Incidences

[Jag01]

Hi all,

I was out trying to do some flight trimming today on a small electric plane that I use for some 3d training and come across a few problems.

Before I explain the problems let me say this is plane (shall remain nameless) has been marketed as a high end aerobatic model, it comes 90% pre-assembled and there is very little that the builder can do in regard to airframe alignment.

During the initial build I did discover a number of misalignment issues that I corrected at the time of the build. However after flying this thing for a while and not being overly pleased with the performance I decided to spend a little more time trimming to see if I could improve it.

So that is where I am at now.

After setting the CG to the recommended settings, I flew the model in a power off, hands off vertical down. I trimmed the elevator and rudder until the plane would descend vertically and not veer to one side or the other and would not pull out of the vertical dive. Having done this I note that the elevator is now carrying significant down trim.

In a vertical climb the plane veers drastically left and pull noticably to the wheels. Ok so then I moved the battery until such time as the plane requires just a little up stick held in to maintain level flight and about the same to hold level inverted flight.

Pull a 45 deg upline and the plane nose just falls aways slightly, roll inverted and the plane climbs inverted.

Now I could put some up thrust in the motor which would probably cure the vertical up pulling to the wheels problem and would also then probably help the climbing inverted on a 45 deg upline.

However that would seem counter productive for the plane to have lots of motor up thrust and the elevator to have lots of down trim!

So I am assuming this really is a incidence issue? Now the plane came with wing tube and anti rotation pins pre - installed, tail plane which fits into a slot in the rear of the fuse, due to the built up nature of the fuse it would be impossible to change the tail plane incidence. The motor box/firewall is all pre-built with thrust angles built in and the motor mounts on the inside of the box so that makes it more difficult to install spacers to adjust thrust angles.

So there are my problems in a nutshell.

Am I on the right track this is a incidence problem? If so am I correct in assuming that given I cannot change the tail plane alignment then I need to rotate the wings to increase incidence of attack of the main wing?

I look forward to your response.

Cheers.

Jeff

[zx32tt]

Unless you have an incidence gage to accurately measure the surfaces, you are shooting in the dark at this issue. I usually balance cg to fly hands off inverted, or with only a touch down elevator. Get Paul Goldsmiths trimming chart, and don't skip any steps.
zx32tt

[tele1974]

Maybe the misalignment issue were supposed to be their.

[Jag01]

I would find it difficult to believe that a plane would have these inaccuracies designed into it, given the flight performance is so poor.

I do have an incidence meter, the reasonn for my post was to see if I was on the right track before making changes.

I have set the cg so as to require both a touch of up and a touch of down stick for both upright and inverted flight. So with down trim in the elevator the aircraft is flying nuetral, however this is as a result of fighting forces, elvator trimmed down, motor thrust line down all artificially creating a positive angle of attack for the main wing. Whilst the plane is flying straight at level flight, in other aspects of flight like uplines and powered downlines the plane is all over the place.

cheers,

Jeff

[exeter_acres]

	Quote: Originally Posted by Jag01
	I do have an incidence meter, Jeff

have you measured the incidence and contacted the manufacturer to see what it should be?

[rchotdoger]

I think it sounds more like a cg issue, the incedences can make a difference. It could however be a compile of both.

[wingwall]

It's Peter Goldsmith. You should be able to find his two part articles with a web search or go to the IMAC website. The first step is to get the CG where you want it. And, I would say based on the trimming for level flight and then going to a 45 degree up line, rolling inverted, and finding that the plane climbs is a stong indication that it is tail heavy. It should just fall away slightly when inverted.

The down elevator trim is also an indication that your plane is tail heavy. If the CG is too far back you need some down trim to create more upward lift on the tail to keep it level. Does that make sense? The tail is heavy, so it needs more lift. To create more lift, down elevator is required, much like flaps.

If you read Peters article, you will see that after getting the CG right and trimmed for level flight, a full power vetical climb tells you whether engine thrust line is off. It appears that yours is. Peter give you a trick to find how much to correct it.

I would say before you go through Peters' trimming steps, check the wing and stab incidences. the vertical climb test wil get your thrust line correct. However, I believe most aerobatic planes with symetrical airfoils are designed with the incidences of zero degrees on thrust, wing, and stab, or what is commonly noted as 0-0-0 incidence. I have found ARCs and ARFs with 1/2 to 1 degree off, and have found differences from side to side. The two wing panels must be the same, and the two stabs must be the same, otherwise you have a built-in roll. You may have to redrill/relocate the anti-rotation pins. It can be done.

[Panther45]

The order of the flight tests is key to the success of the trimming sequence. First is thrust with the full speed into the wind and chop the throttle. Plans should continue and then fall off a little. As stated, then the CG and it needs to be done hands off into the wind at cruise speed trimmed flat. Then invert and should be slight to no down needed to hold it. Your derivation will dial some variability into the test.

Once those are done you are ready for the straight down test to see if the incidences are in agreement with each other. Doing the incidence test earlier will cause you to get a misleading answer.

It is great that you are doing the trim tests, most folks just start flying and wonder why it is hard to fly straight. I have customers that have been in IMAC for years and have never had a plane that flew straight. This takes time to get done and the payoff is good to the expiration date on the plane

Good luck

[wingwall]

Trimming and design of an aerobatic airplane is primarily trying to get a balance in forces as much as possible in all desired attitudes such that the plane takes little or no input from the pilot to keep it on a line; upright, inverted, and knife-edge horizontal lines, and up and down lines.

It is true that trimming for horizontal level flight and then pulling back to idle will give and indication of thrust, but it also indicates forward/aft CG performance. And, I will admit, it is one of the first things I will do. You could have down thrust to keep the plane level, and when you pull off the throttle for the glide performance see the plane climb and stall, drop its nose, dive and then climb and stall again. Two things are indicated. First, in order to hold a horizontal line, the plane needed the down-thrust to stop the plane from climbing and stalling. Second, the climbing and stalling tells you the CG is too far back. So, you have the thrust off to compensate for out-of-balance.

The advantage of vertical downlines is that you can pull back to idle and take most of the thrust vector out of the equation. Then it becomes mostly gravity and drag. Although, note there is likely some drag from the prop, which might be considered reverse thrust, and will increase as the plane picks up speed. If the drag is above (to the canopy side) of the CG, then the plane will try to pull out of the dive to the up side, or canopy side. High-wing planes such as trainers, full size or model, will have this tendency to pullout, and the designers want this in those planes. This will require down elevator to correct to a true vertical downline. Now you could correct the drag by changing the incidences and/or the vertical positions of the wings, but that is likely to change your trim undesirably, in other important attitudes, such as upright and inverted horizontal flight. A easier way, at least for a good aerobatic design, that may help in the other attitudes, is to raise the CG.

On vertical uplines you take most of the lift from the wing and tail out of the balance equation and have mostly thrust (although, including torque), gravity, and, as the plane slows down, less and less drag. This is why Peter Goldsmith would recommend this attitude to adjust the thrust line. As long as you maintain significant forward speed this will give the best measure for the thrust line and get a good vertical track in uplines. So, then you use the trick that Peter recommends to get your thrust line correct, which is you trim the elevator and rudder to give as true a vertical climb as you can. Then, land the plane, measure the angles of the elevator and rudder, and adjust the thrust line of the engine by half of that angle in the same direction the elevator and rudder would have been directing the plane. Now the plane should track a more true vertical upline when the elevator and rudder are back to neutral. The only glitch here is if you have not set the forward/aft CG before this, you may need up or down elevator trim to make the plane fly a horizontal level line again. I believe that is why Peter recommends getting the CG correct before doing the thrust line.

Now, staying with the power-on vertical up line, the vertical position of the CG also comes into play, and always has throughout this trim test. It is a part of the equation all the time in the vertical upline. As the plane slows down though, the drag force from forward motion falls away and the CG will try to settle in below the thrust. If the CG in upright level flight is below the thrust line, then when the plane slows down, say to a hover, the plane is likely to rotate to settle the CG below the thrusting force. The thrust force vector will have a force component that is trying to make the plane move, or rotate, toward the belly. This will require some up elevator to balance the forces to hold the plane in one position. Moving the CG up will most likely correct this. I think most pilots just attach their radio gear down near the bottom of those big cavernous fuselages of giant scale planes and don't think about vertical CG. A lot of this is why many designers of IMAC planes (as well as pattern planes) want to raise the main wing, lower the tail wing; put the CG in a better place and have the drag more centered to the CG. Take a look at some of the better planes that the top pilots fly. I know that QuiQue likes to put the tail wing much lower than the full scale, and he has placed the rudder servos higher on some his recent designs, so that the pull-pull cables are above the stab.

There is also a good lesson in adjusting the vertical CG by Mark Leseburg on an electric foamy for better knife-edge performance in the "Electric Flight School" dvd by Wild Berry Productions. Do an internet search for Wild Berry Productions.

I learned early on, when starting to fly IMAC, and after I asked some unlimited pilots for some trimming advise, that many pilots do all the other trimming without worrying about the pullout tendencies in the power-off vertical down line. Then when they are happy with everything else, they go to the transmitter and put in the required amount of down trim mixed with throttle to idle, to get the plane to track a true vertical downline. They use this to track good 45 degree downlines also, by bringing the throttle up a few clicks to where this down trim is taken out.

Hope that helps, and you're still out there Jeff.

[Panther45]

Good thoughts on the trimming.

The thing that most IMAC folks run into is the inherent design limitations since the plane must be within 10% of the original. The design limits some of the forces ( cap 232 or a Yak) and that can make a large difference. The thrust test then changes due to the design and that gets complicated.

I think you are right about the upline test as long as the other things are close to correct. As a builder, I suppose my customers bring me the planes they seem unable to trim out and those are not close to start with. That becomes a question of where to start. I think your thoughts on the upline test might be a better first test than the level test.
I will give it a try.

Sounds like you are getting lots of thoughts, Jeff, i hope this is helping you. In any event it is interesting discussion

[midwestpilot]

subscribed... I gotst a lotst to learn ;-)

[JRO]

If it's a PA Electric Shock, that's why.

Just my $0.02

J-RO

[RTK]

Interesting thread

[Mike Busutil]

Must be a Giles...... That is as good as they get.