Pulling and tucking in knife edge flight
07-31-2016, 06:38 AM
#1
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Thread Starter
Pulling and tucking in knife edge flight
I'm fine tuning my Intruder for SPA, and while adjusting the CG yesterday I found a strange (to me at least) phenomenon. When I rolled it to knife edge, if I needed right rudder it would pull to the canopy, but if I needed left rudder it tucked to the gear. I suppose this has to do with the spiral slipstream effect, but is it something I can or should trim out mechanically before setting up a mix to fix it?
07-31-2016, 06:07 PM
#3
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Thread Starter
It stays pretty straight. It's been windy all 3 times I've flown it, so it's hard to get a real precise look at it. But it doesn't seem to have a tendency toward pulling or tucking and didn't seem to when the CG was farther forward.
08-01-2016, 06:50 AM
#6
Being that it was windy you will no doubt be using different amounts of rudder for each direction. It's best to test using the same amount of rudder when testing knife edge. I honestly have never had an airplane that would tuck one direction and pitch in the other. All my airplanes are set up so that at 1/2 throttle it takes full rudder to maintain level knife edge. That makes testing and adjustments easier. I doubt you would be able to use the same method for the Intruder however. We have spoke about getting your CG correct. Do you in fact have it locked down to the point of not changing it again? We spoke about the 45 degree inverted upline test and how your intruder wanted to drop the nose fairly quickly. Have you corrected this yet. IMO until the CG is correct there really is no sense in trying to trim anything farther as CG affects everything else. Case in point. I picked up a 2 meter ship a few weeks ago that had been set up for YS 160 power. Don't get me wrong, I love YS engines but truth is I will be more competitive with E power so the entire fuse was gutted. I pulled out 3 lbs of " junk " the airplane didn't need and did my conversion with the appropriate strength of components. The airplane was test flown on Friday. During the first flight I did the 45 degree inverted CG test. It was holding track hands off. Ok let's try a knife edge both directions. Sure enough both directions tuck to the gear. I took the RX battery and moved it from the wing tube to as far forward as I could get it. Back up in the air, re trim the elevator for the now slightly forward CG as compared to earlier. Now the nose drops ever so slightly on the 45 degree test and knife edge is locked on but with just a touch of proverse roll. I also found that it needs a touch more right thrust which will be done before and more trimming adjustments are made.
point being is that trimming must be done in a sequence of events starting with CG first, then thrust adjustments, then incidence changes and lastly maybe 20-30 flights into trimming you may be fine tuning the final 5% with some mixing. You may also get some good info by posting this in the classic pattern forum. Keep in mind as well that you will only get so far with an Intruder.
point being is that trimming must be done in a sequence of events starting with CG first, then thrust adjustments, then incidence changes and lastly maybe 20-30 flights into trimming you may be fine tuning the final 5% with some mixing. You may also get some good info by posting this in the classic pattern forum. Keep in mind as well that you will only get so far with an Intruder.
08-01-2016, 06:52 PM
#8
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Thread Starter
I think the CG is where it needs to be. It still pitches some in the inverted 45 line, but at that setting it had no tendency to drop its nose on the landing, which caught me by suprise when it tip stalled during the approach. I don't want to go all the way back to a neutral CG, partly because of that little scare and partly because a stable plane is a big advantage in the Texas wind. I also figured this weird knife edge behavior was indicative that moving it farther back wouldn't help.
I was using the same amount of rudder both ways. I have the deflection set the same and was using full throws. I know there are several aerodynamic factors going on here between spiral slipstream, different p factor caused by down and right thrust, and any warps along with the quirks of this particular plane. So is this is a known quirk with some planes that has a common fix, I hope to find it. But if not, I'll mix it out and get to work perfecting the maneuvers.
I was using the same amount of rudder both ways. I have the deflection set the same and was using full throws. I know there are several aerodynamic factors going on here between spiral slipstream, different p factor caused by down and right thrust, and any warps along with the quirks of this particular plane. So is this is a known quirk with some planes that has a common fix, I hope to find it. But if not, I'll mix it out and get to work perfecting the maneuvers.
08-01-2016, 09:15 PM
#9
One thing to remember when setting up an airplane to use for sequence flying in the lower classes is use just enough throw to get you through the sequence. In this particular case reducing your rudder throw down to the point of just being enough will also reduce the pitching. The reality is that except for a stall turn you realistically only need 20-25 degrees of rudder each way to get you through the sequence.
08-02-2016, 05:10 AM
#10
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Thread Starter
That's good to know. I'll be turning it down a bit next time I go out. I have 20 degrees of rudder set up for my low rate which currently has too much elevator and aileron, then a hi rudder flight mode that will have the pitch mixed out for slow rolls, 4 point rolls, and stall turns, then a high rate setting for spins. But I'll experiment with less rudder to see how it does. With the maximum throw, it does gain a little bit of altitude in knife edge and will probably gain more once I get the right prop on it to let the Jett muffler do its thing.
08-25-2016, 07:44 AM
#12
Yes, a controversial comment for sure and in this case could be inaccurate. When flying pattern or IMAC one of the things judges look for is constant speed throughout the sequence. When wind is introduced into the mix this means using less power when flying downwind and more power when flying into the wind. While we are trying to maintain a constant speed from ground perspective the airplane experiences different airspeed going each direction. The result is that the rudder would be more effective while performing knife edge into the wind so less is used. As control cross coupling is not linear the upwind passes will experience it less then downwind passes that would require more rudder input due to slower airspeed. Different amounts of rudder used will result in different amounts of control cross coupling and in some cases such as this could have the cross couple showing up in opposite directions. This is of course all dependent on if the OP is trying to maintain precieved ground speed or not.
08-25-2016, 09:17 AM
#13
Yes, a controversial comment for sure and in this case could be inaccurate. When flying pattern or IMAC one of the things judges look for is constant speed throughout the sequence. When wind is introduced into the mix this means using less power when flying downwind and more power when flying into the wind. While we are trying to maintain a constant speed from ground perspective the airplane experiences different airspeed going each direction. The result is that the rudder would be more effective while performing knife edge into the wind so less is used. As control cross coupling is not linear the upwind passes will experience it less then downwind passes that would require more rudder input due to slower airspeed. Different amounts of rudder used will result in different amounts of control cross coupling and in some cases such as this could have the cross couple showing up in opposite directions. This is of course all dependent on if the OP is trying to maintain precieved ground speed or not.
