Incidence, thrust line or CG issue?

[wollins]

I have a EF 74" Yak 54 and from about the first flight I've had to trim about three clicks UP elevator to keep it flying level, or else it wants to "fly down".

Now that the elevator is trimmed up from neutral from a "matched elevator plane to horizontal plane" standpoint, it flys ok but with increased power wants to climb. I understand that a neutral incidence plane should not change pitch with throttle changes, (or am I wrong?) so I would like to see if I can solve this problem.

Would this be an incidence problem, a CG issue (my CG is just in the first/forward third of the CG range) or a motor thrust line issue? :confused:

Colin.

[rebell]

With the CG forward like you say, it must me the CG. Not that the other issues don't have an influence, a plane is a whole system working together. I would recommend that you move the CG back 5mm at a time and do several test flights until it is trimmed. There is a complete trimming sequence list on the net, I will search for the one I am thinking about and post the link here. It explains exactly what to look for and what to do.

To get a completely trimmed plane might take more than 50 test flights with adjustments all the time. Even more if there are problems.

[Rodney]

Most probably a thrust adjustment is needed. Add a bit of downthrust and I'll bet it improves immensely. Of course you will have to re trim the elevator (take out some of the up trim).

[vintage1]

They are all interrelated. A forward CG rquires uptrim at cruise, and thet translates to more desire to climb asd speed is incerased..which is after all what you want, as otherwise it wouldn't want to pll out of a dive..

Most full size planes are that way too.

They all climb on the throttle.

If its a free flight model, thats what was wanted, and you trimmed any excess with downthrust.

With full R/C you maybe prefer a more neutral model,: Moving the CG back will net you one that changes trim less between full power and the glide, but has the effect of reducing stability..such a model, put in a dive, may well stay there, or in the limit tuck under as the speed builds up.

In your case live with it, or move the CG back a little at a time..

[rebell]

Most probably a thrust adjustment is needed. Add a bit of downthrust and I'll bet it improves immensely. Of course you will have to re trim the elevator (take out some of the up trim).

This is an aerobatic model. Down thrust will not be good when inverted.

[rebell]

They are all interrelated. A forward CG rquires uptrim at cruise, and thet translates to more desire to climb asd speed is incerased..which is after all what you want, as otherwise it wouldn't want to pll out of a dive..

Most full size planes are that way too.

They all climb on the throttle.

If its a free flight model, thats what was wanted, and you trimmed any excess with downthrust.

With full R/C you maybe prefer a more neutral model,: Moving the CG back will net you one that changes trim less between full power and the glide, but has the effect of reducing stability..such a model, put in a dive, may well stay there, or in the limit tuck under as the speed builds up.

In your case live with it, or move the CG back a little at a time..

It is true that it is interrelated. A system working together like I said. As this is an aerobatic model, I will trim it to stay in a dive when put there. I hate it when an aerobatic model pull out of a dive by itself or when speed in increased. It is OK for a Cessna, but NOT for a Yak.

[rofujiyama]

Hi Colin,
I went to see , what plane you were flying, looks really sharp , if it's the one in RC universe review. I did some wingloading calculations , and theirs came out around 23 oz per sq. foot. At that wing loading, I believe, some positive incidence on the wing , is called for, if you plan to be cruising around ,at a mostly reduced throttle setting. Unlike lighter wingloading aircraft, the lift to drag ratio is a little to high to expect the wing to sustain , level flight at less then full throttle, unless you use some positive angle of attack, or up elevator trim, as you have found out. So, If I were you , i would try to get my tail surfaces centered, as I have posted , in the right and downthrust discussion, and/or try to see if you can lighten your plane a little.
Roland

[vintage1]

It is true that it is interrelated. A system working together like I said. As this is an aerobatic model, I will trim it to stay in a dive when put there. I hate it when an aerobatic model pull out of a dive by itself or when speed in increased. It is OK for a Cessna, but NOT for a Yak.




Well Work In Progress, who flies a full size Yak, would tend to disagree with the opinion, but if that is what you want., move the CG back and leave the thrustline dead neutral, till you find a trim to your taste..be warned though, if you have to ask this question at all, you probably haven't been through the nightmare of flying a plane with a CG right back at the limit.

Lots of designers/manufacturers indicate a pretty well forward CG because that means their planes come out predictable and safe..moving it back is for those who want it neutral, and know what the effects will be. If you don't, do it in careful stages..no more than 3mm or so at a time.

[rebell]

I agree that in a full size the CG must be more to the front to be safe. I don't think sitting in an unstable plane would be nice at all. But with a model it is different, I am not sitting in it. :eek: Luckily. :D And then there are the servos that take up the forces, where as in a full size you will have to fight the sticks.

[yoyoML]

If you really want to 3D that plane, you don't want incidences. The CG also has different requirements.

CG fore/aft: You'll want to set the elevator deflection first, where +-35~45 degrees is a good starting point. You need that much because in a hover/high alpha you'll want a lot of deflection so the elevator works well in prop wash.

Then, the CG and elevator expo should be adjusted together: find a combination that gives smooth pitch control near neutral, then either a) move CG back and increase expo, or b) move CG forward and decrease expo. This should be adjusted until you're satisfied with the plane's AoA damping: fly straight and abruptly pull back, release. With a forward CG the plane will snap back to its flight path, but with an afterward CG it returns slower.

An afterward CG helps with hover stability, too, while it hurts normal flight stability.

CG up/down: Hang the plane by supporting only its prop blades, which are rotated parallel to the wings. You'll prefer the plane to point straight up, indicating a CG that lies right on the thrust line (you did set zero up/down thrust didn't you?). Most likely the CG is lower than the thrust line, you can a) move the CG up to the thrust line, or b) add some up thrust so the thrust line passes closer to the CG.

This is quite important if you plan to do any hovering, because if the CG is not on the thrust line you'll need to apply lots of elevator to keep the plane straight up.

If you don't plan to 3D that plane, ignore me. ;)

[rebell]

Here is the aircraft flight trimming chart I was looking for:

http://www.geistware.com/rcmodeling/flight_trimming.htm

[Ollie]

First do Flight Trimming flights. Then consider your flying style for final adjustment. Only you know about your personal flying style. Don't assume other people's flying style. Maybe you like other people's flying style?

[bob_nj]

yoyoML -> Your CG on the thrustline interests me. I have a 40% Cap that will harrier really well, but when I try to get it in a good hover, it keeps wanting to fly forward with 45° elevator deflections in it.

It also flies level with down trim in it. I reduced the downthrust a little at the field the other day just to see what influence it might have on the hover thing.
By then it was windy and although it seemed a little better, I couldn't really tell. I did have to give two clicks of up trim though. It still has noticeable down trim in it.

I was so frustrated that I just wanted to sell it.
After I calmed down, I decided that since I didn't build it, maybe the incidences and/or engine thrusts could be way off, and they might be worth looking in to.

I ordered one of the AeroPerfect meters and am waiting for it to arrive.
It's pricey, but I have this 40% to set up, an un-assembled 102" QQ Yak to get together, and am building an OMP 80" profile Edge kit.
The tool will get a good initial workout, and then have many uses in the shop for a good long time.

I'll try to find a way to really check the cg while I'm at it.
I could swear it flies a little tail heavy, but am not 100% sure.

It should be a fun project once it gets going.

Here you say ->
"CG up/down: Hang the plane by supporting only its prop blades, which are rotated parallel to the wings. You'll prefer the plane to point straight up, indicating a CG that lies right on the thrust line (you did set zero up/down thrust didn't you?).
Wouldn't a heavy wing give the impression of the cg being up or down too?
What makes the CG go up or down? How does that happen?

Please elaborate, and thanks.
Bob

[yoyoML]

Here you say ->
"CG up/down: Hang the plane by supporting only its prop blades, which are rotated parallel to the wings. You'll prefer the plane to point straight up, indicating a CG that lies right on the thrust line (you did set zero up/down thrust didn't you?).
Wouldn't a heavy wing give the impression of the cg being up or down too?
What makes the CG go up or down? How does that happen?

Please elaborate, and thanks.
Bob

Hmm... what does a "heavy wing" and "impression of..." mean?

Most of the time the CG is pulled down by gears hanging below fuselage together with strengthened structure, batteries stuck to the belly, a low main wing... Or it may be that the prop axis is mounted higher for sake of prop clearance.

Heavy up elevator in a hover most likely means 1) high thrust line 2) down thrust 3) low CG...

[bob_nj]

By my response you can tell that I just don't understand the concept of a high or low CG. I'm hoping you can elaborate on that and say something that will light my bulb.
Are you saying that if you hang the plane by the prop blades the wings should stay "level" indicating the cg lies on the thrustline?
If so, what I was asking is wouldn't a heavy wing also keep the wings from staying level giving the "impression" that the cg is not on the thrustline?
When you said the plane should be "straight up", I took that as level.

Let's assume for a minute a plane where the cg is perfect in my world.
Let's also say that if it is perfect the nose is level and the batteries are on the thrustline.
1. If I move my batteries above the thrustline, what happens?
2. If I move my batteries below the thrustline, what happens?
3. If I move my batteries left of the thrustline, what happens?
4. If I move my batteries right of the thrustline, what happens?

If these questions don't make sense, just give me some theory on why it makes a difference if weight is moved above or below the thrustline even though the plane in my mind should still have a perfect CG?

We are GMT -4 here, how about you?

Thanks for the replay.
Good thread!
Bob

[nmasters]

just give me some theory on why it makes a difference if weight is moved above or below the thrustline even though the plane in my mind should still have a perfect CG?

Have a look at this (http://www.rcgroups.com/forums/showthread.php?t=656724) . I'll post a more detailed drawing here later

[yoyoML]

By my response you can tell that I just don't understand the concept of a high or low CG. I'm hoping you can elaborate on that and say something that will light my bulb.
Are you saying that if you hang the plane by the prop blades the wings should stay "level" indicating the cg lies on the thrustline?
If so, what I was asking is wouldn't a heavy wing also keep the wings from staying level giving the "impression" that the cg is not on the thrustline?
When you said the plane should be "straight up", I took that as level.


That would actually mean vertical instead of level. I mean if you hang the plane by the prop, chord line/thrust line should all be vertical. It isn't possible to hang by the prop and have the plane level, is it?


Let's assume for a minute a plane where the cg is perfect in my world.
Let's also say that if it is perfect the nose is level and the batteries are on the thrustline.
1. If I move my batteries above the thrustline, what happens?
2. If I move my batteries below the thrustline, what happens?
3. If I move my batteries left of the thrustline, what happens?
4. If I move my batteries right of the thrustline, what happens?

If these questions don't make sense, just give me some theory on why it makes a difference if weight is moved above or below the thrustline even though the plane in my mind should still have a perfect CG?


As you know, if a force does not pass through the CG, the force also exerts a torque. That force is just your thrust line. So if the thrust line passes above the CG, you get a nose-down torque.

So the answers specifically:
1. CG moves up, requiring less up elevator (more down elevator) in a hover.
2. Opposite of 1.
3. CG moves left, requiring right rudder in a hover.
4. Opposite of 3.

But then, I don't know what means a perfect CG in your mind. How do you find a CG perfect?


We are GMT -4 here, how about you?

Thanks for the replay.
Good thread!
Bob

GMT +8

[nmasters]

Most modelers, and model plans, only consider the CG position in the top view. It should also be marked in the side view because we're dealing with 3 dimensional objects that have 3D mass properties. In fact there are two 3D points that influence the pitch behavior of your airplane. They are the centers of mass and drag. Their side view positions relative to the thrust line generate pitching moments that vary along with airspeed and acceleration. The drawing shows a very poor arrangement of these two points and the thrust line.

In the case of the center of mass AKA "CG" our newbie has built his wing hell-for-stout and left off the landing gear. Both of those design decisions tend tend move the CG up. He has also not put any down thrust into the motor mount. It's possible to balance and trim this plane so it shows neutral stability in the dive test (with constant power or gliding) but when you increase power in level flight it will nose up because of the moment generated about the center of mass. The center of mass is simply that point through which the momentum acts. When you accelerate an object by puling a string attached to a point it will rotate until the center of mass is in line with the string. In the situation illustrated here the airplane will rotate until the AoA of the stabilizer generates enough force to balance the nose up moment. A little of this is okay and actually good because it turns excess power into climbing instead of letting the airframe over-speed. However in this case it's way too much. The solutions are to either move the CG down or tilt the thrust line forward. The CG can be moved down by putting the electronics closer to the floor or making the wing lighter. Also landing gear would move the CG down by adding mass below the floor. Down-thrust tilts the thrust line forward thereby reducing the offset of the center of mass and its moment. However this plane would require 11 degrees of down-thrust to completely eliminate the pitching moment due to inertia so some of each would be the best bet.

In the case of the center of drag the moment is always present while the motor is on and proportional to the distance of that point from the thrust line. The pitching moment due to drag is actually pretty small compared to the moment the stabilizer can generate so it's usually ignored because it is directly proportional to airspeed^2 and thus easy to trim out. However if you really want that last bit of neutral stability you need to get that point on the thrust line and near, but still behind, the CG . The first thing would be to go to a mid or low-wing position. The drag of a tall vertical fin lifts the point up a bit and moves it aft. A low wing moves it back down. Landing gear is also pretty dragy so that also moves the center of drag down and forward.

Now for that hanging by the nose test that yoyoML described. With the blades parallel to the wing (you have a 2 blade prop, right?) hang the airplane by the propeller. By looking at the side you'll be able to see if the plane is balanced by noting how far the tail swings away from vertical. You may want to hang a plumb bob between yourself and the model for reference.

-- Norm

[bob_nj]

Wow, where do I start?
By saying thanks to you guys for being so supportive and helpful.
You are truly an asset to this board and people like me who are stuck in a two dimensional mindset.
I can see that my brain needs to open up to that third dimension (3D), and it slowly is.

yoyo -> I totally misread the idea of hanging the plane by the prop until I saw the picture from nmasters. Again, I was thinking in a straight and level two dimemsional picture. I thought you were trying to make sure the plane sits level if supported upright. My thinking was that if the cg was more to the right or left, it would cause a wing to drop and the plane wouldn't be level in an upright attitude. Now I see that a heavy wing and a high or low cg can be and are in fact different.
My reference to a "perfect" cg was one where the plane is flying well, and the cg is neither up nor down, left or right. My question then was, if it was "perfect", what affect would moving it be?

Earlier I mentioned that my Cap harriers really well, but wants to keep moving (walking) forward when I want to stop it to hover. By hover, I want to keep it at less than a 90° because it will start torque rolling.
I can stand it up, but that's not what I want to do. I've seen planes sit still at between 70 and less then 90° in front of the pilot and just "hover".
That's what I'm after.

So if my plane wants to not stay between 70 and 90°, and just either harrier or be straight up, by your explanation, and nmasters picture, it may have a cenario as in your #2 answer -> CG moves down, requiring less down elevator (more up elevator) in a hover?
Does that seem like the case?
From the picture, it says that the plane needs more "up trim" in a hover.
I can't get any more up "trim" than full elevator deflection in a harrier.
Does this mean more up "thrust", or a more "up" cg?

nmasters -> It's going to be difficult if not impossible for me to hang my 40% Cap by the prop. I really wish I could, but I don't think so. Dag :(
What to do, what to do?

Do you guys have any pictures of fixtures or gadgets to check the cg on a big plane? I am going to start with finding out what my engine thrust is, and where the cg is.

Once again, thanks so much for the insight. Problem now is, I'm coming up with a whole new set of questions.

First thing that comes to mind is;
What affects are noticed if the engine is above or below the thrustline on an aerobatic or 3D plane such as the Cap?

Next is, what affect do incidence and decalage have on my plane?

So many questions, so little time
Bob

[bob_nj]

Did I not make sense or something...

[nmasters]

You've been slapped with a fairly large piece of information. It'll take a while to digest it. The plane in the drawing can't truly hover. Since the pendulum effect of the CG forces the thrust line to be angled a pilot trying to hover it would put in a lot of down elevator to try to counter the tendency to drift off toward the top of the plane. The problem here is that that elevator input is producing lift in the same direction that the skewed thrust line is pulling. The result is that the almost hovering plane drifts off in the direction that the center of mass is offset from the thrust line. When the CG/thrust line relationship is right you shouldn't need elevator trim to hover.


PS I'm a great big fraud. I only build gliders :D

[ram3500-rcu]

Great stuff. :D