Tail lift

[Real Old Salt]

What should I be looking to "tweak" for an aircraft (not tail dragger) who's tail lifts during take off acceleration? If I were to let the plane continue with out back pressure on the stick the main gear will come off the deck while the nose gear stays down.

[fliger747]

That's a cute one!

The obvious would be to czech the COG and trim settings.

I know, it's like calling the computer help desk and the first question is always "is it plugged in".

What sort of flier is this?

Tom

[Real Old Salt]

This is a single engine jet fighter, cog is at 24.7% full. Trim is neutral

[sparks]

'If I were to let the plane continue with out back pressure on the stick the main gear will come off the deck while the nose gear stays down.'

This tells me the problem is one of the Cm coefficients. Could be the CM-AoA curve (section 473), Cm0, Cm flaps, or Cm landing gear. One of them is a positive number (causes nose down rotation) and it's probably off by an order of magnitude.

[fliger747]

Swept wing?

[sparks]

Check section 433 too, mach effect on Cm0...

[Real Old Salt]

Thank you gentlemen, I'll dig into it later today when I have the chance.

[sparks]

One other possibility is the location of the aerodynamic center in section 1534. This section isn't always present, but if it's there and the longitudinal value is aft of the CoG it'll have the same effect as a positive Cm coefficient.

[Farmboy]

One other possibility is the location of the aerodynamic center in section 1534. This section isn't always present, but if it's there and the longitudinal value is aft of the CoG it'll have the same effect as a positive Cm coefficient.

Or horizontal tail incidence/trim pitch moment. How does the plane work if you wheel in a few degrees of nose-up trim before taking off?

[Real Old Salt]

Well, I found one glaring error (Pitch Moment Coeff at AoA) But that still hasn't corrected the problem entirely. In fact making an adjustment there has changed other aspects ( of course! ) I think what I need to do is break out the work sheet and start from scratch.
This for the AH's Panther which was the first airfile I ever tried to tweak. I recently came in touch with a pilot from our old squadron who actually flew this airplane and want to send it to him. So, before I did I rechecked the way I set it up. Boy, was I green three years ago.
Edit: Farmboy, putting in about 5* of positive trim will reverse the effect when the plane has exceeded stall speed for the flap conditions. (Somewhere I have been lead to believe that a properly set up aircraft will try to lift at or about it's stall speed during take off.) but prior to the reversal this plane tries to plow a furrow with it's nose wheel. Kinda like a three point bottom plow with it's back jacked up.

[Henry]

Boy, was I green three years ago.
You too hey!:costumes:
H

[Real Old Salt]

You too hey!:costumes:
H
Still am, don't tell anybody.

[fliger747]

ROS:

Fun isn't it! Just one darn thing after another.

Most nosedraggers that I have flown, including jets, require some positive rotation to lift off. In Gen Av aircraft they say "lift the nosewheel and let it fly off". In more high performance aircraft with more variable weight ranges, speeds are usually computed for the weight and a positive rotation and climb away is initiated at a computed airspeed. The exception I have seen to this is in light aircraft with a full flap takeoff, where "Wheelbarrowing" can result if rotation is not initiated, giving a result as you describe. Some FS aircraft seem to behave a little strange in this flight regime, requiring a pull off and then settling into a nicely trimmed climb. Typically an aircraft should pretty much be ready to fly and take little nudging to become airborne at the correct speed.

Haven't screwed around with the Panther, but it is a nice plane and should be a straight forwardish configuration! Thanks for the interesting question and I for one am interested in learning a bit about this one. I did see a little similar charcterisics in a B-47 a while ago, which would like to drive along on the front gear just before liftoff, though it wasn't too objectionable.

Best regards: Tom

[Real Old Salt]

Thanks for the feedback, you've kept me from going insane! When I first started using this model again I noticed that the distance between the horizon and the top of the gunsite kept shrinking the faster the plane went on take off. Then the next thing I noticed was that my left wing was lower than my right one. Realizing that something was "amiss" I shouted "Shazam" and went outside in my lighting stripped red underwear, to fly along side the plane and investigate. Well, OK, I went to spot view. :costumes: Both main gear were about a scale foot off the deck and the nose wheel was firmly on it.
Back when I was in the Navy, I worked on F9F-6 Cougars that had been converted to drone use. In the avionics shack we would set them up to rotate at 118 knts. I don't recall how much positive pressure was set up, but I'm sure it wasn't the equivalent of "stick in the crotch" Which is what I was needing to get the (similar characteristic) Panther airborne.
I found, with direction from Jerry,that the main culprit was "Pitch Moment Coeff at AoA" This was set to a positive figure of 78. Everything I looked at for examples were in the negative range. Changing that got me going in the right direction. But then of course
the chain reaction effect needed to be dealt with. A little cheating here and a little cheating there and it's flying pretty good right now.
The thing that has been keeping my bubble off center until now, is that the nose still creeps downward during acceleration. From what you say, that's sort of normal. Which makes me a happy man. Gonna put this project to bed for now and go lite up a cigar!

[fliger747]

Glad it's coming along! Any idea what in the workbook, or elsewhere, caused the Pitch Moment Coeff at AOA to be off?

[Real Old Salt]

Sorry, I sure don't. Like I said in an earlier post, the first time I played with this one was about 3 years ago. Even if I knew then, which I doubt cuz I still have a long way to go with air files, I 'm sure I wouldn't recall now what I entered that could have messed it up.
It wouldn't surprise though if Sparks were to pop in here and say that I must of did this or that.

[sparks]

I've just about used up my quota of educated guesses for the week. I'd have to get my hands on the problem files to figure out the rest of the story on this one. :d

[Real Old Salt]

And here I thought you hid behind an emerald curtain and had all the answers. :costumes: