Trimming Solution?
 

A little help, guys, please.

I have a sport plane: symmetrical airfoil, lifting horizontal stab., shoulder wing, engine slightly below the main wing centerline, everything at 0-0-0. Pics available if needed.

This plane climbs at full throttle. When trimmed for level flight at full throttle, it dives at low throttle. What would be the best way to trim this out so it will fly level at all speeds?

Thrust angle changes? Stab. or wing incidence changes? What say you all?

Thanks!

Dr1

RE: Trimming Solution?
 

sounds like its acting nose heavy because of cg or stab incidence at low speed the stab wont hold it i'd try moving cg aft a lil first then trim its dropping like a rock i assume sorry aint awake yet i meant add a lil wieght aft or flayyen that tail out sheesh im going back to bed cant even type flatten hor stab

RE: Trimming Solution?
 

Sounds like it's doing what it is supposed to. Any airplane that is trimmed for level flight at full throttle will descend when throttle is reduced, unless it is WAY out of trim, such as an extreme amount of down-thrust, which isn't the case with your plane according to your post. When you write that it "dives" at low throttle, how much does it "dive"? Is it extreme?

If it's just a matter of gently nosing down and losing altitude as you reduce power, use the elevator if you want to maintain altitude. Even pattern planes do that. It's what full-scale pilots do. There is no airplane in existence that I know of that will maintain altitude/attitude at all power settings/airspeeds without some elevator adjustment, either by trimming or holding elevator pressure. And please (in case you were thinking about it), I suggest you DO NOT try to electronically mix up elevator with low throttle, unless you are setting a landing flight mode--it could get you into trouble at some point.

How do you know the stab is lifting ( I assume you mean positively)? It's actually somewhat involved mathematically to determine with certainty whether a stab is generating positive, negative, or zero lift based on a given CG, unless you mount a strain guage to the tail or use a wind tunnel. Does the designer assert that the tail lifts positively (even that is not a guarantee)?

RE: Trimming Solution?
 

treedog,

Thanks for waking up and answering my post. I'll check the CG.

mesae,

Thanks for the very informative post. Your comment of "even pattern planes do that: is well taken. Yea, it's more of a "nose down-hold elevator" situation than an extreme dive. Actually, it seems to need down trim to fly level at high speed, which causes it to sink at lower throttle settings. The designer (me) knows the stab is lifting. It's a flat bottom airfoil. [8D]

It's not a major problem, I just wondered if it could be trimmed out.
Dr.1

RE: Trimming Solution?
 

Not to be disputatious, but just because the airfoil is shaped a certain way is not proof that the stab is lifting positively. In fact, the airfoil shape of the tail has little to do with the direction the tail is lifting at any given time. If the center of gravity of your model is forward of the main wing's aerodynamic center, then the tail is producing negative lift at equilibrium*. If the CG and AC are in the same position, the tail is producing zero lift at equilibrium (this is the most efficient), and if the CG is aft of the AC, the tail is producing positive lift. It's quite possible, even likely based on your description that it climbs with high power, that your CG position is causing your tail to lift negatively, despite the positive camber of your tail airfoil. This is equivalent to flying a flat-bottomed airfoil inverted. I canot say conclusively that this is the case with your model, however.

One might predict that with certain configurations of true lifting tails (with the CG actually aft of the wing's AC), that above a certain airspeed, the tail would rise and the nose would drop. I have observed this effect with a Telemaster (a true lifting-tail design). If you fly it too fast, as in a power-on dive, the nose "tucks" markedly and it takes nearly full up elevator to recover. I suspect that if I had flown it much faster, it would not have recovered. The nose continued to drop as the speed increased, and only when I reached nearly full up elevator did it very abruptly pitch up and recover. It was alarming and unexpected, to say the least, and I kept it real slow after that![sm=eek.gif]

This discussion does not consider stability or control requirements, only tail-lift.

* This is a first-order approximation, and does not consider the lift-stability contribution of other parts of the airframe such as the fuselage, engine cowl/s, landing gear, etc.

If you have already made this comparison, then please accept my apologies for over-explaining.

RE: Trimming Solution?
 

No apologies necessary, mesae. I'm learning things. :)

How do you determine the AC of the wing? The plane currently balances at about 30% chord.

Here's a pic of the plane.

Dr.1

RE: Trimming Solution?
 

I haven't tried this but it looks good:

http://perso.wanadoo.fr/scherrer/mat...glish/mce.html

There are others.

It's in the aerodynamics tools, calculators and test links thread.

Or you can model your wing panel in CompuFoil and it plots the AC automatically.

For your constant-chord, unswept symmetrical wing, the AC is 25% behind the LE. So it is possible with a 30% MAC CG that your tail lifts, but we don't know for certain without accounting for fuselage/landing gear/propeller effects, which can be substantial and difficult to quantify. If the CG moves as fuel burns, this could be different with a full tank.

The AC of any subsonic (incompressible flow) wing is very near the quarter-chord point (at low AOA) but you have to account for sweep and multi-panels.

RE: Trimming Solution?
 

And it's also possible that, with the thrust line below the wing centerline, the asymmetrical thrust is pulling the plane upward at full throttle. Looks like moving the CG back is an easy modification and might do the trick. Maybe a little down thrust, too. A degree or two. Thanks for all your help.

Dr.1

RE: Trimming Solution?
 

You're welcome.

You will get minimum pitch variations with power and airspeed if the (three-dimensional) aerodynamic center of the whole airplane, the three-dimentional CG and the thrust line are all in line (the CG and AC should be co-located, not just in line). This probably never occurs perfectly in real life, but the Yak 54 comes close.

Remember that down-thrust has the effect of moving the center of gravity forward while the propeller is producing thrust, which is counter to an aft CG adjustment. You might want to try just down-thrust first, before making any CG adjustments.

Have fun!

RE: Trimming Solution?
 

DR1, the engine being in a low position will produce some pitching up that you can counter with some downthrust.

But before that I'd suggest you set the CG to as close to the neutral point as you want by using some dive testing.

For this sort of model it's easy. Trim the elvator for level hand off at about 1/3 power (power, not stick position) and when happy push the plane over into a 60 degree nose down dive and let go of the stick. If the model is nose heasy and the tail is compensating the model will tend to pitch back to level quite quickly. If it's close to perfect there will be just a hint of pitching back to level. If it's neutral it'll ontinue on a perfectly straight line into the dirt. If it's too far back it'll try to tuck and steepen the dive.

Move the CG, retrim for level flight and try again until you have it just as you wish. Once this is set then I would alter the thrust angle to compensate for the lower thrust line if it's still needed.

If you ignore the CG and just use downthrust then yeah, you can compensate for the overall speed issue. But you'll find that when it's inverted you'll need more down elevator than you should require if you cover up the CG locaton with downthrust. In the end I think you'll find the model works better if you deal with each issue in the proper order.

I prefer the real world dive testing for this as it allows for all the little things that add up that you don't see when you do some math that only covers some of the issues. For example it's hard to factor in the location of the drag center of the wing but if you do the dive testing like this then it's automatically taken into account.

RE: Trimming Solution?
 

Hey, I see B chimed in with a different suggestion while I was editing my last post. Either approach will work, but I suspect he will end up with a bit of down-thrust to compensate for the low thrust line. The thrust-line/AC relationship is preserved in any attitude so it might not hurt inverted flight to add a little down-thrust.

RE: Trimming Solution?
 

Thanks to you, BMatthews, and mesae both. I'll let you know what happens.

Dr.1

RE: Trimming Solution?
 

Good thread guys!

I was thinking about that in these days, if I can I would go deeper with a question. I preface I'm a pattern guy, so let's talk about aerobatic planes :)

Mesae,

you say that even pattern planes exhibit such behaviour... mmh... I don't agree. The trimming process is aimed to overcome that aspect, so it is actually possible to end up with a plane that can be trimmed for 1/3 throttle and it will stay S&L even at full throttle! I've spoken also with some successfull IMAC pilots that said the same thing...

Now I ask you... HOW that could happen? It is known that for straight & level flight the wing must produce some lift, and lift = weight. To do that, some positive AOA of the wing sould exist. We all know that lift is a function of speed, so how can a plane keep S&L going much more fast, with respect to the speed at which it has been trimmed?

My understanding is that down thrust is the answer here. So, with more power the nose-down moment induced by the down-thrust can keep the plane S&L?

To me it is a bit strange, since the lift acts near the cg, so it is a force towards the sky, whereas the down-thrust generates a nose-down moment... [&:]

What do you think guys?

RE: Trimming Solution?
 

Take a look at this link, it is quite interesting:

http://home.tiscali.be/comicstrip/Th...0on%20Trim.htm

RE: Trimming Solution?
 

Down-thrust can be used to partially compensate for the difference between an airplane's aerodynamic center and it's center of gravity, and in fact that is what it is most often used for, even if most pilots don't realize that is the basic reason for it (full-scale or model). If it is used to cause the plane to maintain altitude over a broad range of power settings, it will only work when the airplane is upright, and will make things worse when inverted.

Lift acts normal to the free-stream velocity, which has nothing to do with "toward the sky", unless the airplane is flying straight and level.

We have to carefully define what we are talking about before we can disagree. I know about pattern airplanes and IMAC: I have done both. I also know about the effects of downthrust. It can be used to approximate what you are talking about, either upright or inverted, but not both. Down-thrust has several different aspects. When people talk about models maintaining altitude, and flying perfectly straight lines, they mean it looks like that from the ground. That doesn't mean that if you put an altimeter on the airplane, you wouldn't see a much bigger variation in altitude than you thought was going on. The fact is, human eyes are just not that good at telling whether an aircraft is climbing, descending, or maintaining exact altitude, while watching from the ground.

It is physically impossible to leave the elevator fixed and have an airplane fly at the same flight path at all power settings. As the airplane slows down, lift is reduced, if the AOA is not increased, and the airplane will descend. Maybe not enough for a pattern/IMAC judge or the pilot to see over the distances typically flown in a costest, but it will descend.

It is overstating one's visual observation abilities to definitively say he or she can observe whether an airplane is flying a perfectly straight line or not over any distance greater than a few hundred feet.

RE: Trimming Solution?
 

Thanks mesae for the reply,

I agree with you, from the physical point of view it is not possible...

Thanks again.

RE: Trimming Solution?
 

Interesting point, could you explain in greater detail this concept? What would happen during inverted flight?

RE: Trimming Solution?
 

Quite right Mesae. Once the CG has been set the thrust line can be set to compensate for any "residual" effects. And because it's not being used to cover up any CG issues it'll work equally well inverted as upright. (just another way of saying what you said to help out anyone that's trying to figure this out)

RE: Trimming Solution?
 

The picture shows a case where thrust angle can be (and is) used to compensate for the vertical difference between AC and CG.

If CG and AC are already in the same longitudinal line, or very close, as with a pattern airplane, and down-thrust is added for the purpose that as power is reduced, the CG effectively moves aft, causing the nose to rise, and the airplane to approximately maintain altitude (to a point), then if the same maneuver is performed from level inverted, as power is reduced, the effective center of gravity will move forward, and now both the elevator trim and the more forward CG will cause the airplane to descend toward the earth more rapidly than it would have, had the thrust angle been set neutrally. So, for an aerobatic airplane, it is better to set thrust to achieve zero nor nearly zero pitching with power changes, and use the elevator to maintain flight path.

RE: Trimming Solution?
 

This is a problem where both factors affect each other. The thrust angle affects CG while the propeller is producing thrust, so the zero-thrust CG is masked or altered. So if you make an adustment to CG, then adjust the thrust angle, the CG will probably have to be re-adjusted, depending on how persnickety you are. But if you adjust the thrust angle first, you will then have to re-adjust the CG.

RE: Trimming Solution?
 

down thrust can also be a cheap and easy way to be lazy. The reason many airplanes have a particular airfoil on the H-tail is so that the forces on the tail can be tailored to develop the required longitudinal trim force. Typically, higher C-L airplanes will use a tail that has an inverted tail, since at high speeds, a negative pitching moment will be required to keep the airplane from rising. The vehicle may even fly at negative angles of attack if it has a signifacantly high C_L or is traveling at high speeds. Remember, any wing follows the same lift principle (L=C_L*1/2 *density*V^2....its the Velocity term that you are increasing or decreasing) A symetric airfoil has a C_L=0 at alpha = 0, but all other airfoiled planes will experience trim changes at diff speeds unless the tail also has a nonsymentric airfoil......or, the cheaper, easier thrustline is set to nonzero.

RE: Trimming Solution?
 

Hello mesae,

I'm not sure to understand your point...


1) why thrust variations should produce CG movements?

2) How AC vertical position is computed?

RE: Trimming Solution?
 

Down-thrust affects CG dynamically because when you angle the propeller downward relative to the free-stream (flightpath), the thrust vector is split into two components: a large one aligned with the flight path, and a smaller one acting "downward". This downward vector has magnitude T times the sine of the down-thrust angle. T is the thrust vector and the down thrust angle is the angle the thrust vector makes with the flightpath, not the fuselage centerline. This downward vector works exactly like additional nose weight on the airplane, effectively moving the CG forward. This force must be balanced if the airplane is to fly straight, and that balancing force comes primarily from the horizontal tail (up-trim) and/or additional airspeed and/or additional slipstream lift over the wing/fuselage.

As for computing the three-dimensional position of the AC with accuracy: this is more involved than I am prepared to write out in this forum. I would direct you to a book on aeronautical engineering, such as Warren Phillips' excellent Mechanics of Flight. There are others. I recently bought this book and find it very helpful and informative, and Dr. Phillips can explain the more complex subjects better than I can.

RE: Trimming Solution?
 

I've never heard of altering C.G with thrust lines. Did you get this out of your "Mechanics of Flight book"? I think what you mean to say is that the sum of the moments acting on the vehicle becomes more negative. The C.G. stays the same. lets be careful about making up terms ("Down-thrust affects CG dynamically ")...this is what confuses people.

RE: Trimming Solution?
 

No, I didn't get it from the book. I thought I did a good job explaining it to a non-engineer who asked a specific question about it. Didn't think the "sum of the moments" statement would have made much sense to him. Conceptually, it's pretty easy to understand the way I explained it. You are the first to express confusion about it. Stek just hadn't heard of the effect before.