richard i suggest to reduce the inertia. At least in the pitch.

To Bomber i suggest to watch the inertia in the roll of this plane, which can be classified as reasonable. This inertia (maybe a tad less) is what i am looking for in your model. (Roll, neutral, and the plane continues to roll just a bit.)

Kind regards, Vincent

[Richard]

@sanhozay - thanks for the testing. The engine / prop may be overpowered (or it could be my basic form drag that is wrong). I'll look into the points about overall 'aerobatic' performance to see if I can find any information that indicates what it should be like. I would expect that it won't always fly dead straight and level if there is any wind at all - and it might be that the gyro effects of the propeller are enough to skew things.

However I think I've got a weight and balance problem; (i.e. CG/AERORP) which is what I'd expect given that it's one of the things that has been the hardest to figure out.

In terms of licencing - if the experiment is a success and the result is an aero model to be proud of then I'll release it as GPL/CC - otherwise it'll go into the great bitbucket in the sky. I'm not going to get into a licencing/copyright debate either.

The point of this experiment is not necessarily to produce a better Beagle Pup aero model - it's to explore the possibilities of working together with aero modellers, test pilots and any other help that can be given. What I'm interested in is really to see if we can find the future of aerodynamic modelling. If it's not VSPAERO then I'll be disappointed. I can sit down with a picture, some documentation and a vivid imagination and make a flight model for any aircraft - that's easy. What's hard is to get something that actually flies like the real thing.

@Simon - I feel your frustration. The engines, propeller and startup sequence aren't mine - however you can autostart on the ground easily enough; and if that doesn't work just turn all of the switches in the lower centre console on, and push s and m at the same time until it starts. It took me about 20 flights before I figured this out. However you can just remove the

@Vincent can you please try this revised FDM. I've tuned the pitch moment to be what I think is more suitable - the point of this is to see if this is what you mean by inertia and control problems with pitch: https://dl.dropboxusercontent.com/u/728 ... up-vsp.xml

[sanhozay]

Just a tiny pitch up and immediate neutral after that, results in the plane continuing way too long pitching up more. To adjust a rather big counter elevator action is needed, resulting in an overreaction. Necessary to stop the continuing pitch up, but immediately causing a too strong pitch down when "over the top". So i have been flying a sort of roller coaster.

This is the pitch instability I was talking about when I said it was "nodding". Vincent explains it better.

I just had another couple of flights with it I think the roll instability I'm getting at about 120-130kt at 1500ft is a dutch roll. I've never had another FG aircraft do that, so it is interesting in itself.

I'm just trying to make Bomber's FDM work and do some testing of that one. (The FGMEMBERS NON-GPL download is missing the engine and propeller files).