Dealing with a thrust/drag pitch couple
10-06-2014, 05:35 AM
#1
Thread Starter
Dealing with a thrust/drag pitch couple
These thoughts were offered on an airplane specific thread but might be of interest in this forum. The plane discussed is a Seagull Spacewalker II.
A poster comments:
my maiden was extremely distressing, first, the plane wanted to nosedive which I have corrected with 7-8 clicks up elevator
My response:
... You should not feel alone in thinking the plane was initially nose heavy due to the up elevator trim needed (in my case 1/4 inch). What I soon discovered however was that the up trim needed for powered flight, did not match the trim needed for a proper glide slope for landing, it needing close to neutral elevator. I'd checked the incidence numbers during the build and found them to be zero/zero for wing and stab and rechecking verified it. That left the question of what was at play causing the issue.
Having been into RC going back to the mid seventies and around mentors who cut their teeth on free flight who taught the discipline that a plane was trimmed for power off and then thrust compensated for power on, that principle was applied to the Spacewalker II. The problem of course is that up thrust in an engine is not common and the Spacewalker's cowl aligns with the spinner and that means that up thrust would be a major headache.
We now of course have the alternative to mix trim instead of changing thrust angle, so it was a matter of mixing up trim with throttle (or down trim with low throttle). When going full throttle, the elevators were trimmed up the 1/4 inch. When cutting throttle to land, they of course trimmed near neutral for a proper glide slope. That seemed to work fairly well albeit not perfectly, which will be explained shortly but first, some words about what was probably going on.
The Spacewalker has a generous and fairly thick low wing and a high engine thrust line. That seems to create a lever couple between the drag of the wing and the thrust of the engine that pitches the plane down. To counter the pitch down forces, up trim is needed, but when engine power is cut, the pitch down force goes away, leaving too much up trim and the model wants to linger aloft down the runway producing long landings where the plane then slows too greatly above the ground and plops down hard and bounces yielding three landings during one approach, hence why the mix is needed.
Are there any negative affects from the mix? Yes. The most notable is when going vertical. As the plane slows, the drag of the wing goes away, now leaving an up trimmed elevator with no force to counter so the plane pitches to the canopy. Such is likely not a problem to the average sport flier and if that is the case.... provide the throttle to elevator mix so that power on is in trim and power off is in trim and enjoy and stop reading.
If however, the canopy pitch when going vertical is disconcerting, there is a fix. Trim the elevator neutral under full power. That is done by adjusting the decalage and in the case of the Spacewaker which has a fixed horizontal stab, doing so must be done at the wing. Give the wing a hair of positive incidence. What worked for me was to glue a sliver of 1/64" ply to the bottoms of the forward wing positioning/retaining bolt holes and the tops of the aft. Then using a round file, slightly dress 1/64" from the other side of the holes. That was all it took on mine to achieve a neutral elevator under full power and solve the canopy tuck when going vertical. I also found that a throttle mix was no longer needed for landing trim albeit your mileage may vary on that. I'll explain.
I personally am uncomfortable having to give down elevator to produce a proper glide slope so will always do a throttle to elevator mix if it is needed. But, if the plane needs a bit of up trim to achieve a proper glide slope, I'm comfortable doing that with the stick while landing, after all, pulling on the stick is part of the flair process and seems natural.
I hope my comments were understandable and perhaps helpful or at the minimum were interesting.
A poster comments:
my maiden was extremely distressing, first, the plane wanted to nosedive which I have corrected with 7-8 clicks up elevator
My response:
... You should not feel alone in thinking the plane was initially nose heavy due to the up elevator trim needed (in my case 1/4 inch). What I soon discovered however was that the up trim needed for powered flight, did not match the trim needed for a proper glide slope for landing, it needing close to neutral elevator. I'd checked the incidence numbers during the build and found them to be zero/zero for wing and stab and rechecking verified it. That left the question of what was at play causing the issue.
Having been into RC going back to the mid seventies and around mentors who cut their teeth on free flight who taught the discipline that a plane was trimmed for power off and then thrust compensated for power on, that principle was applied to the Spacewalker II. The problem of course is that up thrust in an engine is not common and the Spacewalker's cowl aligns with the spinner and that means that up thrust would be a major headache.
We now of course have the alternative to mix trim instead of changing thrust angle, so it was a matter of mixing up trim with throttle (or down trim with low throttle). When going full throttle, the elevators were trimmed up the 1/4 inch. When cutting throttle to land, they of course trimmed near neutral for a proper glide slope. That seemed to work fairly well albeit not perfectly, which will be explained shortly but first, some words about what was probably going on.
The Spacewalker has a generous and fairly thick low wing and a high engine thrust line. That seems to create a lever couple between the drag of the wing and the thrust of the engine that pitches the plane down. To counter the pitch down forces, up trim is needed, but when engine power is cut, the pitch down force goes away, leaving too much up trim and the model wants to linger aloft down the runway producing long landings where the plane then slows too greatly above the ground and plops down hard and bounces yielding three landings during one approach, hence why the mix is needed.
Are there any negative affects from the mix? Yes. The most notable is when going vertical. As the plane slows, the drag of the wing goes away, now leaving an up trimmed elevator with no force to counter so the plane pitches to the canopy. Such is likely not a problem to the average sport flier and if that is the case.... provide the throttle to elevator mix so that power on is in trim and power off is in trim and enjoy and stop reading.
If however, the canopy pitch when going vertical is disconcerting, there is a fix. Trim the elevator neutral under full power. That is done by adjusting the decalage and in the case of the Spacewaker which has a fixed horizontal stab, doing so must be done at the wing. Give the wing a hair of positive incidence. What worked for me was to glue a sliver of 1/64" ply to the bottoms of the forward wing positioning/retaining bolt holes and the tops of the aft. Then using a round file, slightly dress 1/64" from the other side of the holes. That was all it took on mine to achieve a neutral elevator under full power and solve the canopy tuck when going vertical. I also found that a throttle mix was no longer needed for landing trim albeit your mileage may vary on that. I'll explain.
I personally am uncomfortable having to give down elevator to produce a proper glide slope so will always do a throttle to elevator mix if it is needed. But, if the plane needs a bit of up trim to achieve a proper glide slope, I'm comfortable doing that with the stick while landing, after all, pulling on the stick is part of the flair process and seems natural.
I hope my comments were understandable and perhaps helpful or at the minimum were interesting.
