Overhead Tension

I originally posted this question in the Rudder Trim Thread but suspect that it got buried and not seen.

Beginner question here.

I have followed most of the discussions of leadout position, rudder offset, and engine offset, as well as some of Mr. Bucks and Ted Fancher's trim explainations in recent threads with great interest. But one concept I've heard in several places puzzles me.

Brett Buck (and others talking about yawed-out trim) indicated in a different thread:

"The large yaw angle kills the
overhead tension."

I guess I believe that an outboard yaw angle will generate some tension simply because of the angled thrust vector. What is it about the overheads that makes this not so??

I've tried to visualize the relationship of the various centers and forces but havent found a reason for this overhead change.

Any hints.

Thanks

The model/engine has to support the whole weight of the lines and the model when overhead, and the flying surfaces are no help. It's all centrifugal force, caused by that curved flight path. When the model's speed falls slightly due to drag of maneuvers and wind influences, the tension is much reduced. Having a LOT of power will counteract this speed reduction, and if suited to the engine, a lower pitched prop will too. Yaw creates more drag and load on the engine and just makes the problem worse. That's my theory and I'm stickin' to it! Now, why is it that extra 1/4" pitch in the prop tips helps overhead tension? Steve

I think you are right about the overhead tension, issue, Steve. I have seen a lot of engines sag and just plainly run out of "oomph" when going overhead--even a lot of pipe ships. It has nothing to do with the engine. It is the way it is set up. The big knock on pipe setups when they first came out was that they had no overhead tension. That's because too many tried to run them "over the curve" so that if the engine ran faster it lost power. It was great for slowing the airplane when going down hill. Lousy for going up hill and overhead. But if the engine doesn't put out power, it is going to lose that overhead tension.

We may not notice this as much when flying level, and, especially, when we have assymetry helping to overcome line weight. And even one "G" when flying level is "adequate" line tension (for some, not for me.)

But, now put the plane overhead, and assymetry goes out the window and gravity totally eliminates that one "G". That is why I don't like either of them. I can't eliminate gravity, but I can eliminate assymetry. And, if you have any excess drag, then overhead it is going to be accentuated, hence, loss of tension.

Let's put it this way. Tie a line to the tail of your airplane and hold onto that line while the airplane sits on the ground with the engine running at flying speed. Feel the pull. With the engine pointed straight ahead and the rudder pointed straight ahead and the plane properly trimmed out and flying at a correct lap speed, you will have *much* more pull on the lines than you did on the ground with the engine pointed straight out. Tilting the engine just slightly out isn't going to add much of anything, but it will drag, and that will translate to yaw, and loss of efficiency, and loss of speed overhead, and loss of overhead line tension.

Just remember, just because you have proper lap speed while flying straight and level, when the airplane goes up and down that speed can change, which means tension can change. And the place where the loss will be felt at its greatest will be directly overhead where maintaining lap speed is most important and gravity is being felt at its worst.

>Now, why is it that extra 1/4" pitch in
>the prop tips helps overhead tension? Steve

I touched on this on another forum, but the subject got way off the original question (prop pitching). This does relate to overhead tension, however, because in your first answer you mentioned loss of thrust. Anything that contributes to loss of thrust (engine slowing, added drag--yaw, inefficient prop) will contribute to loss of overhead tension which is where any loss of thrust is felt at its greatest.

My feeling on why it helps to add a bit of pitch (1/4 inch, whatever) to the tips, is that our props flex. *All* of our props flex. Grab the tip of the prop on a Piper Cub or Cessna next time you are out at your local airport and see what happens. Can't twist it? Neither can I. Now do the same thing with the prop at the end of your "Super Sod Buster". Plastic props are worst, but wood props, carbon fiber props, all will twist to some degree with the simple pressure applied between a finger and a thumb. (Carbon fiber props are stiffer, but they are, also, made thinner for efficiency and will flex.)

Now, a little calculation shows that most props are traveling at around (or even beyond) 400 miles per hour at the tips. So, take your favorite prop and hold it out the window next time your are driving 400 miles per hour down the highway and look carefully to see if that prop tip twists under the pressure of the "wind". Don't drive that fast? Well, imagine what it would do. It has *got* to twist. Even if it is only 1/4 inch, it has *got* to twist. Anything that can be flexed on the ground can certainly be flexed under load at 400 miles per hour.

So, since thrust efficiency is most important overhead, it pays us not to throw any away. If the prop is going to flex at the tip, give it a little more pitch at the tip while it is not turning so under load it will even itself out.

Now, empirical testing: We took three identical 13 inch 3-blade props, and pitched them so that the straight and level lap speed was the same at the same launch rpm on the same plane (PA 61 and pipe) no other trim changes. One had a little less pitch at the tips, one had even pitch all across the blade, one had just slightly (maybe 1/4 inch) more pitch at the tips. There may have been a slight difference in pitch elsewhere to compensate, but lap speeds were the same with the same launch speeds.

The first one fell out of the hourglass. That was the first one we started with, but it fell out of the hourglass. The second one worked, but wasn't the greatest. The third one (with the slight increase in pitch at the tips) had the best overall tension, both in level flight and overhead. It was working at the best efficiency, maintained that efficiency throughout the pattern, and kept good tension overhead.

I think the pipe ship with its lower pitch and higher rpm will notice this effect more, but the results should be the same, to some degree, at least, on any set up. But I am convinced.

Oh, yes, Matt learned another technique that helps that hourglass when you don't have tension, but I will leave that for another thread.

Leonard --
>Oh, yes, Matt learned another technique
>that helps that hourglass when
>you don't have tension, but
>I will leave that for
>another thread.
>

You can't get away with comments like that in this forum. Explain!

Curt --

Let's do a mind experiment. Assume the plane is directly overhead and at that point you hop on a motorcycle and instantly match the speed of the airplane. The plan is to drive down a highway with the airplane directly overhead of you at all times.

The airplane has now changed. The fuselage is now the "wing" that provides line tension. The elevator is now a rudder. The rudder is now an elevator that can increase line tension. That gives you one model (yaw angle, etc.) for line tension. This is the aerodynamic model.

The second model is the "rock on a string" model which says that aerodynamics don't come into play when we stand in the middle of the circle (no motorcycle). In this mind experiment, we use prop pitch and engine size to maintain speed to keep the rock out there with some line tension.

In the real world, I don't know the answer.

The "Rock on a String" doesn't fly. I don't care who the person is who says so. Certainly a rock on a string will provide a certain amount of tension, and this is why it is important to maintain lap speed as close to constant all over the hemisphere, but, in truth, we are flying. I have flown planes that had good lap speed and no tension. I have flown planes that had the same or slower lap speed but great tension.

Aerodynamics are going to be a factor, and we must work with them. True, if we slow it down too much, the rock will quit spinning at the end of the string. But I can put an RC plane at the end of the string and fly it at any speed I want, and so long as it maintains adequate air speed to continue to fly, I can fly it in such a way as to maintain good line tension.

We can't change things (as much) in flight as the RC guys can, but we can make sure we have designed and trimmed the model (including the prop) in such a way as to maintain good line tension throughout the hemisphere.

Fortunately for your motorcycle illustration, we don't fly the plane constantly straight overhead. It passes through the point straight overhead and continues to fly beyond where other factors continue to enter the equation.

Leonard --

We agree double aught decimal perfect!

The Ukie model straight overhead, controlled from a motorcycle going the same speed will have serious problems. Line drag much increased (all the way to the handle, instead of progressive toward the model), no arch, no centrifugal force. But it may be possible with yaw, lots and lots of power and a wide "lifting body". A .60 sized profile GeeBee R-2 with a hot .90 would probably do it. Are you riding solo, or gonna ride on the back?
Teehee! And I'm not at all sure I agree with Leonard's prop explaination, but certainly believe the 3 prop test program results. I also recall an article in SN about how the tip shape affects various things. Man, there's a lot of stuff to keep track of. I sure don't want to rediscover all this data the hard way. I just won't live that long. SN is great. Steve

The second Twister I built was stock except for adjustable leadouts, weight box and working rudder. Engine was a Fox Stunt 35, no offset, with ST NVA, tongue muffler, uniflow tank and dresssed up 11 x 5W Zinger. I never got the rudder working right and finally abandoned it. It was straight at neutral and went out some with up and more with down. It always had too much throw and in squares the airplane would yaw out on each corner, then come back to tangent. It was actually kind of interesting to watch. However, the airplane had fantastic tension on the overheads. I could feel the tension drop off when I made the intersections and the controls passed through neutral.

Obviously, with a small amount of control input, the rudder was yawing the airplane out a little, and possibly the Twister body, which is fairly deep, was giving me some aerodynamic lift. After I fixed the rudder at neutral, the tension in the overheads was still adequate. I fly my overheads with my arm fully extended, by the by.