Engine Upthrust?
07-02-2008 | 01:18 PM
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From: Colbert,
WA
Engine Upthrust?
I recently picked up on of the Nitroplanes Falcon50 quasi-pattern planes. My general impression of the ARF construction was that it looked OK, with things like triangle stock reinforcements and fuel proofing already done. It looked like the firewall was set up for some engine downthrust and rightthrust, so I installed a Magnum 70 4S inverted and maidened it last weekend.
We noticed that on the takeoff run it seemed to want to nose over, and that during flight it had a tendency to lose altitude. I can't say it was diving, but you had to hold up-elevator to stay in the air. So when I tried to dial in some up-elevator trim, my thumb cleverly hit the DX7's on/off switch instead of the elevator trim. Massive panic attack, but I managed to get the transmitter back on. The engine quit, but the controls came back and she deadsticked in after a long glide.
The engine probably quit because of either/both a fuel siphoning condition causing it to run very rich, or when the DX7 reset it went to "idle" on the throttle servo and flooded out. So I got the plane on the table last night, took off the cowling with the intention to start leaning down the low end, but when I looked at the engine/fuselage lineup, it looked like the engine was pointed slightly upward, to the point where if I got the fuselage in what looked like a level flight attitude, there was definately some upthrust in the engine alignment. The motor mount beams are all at 90* to the firewall, and the holddown plates on the engine were resting squarely on the beams.
(1) Would upthrust cause the plane to want to dive? If we assume that the prop generates a thrust vector that's parallel to the prop shaft, then upthrust on the engine would push the fuselage and tailfeathers above the thrust line during flight, generating a negative AOA on the wing, which could cause the plane to dive. It seemed to glide just fine, with no tendencies to climb or dive. My CG was farther forward than called for in the plans, but better that than tail heavy for first flight.
(2) Is there some method, absent a set of plans, to figure out where the "reference line" is on a fuselage? What I was considering was putting an incidence meter on the wing, adjusting it to 0* of incidence, and then shimming the engine mount to get a couple of degrees of downthrust.
Thanks in advance for any help.
Fred
We noticed that on the takeoff run it seemed to want to nose over, and that during flight it had a tendency to lose altitude. I can't say it was diving, but you had to hold up-elevator to stay in the air. So when I tried to dial in some up-elevator trim, my thumb cleverly hit the DX7's on/off switch instead of the elevator trim. Massive panic attack, but I managed to get the transmitter back on. The engine quit, but the controls came back and she deadsticked in after a long glide.
The engine probably quit because of either/both a fuel siphoning condition causing it to run very rich, or when the DX7 reset it went to "idle" on the throttle servo and flooded out. So I got the plane on the table last night, took off the cowling with the intention to start leaning down the low end, but when I looked at the engine/fuselage lineup, it looked like the engine was pointed slightly upward, to the point where if I got the fuselage in what looked like a level flight attitude, there was definately some upthrust in the engine alignment. The motor mount beams are all at 90* to the firewall, and the holddown plates on the engine were resting squarely on the beams.
(1) Would upthrust cause the plane to want to dive? If we assume that the prop generates a thrust vector that's parallel to the prop shaft, then upthrust on the engine would push the fuselage and tailfeathers above the thrust line during flight, generating a negative AOA on the wing, which could cause the plane to dive. It seemed to glide just fine, with no tendencies to climb or dive. My CG was farther forward than called for in the plans, but better that than tail heavy for first flight.
(2) Is there some method, absent a set of plans, to figure out where the "reference line" is on a fuselage? What I was considering was putting an incidence meter on the wing, adjusting it to 0* of incidence, and then shimming the engine mount to get a couple of degrees of downthrust.
Thanks in advance for any help.
Fred
