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Tim Conrad's T-33 - Nose heavy? (Read 708 times)
Jun 18th, 2009 at 9:28am

JJH   Offline
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I've recently noticed the aircraft's nose is pitching down during takeoff. The rear comes up first, and the nose only after liftoff. This seems to be a recent phenom as I don't remember this happening before. The only changes I've made to the aircraft.cfg have been small adjustments to wheel contact points and the addition of an autopilot panel. I cant locate anything resembling a centre of balance number. Any advice?

Jim B Huh
 
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Reply #1 - Jun 18th, 2009 at 10:27am

JJH   Offline
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I've found what I think are the Centre of Gravity numbers:

reference_datum_position =  0, 0, 0
empty_weight_CG_position  = 0, 0, 0
CG_forward_limit = 0
CG_aft_limit = -1

I don't know what they represent, but I'll compare them to another period jet (F86) to see whether there are peculiarities and tweak them to see what happens.
 
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Reply #2 - Jun 18th, 2009 at 11:01am

Travis   Offline
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Ummm . . . as far as I know, all taildraggers SHOULD pitch nose-down before they are able to get into the air.

The tailplane must begin to produce lift (just like the wings) before you can get off the ground.  I'm guessing that if you knew the EXACT moment that the tailplane began producing lift, you could get it to lift off without the tail coming up at all.
 
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Reply #3 - Jun 18th, 2009 at 11:04am

Hagar   Offline
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Quote:
Ummm . . . as far as I know, all taildraggers SHOULD pitch nose-down before they are able to get into the air.

The T-33 is not a taildragger.

JJH. It might be best to post this in the Aircraft Design section. That's where the serious tweakers hang out. You could also read the appropriate FS9 SDK.
 
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Reply #4 - Jun 18th, 2009 at 11:11am

JJH   Offline
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Hagar wrote on Jun 18th, 2009 at 11:04am:
Quote:
Ummm . . . as far as I know, all taildraggers SHOULD pitch nose-down before they are able to get into the air.

The T-33 is not a taildragger.

JJH. It might be best to post this in the Aircraft Design section. That's where the serious tweakers hang out. You could also read the appropriate FS9 SDK.


I'll follow both of those suggestions, Hagar. In the meantime, I've changed the "elevator_effectiveness =" from 1 to 3 and that has corrected most of the problem.

Jim B Smiley
 
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Reply #5 - Jun 18th, 2009 at 11:17am

Travis   Offline
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I'm sorry!  Went off a bit half-cocked, there.  Looked up "Conrad T-33" on FlightSim and saw shots of a Zlin!  Must be more tired than I thought . . . Roll Eyes
 
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Reply #6 - Jun 18th, 2009 at 12:44pm

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Hi Jim. Try this before editing any .cfg files. Start your sim and load a flight with the T-33. Once loaded, hit alt+enter to get the sim into "windowed" mode. (Skip it if you already fly that way). Now go to the aircraft menu from the menu bar and select fuel and payload. See how the plane is balanced out and if it isn't, from there you can make any changes needed.  

You also may need to check the plane's elevator trim. Wink

 
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Reply #7 - Jun 18th, 2009 at 1:52pm

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Travis wrote on Jun 18th, 2009 at 11:01am:
Ummm . . . as far as I know, all taildraggers SHOULD pitch nose-down before they are able to get into the air.

You've got to be kidding  Smiley.
 
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Reply #8 - Jun 18th, 2009 at 2:15pm

JJH   Offline
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Groundbound1 wrote on Jun 18th, 2009 at 12:44pm:
Hi Jim. Try this before editing any .cfg files. Start your sim and load a flight with the T-33. Once loaded, hit alt+enter to get the sim into "windowed" mode. (Skip it if you already fly that way). Now go to the aircraft menu from the menu bar and select fuel and payload. See how the plane is balanced out and if it isn't, from there you can make any changes needed.  

You also may need to check the plane's elevator trim. Wink



The Fuel and Payload diagram shows the CG well forward on the aircraft (approximately at the front edge of the cockpit). Adjusting the fuel load only moved the CG left or right. Adjusting the payload (Instructor=150 lbs, student=600 lbs) didnt move it at all. Is there any way of moving the CG backwards toward the centre of the aircraft? Huh

The Learning Center shows those are the only two ways to adjust the CG.
Jim B
 
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Reply #9 - Jun 18th, 2009 at 3:08pm

JJH   Offline
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Found what I think I need in the SDK. Just a matter of tweaking the Reference Datum and the CG offset numbers until I get it right. Tks all. Smiley

Jim B
 
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Reply #10 - Jun 18th, 2009 at 5:08pm

olderndirt   Offline
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Which seat is the 600 lb student occupying?
 
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Reply #11 - Jun 18th, 2009 at 5:10pm

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Travis wrote on Jun 18th, 2009 at 11:01am:
Ummm . . . as far as I know, all taildraggers SHOULD pitch nose-down before they are able to get into the air.

The tailplane must begin to produce lift (just like the wings) before you can get off the ground.  I'm guessing that if you knew the EXACT moment that the tailplane began producing lift, you could get it to lift off without the tail coming up at all.


Uhhh, what?  Shocked

The tail plane produces downward lift, because the main wing should produce a center of lift just behind the center of gravity, producing a nose down action. The tail plane pushes down to counter the nosedown, producing level flight.

Take a good look at the shape of a tailplane, the rounded part is on the bottom, opposite the main wing.
 
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Reply #12 - Jun 18th, 2009 at 8:15pm

olderndirt   Offline
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The old adage that the tail must fly before the wings'll fly is absolutely correct but as the tail flies the nose merely goes from three point attitude to somewhat less but surely not 'pitching down'.  When I read that words like 'prop strike' came immediately to mind.  I'd still like to know about the 600 lb student - might be a factor in the CG being out of limits.
 
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Reply #13 - Jun 18th, 2009 at 10:22pm

Travis   Offline
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Sorry.  What I meant was what Older'n wrote: the plane "levels out" a bit as speed builds, then you are able to actually get the main gear off the runway.  No, I definitely understand that the nose shouldn't pitch "down" past the horizontal, but the tailgear must leave the ground before the main gear, correct?
 
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Reply #14 - Jun 19th, 2009 at 12:00am

olderndirt   Offline
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Probably with full flaps and lots of horses it'd get off three point but you'd need to accelerate before trying to climb and get rid of the flaps nice and easy.  Ideally you want the tail empennage able to do its thing - rudder and elevators before the wings start doing their thing.  What about that 600 lb student?  Must be a tight fit, front or back  Smiley.
 
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