Hello all,
I have a Spadet LC-40. When I am under full power the plane wants to climb a lot vs. when im at say 50% power.
I have to apply basically full down trim to get it leveled out when under full power.

What can I do to fix this?

Any ideas.

Thanks much,
Ryan

Add some down-thrust

thats normal,it's not the elevator that makes an aircraft climb per se but the thrust from the engine.

Sounds like you need more down thrust on your motor. The easiest way to change the thrust would be sticking a small washer between the firewall and your motor mount to angle the engine down more. Put the washers on the top 2 bolts.
If you have beam mounts that are part of the airframe, try sticking the washers under the back two m engine mounting bolts. Between the engine and the mounts to agle the motor down.
You will have to play with the amount and size of washers to use until the plane have little to no corrections needed.

That being said, a highwing plane will tend to climb due to the thrust line being under the wing. It all relates back to the laws of physics. but adding down thrust should cure most of the problem.

Thank you very much for the suggestion.
I have some washers that are ~.030" thick and some thicker ~.060".
How much down-thrust is typically necessary?

Thanks again,
Ryan

I'm not sure on the SAE sizes but I can tell you that a 1mm thick washer is good for approx. 1 degree. Based on you desription I would add 3 1mm washers to each top mount bolt to get 3 degrees of down thrust. You could start with 2 washers, fly it and then add more if needed

Make sure that your elevator is centered using the clevises and no trim in the transmitter.

What engine and prop are we talking about here?

Thanks Bill,
I have an OS .46 SF. I am turning a 12 x 4 prop.

Ryan

Before you start adding downthrust you should consider at what speed you want your plane to fly level.

As you may already have found, if you trim for straight and level at full throttle, when you reduce throttle the plane will start to descent (all other conditions being unchanged).

As a starting point consider trimming for level at around 2/3rds full throttle. This will allow you to control the rate of ascent/descent by varying the throttle from this point.


Terry

a plane that has the correct downthrust should have the trim the same at all throttle positions (with the exception of approaching stall speeds...) add one of your thinner washers to each side on the top and fly it, you can always come back and add more.

"Throttle controls altitude, elevator controls airspeed."

Learn that, apply that in practice, and you'll be a better pilot. It's how things really work. The guys that try to go up by just pulling back on the stick are the one's who spin in their pretty warbird screaming "I've been hit".

Every trainer I've flown, and many other models as well, do climb at least some under power. It's not a defect, it's what they do.

If you add in too much downthrust, you can really get some strange things happening in flight. Like, get trimmed for full power level flight, then pull the throttle off, and watch the plane go hard nose-up. Do it enough and you can go right in to a stall. It can also make a "missed approach" interesting.

Now, all that said, there are a handful of things you can do to a typical trainer to make the pitch trim change less.

First, you want to figure out how much of the pitch trim is actually due to engine thrust, and how much is due to airspeed changes. Take the model up high, pull the throttle to idle, let the plane slow in level flight, then point the nose down 45* and let the plane accelerate by gravity alone. If you're good at deadsticks, you can take it up high and kill the engine, then do the dive. As the plane picks up airspeed, you'll see some tendancy to pull out of the dive. If the plane really pulls out hard, you can do a few easy things to make the plane pull out less.

- Move the CG back. Most trainers are nose heavy anyway. A nose heavy plane will tend to change pitch trim with airspeed. Moving the CG back will also make the plane a bit more acrobatic, and will make landings easier.

- Shim up the TE of the wing. This lowers the incidence angle of the wing compared to the horizontal stab.

- Reflex the ailerons. This is a quick and dirty way to also lower the wing incidence. It also changes the airfoil shape to one that isn't quite so "flat bottomed" as you find on a typical trianer. Actually, re-reading the origional post, I'd start here with aileron reflex. SPADs tend to have airfoils that vary a little here and htere from airplane to airplane. So it's not uncommon to adjust the reflex to fine tune the airfoil. It's also common for people to align the ailerons with the top of the wing instead of hte bottom, creating a "drooped" aileron situation which does lead to a lot of bad pitch behavior. Check this first.

If, and only if, the plane does a 45* power-off dive with only a mild pull-out, but still climbs hard when you hit the power, then I'd consider adding some downthrust. Not that you shouldn't check to make sure you have at least a little in there now, you should. Just that adding more with out first making sure the downthrust (or lack of it) is actually the problem can be counter productuve.

While I agree with Montague for the most part, I still say it sounds like you need to give a litte down thrust. There should be very little difference in the amount of trim needed between full power and power off.
While "Throttle controls altitude, elevator controls airspeed" is correct. Minor changes in pitch, yes, but not haveing to hold full down to stay level.
When a plane is trimmed for straight and level flight, it is trimmed only for the airspeed at which it was trimmed. Lower the power and the nose will drop and the aircraft will try to stay at the airspeed it was trimmed at. Less power means you will have to trade altitude for airspeed. Adding power to straight and level flight will cause the nose to pitch up and the aircraft will then climb maintianing the airspeed it was trimmed for during level flight.

Anywhere from 2 to 4 degrees is typical. I've had planes that needed as much as 7 degrees. The washers are a good idea to get started, but once you figure out how much you need, it's a good idea to sand a wedge from plywood and fuel proof it. Use the wedge to replace the washers.

There are a couple reasons. First, the washers will probably sink into the firewall over time which not only changes the thrust adjustment, but also loosens the engine mounting bolts.

Second, they create a stress point right around the holes in the engine mount. If the engine vibrates enough, it can cause the mount to crack eventually or immediately on a nose-over, for example.

A lot of guys use washers forever, but I think the wedge is a lot better.

In the image here you can see a thin wedge between the mount and the firewall. This plane didn't need much of a thrust adjustment.

I am with MONTAGUE on pretty much everything he stated.... ESPECIALLY with a trainer, speed control altitude plain and simple....

I also agree with CAFEENMAN..... I would build a wedge to shim the motor at an angle.

A good trim chart will also help with determining what is needed and where.

http://www.krcs748.com/trim1.htm

This was taken from one of the Great Planes manuals (I forget which one)

I have flown a lot of SPADS and they all tend to climb under full power. They have flat bottom airfoils which will generate a lot more lift as speed incerases, causing the climb. Even with the wing flat on top the fuselage, the curved leading edge causes the wing to be at a positive incidence. .The first thing to do is to lower the incidence. Make a shim from scrap 4mm Coroplast and raise the trailing edge of the wing.

Fly the plane with the raised trailing edge and trim for level flight at the power setting you fly the most. Land and check the elevator position. If you are carrying down elevator trim, you need to lower the incidence some more. You are shooting for a neutral elevator position.

You may need some down thrust, but I use this as a last resort. You have a plane with positive incidence that is trying the "kite" the plane upwards. If you jump on down thrust immediately you can compensate for the climb, but you have the wing pulling upwards and the thrust pulling down at cross purposes. This can result in a neutral plane, but with a lot of unwanted drag. I'd rather try to lose the extra lift in level flight first.

Thank you all for the suggestions.

Sounds like I have several options to try atleast.

Back to the flying field.

Thanks again,
Ryan

Your airplane is too nose heavy. Shift some stuff to move the cg a little rearward. If you only want to fix the problem you don’t need to read any more. On the other hand there is a lot of misunderstanding about this so for those who might be interested, I will explain further.

All airplanes (of conventional configuration and proportion) that have positive longitudinal stability will tend to climb with addition of power and dive with reduction of power when trimmed for normal level flight. The more stable the more pronounced the tendency. [Some aerobatic airplanes are balanced so that the stability is almost non-existent and so is the climb tendency. This makes them more responsive, but also more demanding to fly.] There are some other things that effect the response to power changes but for typical radio controlled aircraft they are not nearly as significant as cg position.

The location of the thrust line with respect to the center of drag will have some effect. A low thrust line with a high wing aircraft will produce a moment tending to pitch the nose up with added power while a high thrust line on low wing aircraft will produce a moment tending to pitch the nose down. These moments are in addition to the moments due to stability and are relatively small for aircraft of conventional configuration and proportion. When such aircraft are balanced properly down thrust is not usually necessary.

Down thrust is really a carryover from free flight models. The typical flight profile for free flight aircraft since the 1930’s has been a maximum power climb of short duration followed by a glide at minimum rate of sink. After launch there is no means to exert any control on the model. The airplane had to make the transition from full power climb to efficient gliding flight with only its fixed trim settings. To make matters more challenging, the wing was typically mounted on a pylon well above the fuselage and had sharply upswept, polyhedral that raised the center of drag well above the wing mount. The aircraft was usually balanced with the cg well aft to make the glide more efficient as well as assist the transition from powered flight, but the low thrust line and very high center of drag had to be countered, which was done with rather pronounced down thrust.

Another challenge was to keep the aircraft near the launch area and remain in thermals if one was encountered. For this reason the aircraft was trimmed to fly in a circle rather than a straight line. The typical flight consisted of a climbing right turn under power and a gliding turn to the left after the engine quit. This was accomplished by trimming the aircraft for a left turn when gliding and mounting the engine with right thrust to force the right turn during powered flight.

The early radio control models were adapted from free flight models and had rudder control only, which was not proportional. The typical flight profile consisted of a launch followed by some steering around with the rudder until the engine quit after which the model was guided to a dead stick landing hopefully in the vicinity of the launch. Since there was no pitch trim available, down thrust was usually used to facilitate the transition from powered flight to glide, and a little right thrust was used to trim the aircraft to fly straight under power.

With the advent of proportional three-axis control plus throttle, including trim on all channels, there is no need for down or side thrust on a properly balanced airplane. I have nine aircraft currently, and all of them have the engine mounted straight with no down or side thrust, and they all fly well. They include high wing trainers, a shoulder wing aerobat, a mid-wing, a low wing, and two biplanes and range in size from a Norvel.64 to a Saito 80 powered machine. For convenience, some high wing trainers are built with a little down thrust to simplify things for beginners who are not yet comfortable with the trim, but it usually isn’t necessary.

The airfoil shape has virtually nothing to do with stability and the pitch up tendency under power. A flat bottom airfoil will fly just as “groovy” as a symmetrical section if the aircraft is balanced for the same longitudinal stability. The only significant difference between the flat and symmetrical sections will be the attitude and stall angle in inverted flight.

Shimming the wing trailing edge and rigging the ailerons up have no significant effect on the tendency to climb under power. All they do is change the trim setting when the elevator is in the neutral position. There is no effect on stability.

Lou,

I agree that the CG is the biggest factor.

However, I disagree that altering the wing incidence and/or aileron reflex has no effect at all.

I agree that on a glider, you can change the wing incidene all you want and just trim the elevator, and all you do is mess with the drag generated by the stab. But in powered flight, you also have the relationship with the engine thrust line. Sometimes, taking a plane with lots of downthrust and lots of incidence between the wing and stab and moving it closer to 0-0-0 helps, and that's what raising the TE does. After all, for level flight, the wing is always going to fly at the AOA that creates the right amount of lift. So, raising the TE of the wing is really moving the fuse up, reducing downthrust(!!) and reducing the differnence in angle to between the wing and stab all at the same time. The theory might be a little off, but the reality is that I've done this, and it has worked.

Relexing the ailerons is the same kind of thing. In theory, it shouldn't make much difference, but in practice is does. Any long time spadder will tell you what happens when a plane has "drooping" ailerons. It often "hunts" in pitch and is very prone to pitch changes with airspeed. Some of my combat planes do this if I don't have enough reflex in them, a little reflex makes the difference between a plane that grooves and one that you have to keep an eye on. To be honest, I'm not totally sure I know why this is. I *THINK* it's lowering the pitching moment of the airfoil. Airfoils that I've seen that were designed for low pitching moment for things like flying wings often had a sharp reflex at the TE. Raising the TE also has the effect (again, I *THINK*) of reducing the camber of the airfoil, which can make the airfoil less speed sensitive. At least that's that idea. Feel free to shoot holes in these concepts. The theory may be hokey, but the field experience says that it does help in some situations. (I'm not saying it always helps in all situations. Just that it's worked for me)

What I said was “Shimming the wing trailing edge and rigging the ailerons up have no significant effect on the tendency to climb under power.” Compared to the powerful effect of cg position, these other factors are relatively small.

You are correct in stating that reducing the wing’s incidence effects the relationship to the thrust line and it tilts it more upward, which is opposite to adding down thrust. Simply reducing the incidence won’t change the required down force on the tail. Reducing the angle between the wing and stabilizer will just require more elevator deflection to balance the aircraft at a given speed unless the cg is changed also. RhyanO’s problem is that he is running out of down trim, so shimming the wing T.E would give a little more down trim. While this may fix his problem, it isn’t the best fix, because it doesn’t address the basic problem, which is nose heaviness. If nothing is changed but wing incidence, the change in stability is nil.

Reflexing the ailerons has much the same effect as washout at the tips, with some additional quirks. You are on the right track when you refer to its effect on pitching moment. A cambered airfoil typically has a nose down pitching moment about the aerodynamic center (ac). Such an airfoil cannot be both balanced and stable since in order to be stable, the cg must be ahead of the ac, and in order to be balanced the ac must be ahead of the cg. By reflexing the trailing edge, the moment can be changed to a nose up pitching moment that can be balanced with the cg ahead of the ac. This gives a section that can be both stable and balanced (making a flying wing possible).

By reflexing the trailing edge of the ailerons, You reduce the overall nose down pitching moment of the wing and if nothing else is changed, static pitch stability is increased. The effect is basically the same as moving the cg forward a little. At the same time since you have in effect added washout at the tips, the stall is moved inboard also.

I mentioned quirks. Like with washout, the lift distribution has been shifted inboard. This adds drag at any given lift condition. Since most models are grossly overpowered, the additional drag won’t likely be noticed, but it’s there. Cambered and reflexed airfoils both usually exhibit some change in moment with angle of attack, especially at high angles. This change in moment changes the static longitudinal stability a bit. The problem is that with the reflex being accomplished by rigging up the aileron, this factor is unpredictable. For a particular problem, it may work or not.

If the problem involves both too little stability and tip stalling, reflexing the ailerons may be “killing two birds with one stone.” Otherwise, simply moving the cg will accomplish the desired result without the increased drag.

I’m not sure what you mean by relexed airfoils being less “speed sensitive”. The moment is greater for a cambered airfoil than a reflexed, and a symmetrical one. But once a tail has been attached, the stability (and resulting sensitivity to speed) can be identical. It all depends on the cg location chosen.

I have no doubt that you have had good results with these adjustments. It’s just that there may be easier and more effective ways to accomplish the same thing.

Lou,
Great write up. Looks right on to me.