spin, spin, why did it spin
05-08-2003, 07:39 PM
#1
spin, spin, why did it spin
I hope some one out there can shed some light on this for me. Last year I had a Airkill Focke Wulf Tank 152 H-1 I built for combat with a Norvel 25 bb for power and it weight 38oz.. The plane was balanced at the forward balance point shown on the plans for the first couple of flights. The first flight was totally uneventful with no surprises, the flight lasted 15min. And only ended when the engine quit.
In flight number 2 I noticed a bit of proposing in the shallow dives at full bore to the runway for low passes. Figuring it was a high speed stall I climbed to high and did a high G loop. It tracked through approx. 150 degrees of the loop and snapped (just as I had expected), I had to much elevator throw. So I reduced the throttle to about 75% and started to role inverted to do a long split-S to get back on the deck. Somewhere in the role the plane started to spin, I stopped the spin then advanced the throttle to full and gently added elevator. At the first hint of elevator it started to snap in the other direction. This time I let go of the sticks and it stops spinning instantly, and again as soon as I touched the elevator it started to spin in the original spins direction. At this point I realize that we are going to hit the ground so I try to turn the spin to a flat spin. It worked The plane hit the ground and the only damage was the cowl (a total loss on the cowl but no other damage. It took us about 4 min. To get to the plane and it was completely empty of fuel, I was the only one in the air and the engine ran right to the ground. I'd rather be lucky than good any day of the week!
I took it home to check it over and found the elevator servo output arm had split and the servos had broken away from the fuse walls (expected). Went over it real well, put everything back together and ask a pilot with more experience than I to stand with me for the next flight.
Before the next flight we talked about how to handle the next spin, more specifically things to try that I had not yet. Right out of the launch it required a lot of aileron and elevator trim. That has always bothered me because I checked it over very well and the servos went right back were they broke out of. So why would I see a 5-6 click change in aileron and 3-4 in elevator. Anyway Being I was unsure of the plane I did no High G maneuvers, all turns were horizontal turns and just when I and my watcher were getting comfortable in a low G horizontal turn (max. Bank of 45 degrees, and about 7 minutes into the flight) it snapped again, into the turn. I stopped the spin and this time let it track in the dive for as long as possible at 75% throttle before hitting any Elevator. But at the first hint of elevator it snapped the other way and hit the ground.
I'm at a loss to explain this? Do any of you have any Ideas? It would stop spinning, I could aileron turn it around to pull out towards the field and let it track in that dive. But any amount of elevator would induce a spin. I've gone over it and over it for something that was broken that I missed the first time or broke free and moved back to change the CG and have found nothing. Two other people saw it with me the second time and we were all baffled. The plane will never fly again, but the only solis I've been able to come up with is that when I did the high G stall test in flight two I broke something that was well hidden. What do you think?
In flight number 2 I noticed a bit of proposing in the shallow dives at full bore to the runway for low passes. Figuring it was a high speed stall I climbed to high and did a high G loop. It tracked through approx. 150 degrees of the loop and snapped (just as I had expected), I had to much elevator throw. So I reduced the throttle to about 75% and started to role inverted to do a long split-S to get back on the deck. Somewhere in the role the plane started to spin, I stopped the spin then advanced the throttle to full and gently added elevator. At the first hint of elevator it started to snap in the other direction. This time I let go of the sticks and it stops spinning instantly, and again as soon as I touched the elevator it started to spin in the original spins direction. At this point I realize that we are going to hit the ground so I try to turn the spin to a flat spin. It worked The plane hit the ground and the only damage was the cowl (a total loss on the cowl but no other damage. It took us about 4 min. To get to the plane and it was completely empty of fuel, I was the only one in the air and the engine ran right to the ground. I'd rather be lucky than good any day of the week!
I took it home to check it over and found the elevator servo output arm had split and the servos had broken away from the fuse walls (expected). Went over it real well, put everything back together and ask a pilot with more experience than I to stand with me for the next flight.
Before the next flight we talked about how to handle the next spin, more specifically things to try that I had not yet. Right out of the launch it required a lot of aileron and elevator trim. That has always bothered me because I checked it over very well and the servos went right back were they broke out of. So why would I see a 5-6 click change in aileron and 3-4 in elevator. Anyway Being I was unsure of the plane I did no High G maneuvers, all turns were horizontal turns and just when I and my watcher were getting comfortable in a low G horizontal turn (max. Bank of 45 degrees, and about 7 minutes into the flight) it snapped again, into the turn. I stopped the spin and this time let it track in the dive for as long as possible at 75% throttle before hitting any Elevator. But at the first hint of elevator it snapped the other way and hit the ground.
I'm at a loss to explain this? Do any of you have any Ideas? It would stop spinning, I could aileron turn it around to pull out towards the field and let it track in that dive. But any amount of elevator would induce a spin. I've gone over it and over it for something that was broken that I missed the first time or broke free and moved back to change the CG and have found nothing. Two other people saw it with me the second time and we were all baffled. The plane will never fly again, but the only solis I've been able to come up with is that when I did the high G stall test in flight two I broke something that was well hidden. What do you think?
12-09-2004, 10:08 PM
#3
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RE: spin, spin, why did it spin
well the real things did snap easy, but dang not that easy.
where was the CG located after the the crash, was the fuel being empty causing any problems.
cant think of anything else.
where was the CG located after the the crash, was the fuel being empty causing any problems.
cant think of anything else.
12-10-2004, 08:05 AM
#4
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RE: spin, spin, why did it spin
Of course this is all speculation but I think SBR might be onto something. As the fuel is being used the CG naturally moves aft. Maybe the CG should have been a little more forward. (??) I know on my small planes I dont use a lot of throw when Im burnin up the sky. I flew a small ugly stick (30" ws) with a norvell .074 and it has a rediculous roll rate. You said it was porposing, I do a CG test on maiden flights that is basically a 45deg dive at 1/2 to 3/4 throttle and let go of the sticks to see what happens. If it continues its track straight, I consider the CG where it should be. If it noses down steeper, nose heavy. If it pulls up, tail heavy. The variances are usually not very pronounced and its a judgement call on your part as to whether or not it will be an issue. Got that tip out of RC Report. That along with testing for slow flight are two of the firsts tests I have to do when I do a maiden or test flight. But that one sure is a head scratcher.
Edwin
Edwin
12-10-2004, 11:36 AM
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RE: spin, spin, why did it spin
Could be a number of things.. A spin or snap is caused when one wingpanel stalls before the other. Working backwards..were the wings straight and true according to the plans? Was there any built in washout in the wings? Is(was) the fin and rudder straight and true? Was the fuse straight (ie no "banana" )?
Other things to look at:
-wing incidence
-decalage (the difference in incidences between the wing and stab)
-aileron and elevator linkage and hinge stiffness
-gaps in ailerons and elevator
- leading edge profile (any sharp edges?)
- center of gravity is definitely a suspect...as mentioned above, as fuel burned off, the problems began, indicating the rearward shift of cg as a trigger.....
Other things to look at:
-wing incidence
-decalage (the difference in incidences between the wing and stab)
-aileron and elevator linkage and hinge stiffness
-gaps in ailerons and elevator
- leading edge profile (any sharp edges?)
- center of gravity is definitely a suspect...as mentioned above, as fuel burned off, the problems began, indicating the rearward shift of cg as a trigger.....
12-10-2004, 12:01 PM
#6
RE: spin, spin, why did it spin
Edwin, I had to go back and look at my notes on this one. Apparently I was thinking along the same lines as you because I had added 2 oz. of managers taped lead weights to the firewall of the model prior to the last flight. I also planed to land at 9 minutes to avoid the empty tank. I also noted that the extra weights were found still attached after the final crash. I also noted that with the fuel tank full the CG was were the LE met the fuse and with the extra two oz. the CG was ahead of the wing LE by half a fat finger width. Since then I calculated the CG and found the recommended CG was at 17 to 23 % of cord.
I have concluded that the forward CG (as compared to other combat models) was a hint that the platform was not stable. Purposing is common in nose heavy planes and this plane would purpose in a shallow dive at full bore. So I can’t help but think the forward CG had something to do with it. But I can’t help thinking I had 30 minutes of flight time (including running it dry the first flight) on it before I did the high G loop. It violently snapped out of that and before I got another 10minutes of air time it had experienced drastic trim changes and snapped in twice. So I figured that the problem was in the wing.
Before giving it to the women in the house to finish the dismantling I completely band sawed the wing and hori. Stab and found nothing broken or out of the ordinary. I’ve always prided myself on being able to figure out what went wrong so that I could avoid making the same mistake twice. But I’m at a loss on this one?
I have concluded that the forward CG (as compared to other combat models) was a hint that the platform was not stable. Purposing is common in nose heavy planes and this plane would purpose in a shallow dive at full bore. So I can’t help but think the forward CG had something to do with it. But I can’t help thinking I had 30 minutes of flight time (including running it dry the first flight) on it before I did the high G loop. It violently snapped out of that and before I got another 10minutes of air time it had experienced drastic trim changes and snapped in twice. So I figured that the problem was in the wing.
Before giving it to the women in the house to finish the dismantling I completely band sawed the wing and hori. Stab and found nothing broken or out of the ordinary. I’ve always prided myself on being able to figure out what went wrong so that I could avoid making the same mistake twice. But I’m at a loss on this one?
12-11-2004, 12:26 AM
#8
RE: spin, spin, why did it spin
Lake, you may have somthing there. Because with the length of those wings, how thin the wing was, its speed, and the speed it s snapped in the max G loop I was surprized to find the wing tips still attached when it stopped spinning. Specially with those voltz servos out in the wing. Once I got my wits about me, coming to the realization of how stupid what I had just done was I was at imediat turn range. So I rolled inverted and think I throttled back (because I thought I might have broken somthing internal) and began a half loop. About the time I was vertical was the time it began to spin. I onestly do not remember any more what I did with the throttle. But the drill is 3/4 throttle (the less than full throttle acts as down elevator on a trimmed plane) so I'm sure I did that. As soon as I neutralized thr sticks it stopped spinning, I rotated the wings to pull out twords the field and added elevator and it snapped again. I whent through this exersize a third time before giving up and trying to flat spin it to reduce the impace velocity.
While getting it ready for its next and last flight I tryed to check the washout. But the wing was so thin that it could not be measured accurately. The amount of aileron trim required to retrim still screams somthing was broken in the wings. The extra elevator could be explained away from the weight I added to the nose. But the one thing that really confuses me is it was at reduced throttle and for at least two turns prior to the one it snapped in, but it snapped to the inside of the turn! The usual turn behavior I've seen is to snap out of the turn. The other thing that struck me is that it looked like a aileron roll rotating about the fuse. the radio and servos have about 10hrs air time without incident since this.
After the final dirt nap it was missing both wing tips, we found the one with the servo in it, and I was not going to spend any more time looking for parts for a plane that was never going to fly again, once I has the big $ peaces.
Joe
While getting it ready for its next and last flight I tryed to check the washout. But the wing was so thin that it could not be measured accurately. The amount of aileron trim required to retrim still screams somthing was broken in the wings. The extra elevator could be explained away from the weight I added to the nose. But the one thing that really confuses me is it was at reduced throttle and for at least two turns prior to the one it snapped in, but it snapped to the inside of the turn! The usual turn behavior I've seen is to snap out of the turn. The other thing that struck me is that it looked like a aileron roll rotating about the fuse. the radio and servos have about 10hrs air time without incident since this.
After the final dirt nap it was missing both wing tips, we found the one with the servo in it, and I was not going to spend any more time looking for parts for a plane that was never going to fly again, once I has the big $ peaces.
Joe
