Pitch Stability in Indoor Models

 

By Steve Gardner

As published in Indoor News and Views

---

 

Your model is up there near the rafters doing great! All you need is another two minutes and it is still all the way up there. You can't miss. You are still watching it very closely thought, because it is not the best model for bouncing around in the clutter up there. You enter the last minute that you need to win, and the model bumps something. It is slowed a bit too much and this lets the nose down ten or fifteen degrees. The model speeds up as it dives and it looses the nice tight turn that has kept it in the center of the building all this time. One of two horrible things happens now. The model flies straight for too long before it starts to circle again and it gets into a wall, or it continues the dive until the wing starts to twist which increases the dive angle and spirals the model to the floor. Fifteen seconds too early. Rats!

 

What went wrong? It was just a bump. It got away from whatever it hit cleanly with the nose down only a little. You own and have seen other models that would pop right back into their flight pattern without any problem after such a bump, but this model has a real problem with recovering from disturbed flight. Why doesn't it behave like the other ones? Can it be fixed?

 

 

 

Chart 1

 

 To start with we need to understand what went wrong. Why do some models pop their nose right back up after being disturbed and some do not? What makes a model "stable"? Look at the simple force diagram in figure 1. Imagine the balance point, or center of gravity as simply being the models weight. The wings have to hold this up for the model to fly. From the drawing you can see that the stab also helps hold up the weight, so there is lift from both the wing and the stab. When the model is in steady flight the lift from the wing and from the stab are balanced so that the weight is just supported and there is no tendency to raise of lower the models nose. The numbers indicating the lift of each surface are simply used to compare the proportions of lift from the wing and stab, and are not related to any real lift values. In this example the balanced lift condition happens when the wing's lift value is 1.27 times the stab's lift value. (the wing carries more of the weight than the stab). If this number goes up, the wing is then lifting more than its share of the weight and so the nose comes up. The larger the number, the faster the model pitches up. Looking at the second set of points on the chart marked B, BB we can see the lift numbers for the same model just after it has been disturbed and is diving as shown here:

 

 

The wing is now lifting 1.55 times the stab and this will pretty quickly raise the nose of this model. From the lift chart you we can see that the lower the angle the model is flying at, the larger the nose up tendency. This lower angle is not the dive angle itself, but a diving model will have a much lower angle of flight, it can get close to zero in very steep dives. This chart, Chart 1, is for a model with 4 degrees of decalage. Decalage is the angular difference between the wing and stab. It has nothing to do with the angle of incidence, which is simply the surface angles compared to the models centerline. When you trim your model out you adjust the wing or stab incidence to get thc model flying nice and nose high. Once you have the model trimmed out there will be a certain angle of decalage between the wing and the stab. In the next ;et of diagrams we show what happens when the decalage angle is too small.

 

 

The model in these diagrams has a decalage angle of 1.8 degrees, which is very small. This model will fly, well as long as it doesn't get too far from its trimmed speed and angle. In steady flight it has a wing to stab lift number ratio of 1.04. Watch what happens when the nose gets down for any reason.

 

 

The lift ratio now goes to l. 11, only .07 from the steady flight. The 4-degree decalage model had .28 difference between steady and diving flight, four times as much. This model may or may not get its nose up before its wings begin to warp from the speed. In any case it will end up much lower than the model with more decalage.

 

So, all we have to do is make our models with more decalage. Right? Mostly, but we have to figure out how to do this, and how much more we need, too. There is a drawback to decalage. The more you use, the less work the stab does. A model with none will fly with the wing and the stab at the most efficient angle for the most lift, and this will maximize endurance. This model will also have to be launched perfectly, and must not run into anything at all that might disturb it. It has no margin of stability at all and a gnat's wake will send it crashing. It will just not work at all. On the other hand a model with say, six degrees of decalage will be stable even outdoors in the wind, but it will just be draggling the stab along for the ride. An indoor endurance model can not afford to give this much efficiency way. To make matters a little more complicated yet we must remember that the tail boom of many indoor models is not perfectly ridged and so the decalage can change in flight.

 

 

Things to try

 

 Part of the problem with this "solution" is that we can not just make the decalage any amount we like. We  test fly our models and move the surfaces so as to make the model fly at what our experience says is the  best speed. Once the model is flying the way we feel it should then the decalage has been determined. If  we mess with the angles now it will make the model fly too fast or stall the model. Now we just fly the  model into the rafters to see if it will behave well or not. Let's say this one does poorly, are we really stuck  with a lemon? Not necessarily, here are some things to try.

 

1.      Move the wing back just a bit on the motor stick. This will effectively shift the center of gravity     forward and so the model will need a bit more decalage. Make this change in small amounts so that     you do not over do it. 

2.      Add a bit of down thrust to the model's nose bearing. This will also result in the model needing more     decalage at a small performance cost. A possible advantage is that the down thrust will help prevent     the model stalling during the initial climb phase, yet allow the decalage to be set so as to get the model     nice and nose high during the cruise portion of the flight. 

3.      Use a stiffer tail boom, If the tail boom of your model is a bit too flexible it will actually let some of     the decalage bend out of the model. Look at the model in cruise flight and make note of the upward     bend of the boom caused by the lift coming from the stab. Now watch the model just after it has     bumped something and is starting to dive a bit. If the bend in the boom stays much the same and the     model gets its nose up right away, fine. If on the other hand the bend relaxes a great deal and the     model dives for an extended period, or even speeds up and spirals in, you need a stiffer boom.

4.      Make the tail boom longer. I like this one. The longer tail boom gives any difference in the lift    between the wing and stab a greater lever arm to act through. A smaller amount of decalage will work    well enough if the tail boom is long enough. Remember number three though when you do this.

5.      Make the stab area larger. This lets the stab carry its share of the weight at a lower angle which means      less decalage. This fix is not too practical because most flyers are using the largest stab the rules allow      anyway. Just another reason to do so.

 

Why the model goes for the wall when it dives from a girder bump

 

One of the most aggravating things about bumping the ceiling is the model taking off for the wall. It will hit the girder or whatever and the nose will get down a bit and the speed will pick up some, then it will proceed to quit circling and fly straight for an extended time. lf you are flying in a small area this will make it necessary to steer the model if you can. If you are way up there in a large site you may just have to watch while your model leaves the sweet spot you launched it into and heads for trouble. What is happening here? Why does a model that flies happily with a circle of 40 or 50 feet decide to open up the turn when the speed gets up a bit? Can you stop it, or at least minimize the effect?

 

We are kind of in a fix with this one. The reason our models do this is related to how we must trim them to get the best duration while staying within the confines of a building. We need a fairly tight turning circle without a great deal of bank angle while flying very, very slowly. The adjustments we must use to get this work well only when the model is at or very near the trimmed speed. When the speed gets up above this certain level the adjustments we use start to work against us. Imagine a hang glider flying along in level flight. The pilot decides to turn to the right. How does he do this? He pushes his weight to the right to get that wing down. Our models fly with the left wing longer than the right. This is exactly like the hang glider pilot pushing his weight to the right. He gets a right turn for his action. What do we get? If the model is flying fairly slowly we get a nice left turn. What is the difference? We have the added complication of torque, the "P"-factor, thrust line effects, stab tilt effects, and mm radius effects. The torque tends to lower the left wing and if you think it is a small force you do not fly mini-sticks! The "P"-factor tends to yaw the model to the left and its strength is directly related to the amount the prop disk is tilted up when the model is flying. The thrust line also yaws the model to the left because that is the direction we point it. The same is true of the tilt of the stab. The last factor comes from the fact that a

 

 model with a mm radius of 20 feet and a span of 18" has a right wing flying about 7.5% faster than the left wing. This makes the right wing lift about 15.5% more per unit area than the left, causing a roll effect to the left. Whew! Complicated!

 

So what is going on with our model? The torque that is applied to the model is fairly constant, causing a left roll tendency. When the model is flying at the proper slow speed the nose is up and so the "P"-factor is helping turn the model left. The slower the flight the harder the prop pulls, so the effect of the thrust line is greatest then giving use more left turn. The effect of stab tilt is related to the lift the stab is giving, and from the previous diagrams that is highest when the model is flying at high angles of attack (slowly), this effect is to turn the model left. The turn radius effect is to roll the model to the leg. No wonder we need a longer left wing to hold that wing up! All that left stuff going on! So what happens to the model to make it dive straight or to the right? Imagine the model with the nose down and the speed up. The angle of attack is very low, so that the "P"-factor disappears. The stab tilt is also at its weakest point. The model is now flying faster' than the prop is pitched to go, so the thrust is way down and so is the effect of the thrust line. All this begins to open the turn up, and this removes the turn radius effect. What remains is the torque and the long left wing. If the wing were the only factor we would turn right just as the hang glider does, but torque helps us out now and we end up with sort of straight flight, unless the speed gets up any higher. If it does then look out! If we have done what we can to get the model to pop the nose back up then this set of effects will quickly return the model to the nice left turn. If we have a model that takes its time getting the nose up then the model will go wandering whenever it bumps anything. It will almost never wander into a better spot than you started in, so see if any of this stuff helps you get a better flying, more consistent model.

 

TURNING AND

ROLLING FORCES
ACTING ON AN
INDOOR MODEL