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Your Nose Is Too Long!
Your Prop Is Too Small!
The Barbell Effect
The Fly Swatter Effect
YOUR NOSE IS TOO LONG!
Whoops, not you! Sorry about that. I meant your airplane. A lot of airplanes flying today are nose heavy. And it usually happens because a larger or heavier engine was put on the plane than what it was originally designed for. If you know ahead of time that this is going to be the case, or if it even looks like the nose is going to be too long for the airplane (because you are going to build the tail lighter), dare to be different! Shorten the nose. (If you are building a "Classic" or "Old Time" aiplane for competition, then you may be stuck with the nose length. But in that case, make sure that you use an engine that is as light as the one originally used on the design or be prepared to add tail weight for balance.)
The length ofthe nose is for balance only. That's it. Period! There is no such thing as certain "numbers" that must be adhered to concerning nose length, as if a certain nose length is needed with a certain wing area or to offset a certain tail moment. I have seen articles where the writer says he measured a number of airplanes and "averaged" the "numbers" to come up with, among other things, the nose and tail moment he used on his airplane. And he wrote the article in an almost sacred way, as if a certain nose length was mandated for the airplane to fly the pattern properly.
I'll say it again: The length of the nose is for balance only.. That's it. The only reason to use a long nose on a stunt airplane is if you have a light engine and a long or heavy tail. Did you ever hear someone say, "Keep the tail light"? Well, it is true. Keep the tail light and you can make the nose shorter, and save weight all around.
So, let me state it in another way. When building a plane by the "numbers", there is no such thing as "numbers" for the nose length. If you are following a plan for a plane that someone else has designed and built, the only reason to build your airplane with the same nose length is if the original plane came out at the proper center of gravity AND you are going to use the same weight engine and the same construction methods. If you use a heavier engine, shorten the nose. If you are going to use a lighter engine or build a heavier tail (? Maybe your selection of wood isn't as good, or you feel your finish may be heavier), then, and only then should you lengthen the nose to maintain the proper balance point. On most designs you will probably find that the nose is long enough already. Maybe too long. Have you checked your nose lately? Your nose may be too long.
HOW DO YOU KNOW IF YOUR NOSE IS TOO LONG (or the plane is nose heavy?)
First off, if your plane is too heavy for its size or power, adding more weight in the tail isn't going to turn a dog into a pussycat. But if it has a reasonable wing loading, and is properly proportioned otherwise, try adding a little tail weight (like a half an ounce at a time) if:
1. The plane doesn't turn as tightly as you would like. Or
2. It needs too much control input to make it turn.
Remember, when you change the center of gravity will also need to change the lead-out location and maybe even the amount of tip weight. And, as you change the sensitivity of the airplane, you may need to narrow your handle spacing as well.
The CG location on the plans is a good starting point. But, as with everything else, you want to adjust it for what works best for you.
If you look at the picture of the Stuka97 on this web site you will notice that the plane has a very short nose compared to many designs of the present day. And the balance point came out near perfect with its OS 35 FP engine and tube type muffler (and a 45 ounce total weight airplane). After we trimmed the plane and narrowed the handle spacing and noticed how easy it was to get snappy square corners on this design, we measured the maximum control we were giving the airplane and found it to be about 15 degrees maximum in either direction for both flap and elevator...and that was for the squares. Rounds were less. Fifteen degrees up and fifteen degrees down would snap the corner because the nose was short and the center of gravity was set far enough back (in this case 3 1/2 inches behind the leading edge of the wing) to allow the plane to turn. But this has not affected the stability of the plane to fly straight and level.
There are a number of other factors to consider here, but if you find that adding tail weight improves the ability of the airplane to turn tightly without hurting either its stability in the rounds, or the glide after the engine quits, or the landing (and all three of these can be affected by getting the weight too far back) next time don't add tail weight. Try shortening the nose.
(The Stuka97 was the fourth build of this design of airplane. Although areas and tail moments always remained the same, each successive fuselage was built with a shorter nose than its predecessor, until we finally reached the point where we didn't have to add tail weight.)
YOUR PROP IS TOO SMALL
There are a number of "old wives tales" that continue to float around the stunt circles. You have heard most of them. You may have even believed some of them. (Sometimes it is hard to separate truth from fiction.) But one of these is that with a .35 size engine you need to think in terms of a 10 inch prop. Another is that with a large prop the airplane won't turn a sharp corner. If that is what you think, the truth be known, YOUR PROP MAY BE TOO SMALL!
I grew up in a small town in Iowa. This was in the days before television (well, not EVERYONE had one yet). But we never complained that there was nothing to do. A number of us kids built and flew model airplanes. And a bunch of the towns people would come out on Sunday afternoon and fill the bleachers at the local high school football field to watch us fly. (Yes, they even let us fly there.) This was in the days when the Fox 35 and Veco 35 and K & B (Greenhead) 35 were king! I had a Veco and a couple of K & B 's, and we flew them with 10-6 nylon props and the engines screaming! We didn't measure lap times then, but the planes were probably doing 4 second lap times on 60 foot lines and we thought that was the way they were supposed to fly! I still have a couple of those planes and engines. But we don't fly our planes that way anymore.
One thing we have learned is that the size prop you use on the engine can make all the difference in the world. And it is not just pitch. It is diameter. Pitch tells you how fast the air moves through the prop (or the prop through the air...in effect, the speed of the plane.) Diameter tells you how much air is moved by that prop at that speed. In other words, diameter rules when it comes to thrust. A speed plane needs a higher pitch and smaller diameter (allowing for the higher pitch) prop. The diameter, in this case, is just enough to develop the thrust needed to propel the plane. The pitch, in this case, is as much as you can get to develop the speed with that engine and that diameter of prop. (That is oversimplified, I know, but this is a small web page.)
For a stunt plane, though, we want thrust, not speed. We want enough pitch to develop the speed--just enough--for the lap times that we need to get the best combination of lift and tension and maneuverability (and to stay within the reaction time of us older geezers). And then we want as much thrust as we can develop with the engine/plane combination. And thrust is a product of diameter. Now again, there are other things involved, but the amount of thrust that an engine produces, with everything else remaining the same, is directly proportional to the square of the diameter of the prop. That means that if everything else is equal (such as power and speed), a 12 inch prop will have 44% more thrust than a 10 inch prop. Even going from a 10 inch to a 10 1/4 inch prop, if that is all that your engine is capable of turning, will make a noticeable difference in the amount of thrust that your engine will be capable of producing.
The moral here is to try different props to see what works best with your engine. Try different makes--not all makes are equal. Try different blade shapes. Try narrowing the blades or thinning the blades to see if your engine will then be capable of turning a larger diameter prop than what it was turning before. Don't be afraid to experiment. When it comes to trimming (and trying different props is a part of trimming) you are never done until you fly your last flight with the plane.
Oh, yes, about that part where someone says "the plane won't turn with a larger prop."... Probably their nose was too long. The answer to regaining the maneuverability of the airplane when using a larger prop, can be as simple as moving the center of gravity back. It doesn't have to go back far, because the prop doesn't affect it that much. Just a little. Move your center of gravity back, and any turn ability lost can be immediately regained.. And if that DOESN'T solve the problem (or the center of gravity is as far back as you feel that you should move it), widen the handle spacing a little. Again, a little is all it takes.
The problem of turn stems from the gyroscopic tendency of the mass of the turning prop to want to stay put where it is. The heavier the mass and the faster it turns, the more it will want to stay planted. (For those who like to turn smaller props at higher speeds, as with piped engines, remember that part about higher speeds. Small props turning faster can have the same effect.) But it also works to advantage in keeping the plane flying straight and level. One offsets the other. And what little tendency there is to decrease the turn of the plane can be easily offset by slightly moving back the center of gravity of the plane or widening the spacing on the handle. Just remember, when you lengthen the prop, you may want to shorten the nose.
THE BARBELL EFFECT
A friend of mine who is an engineer told me that the proper term for what I am about to write about right now is "the polar moment of intertia." Yeah. Uh huh! I'll still call it the "Barbell Effect", OK? (That's the advantage of having your own web page. You can call it what you want.) The key point here is it is not just how much you weigh, but where you put the weight that counts. We men seem to put on all our weight around the middle ,and then we say, "I am not too fat, I am just too short." And it might be true. We treat our planes in the same way. We say, "They are not too heavy. They just don't have enough wing area." And if we had built them with a bigger wing area, would they have weighed the same amount? Probably not. Well, there are some modelers who always seem to build their planes light, but if you happen to be one who always seems to builld heavy, then read on.
The first question to ask if you have a heavier airplane is, "Can the engine carry the weight?" Yes? Then, "Does the plane maneuver well, and corner well?" If not, is it too heavy for the wing area? Would adding more wing area be the answer? Is there some way that we can remove some weight? Hey, wait a minute, maybe it is not the amount of weight that is at fault at all. Maybe it is where we put that weight in the airplane. Maybe we have fallen prey to the "Barbell Effect"
Have you ever noticed how some airplanes with a certain wing area and a certain weight fly well, but others of the same weight and wing area don't? Maybe the problem isn't the weight of the airplane at all, but where we put it. If only we could build our plane like a man--keep the weight in the middle--then maybe we wouldn't have a problem.
One of our key problems in our building today is in our power plants. Power is addictive, and as people have gone for more powerful engines, they have also gone for heavier engines. More powerful engines need bigger tanks, meaning longer noses, and then we have needed to add a longer tail moment for balance and what we have ended up with is a barbell. Take a yardstick and place about 8 ounces of clay in the very center and then try to twirl it and stop it. Even with that extra weight, it is fairly easy to start and fairly easy to stop because it is light at the ends. Now take the clay from the center of the yard stick and move half of it to one end of the stick and half of it to the other end and try to twirl and stop it again. The total weight of the yardstick remains the same, but you will notice that it has become harder to start twirling it, and harder to stop the twirling once it is started. What you have done is made a barbell. You have put the weight at the end of the stick, and the heavier the weight at the end of the stick, the harder it is to overcome the built in inertia and start a turn...or to stop a turn once it has started. Barbells just don't turn as well as sticks with the weight more evenly distributed. Airplanes with the greater weight at the ends have the same problem.
When Al Rabe built his "Mustunt" design, he built a 40 ounce plane with a thick airfoil that he flew with a Fox .35. Then, realizing that most people wouldn't build as light as he did, he added 16 ounces of weight to the CG and said the plane still flew great but that he burned up a perfectly good Fox .35 trying to fly it. His conclusion: "If you tend to build heavy, put on a bigger engine." So a lot of people built heavier "Mustunts" with bigger engines and they didn't fly as well. Al forgot about the barbell effect. When he added the weight to the center of gravity, he didn't affect the turn. But when bigger (and heavier) engines were added to the nose and heavier constructon methods were added to the tail, the "barbell effect" struck. Although they didn't weigh any more than Al's 56 ounce added weight "Mustunt", they just didn't fly as well.
The moral here is, if you are going to build heavy, keep the weight in the middle. If you are going to use a heavy engine, keep the nose short. It is not just how much you weigh, but where you carry the weight that counts! For men chasing girls, or men flying planes, it is all the same. Be conscious of the "barbell effect."
THE FLY SWATTER EFFECT
Did you ever wad up a newspaper and swat a fly only to hit the spot where the fly was sitting and miss? Well, maybe the fly didn't fly all that fast. Maybe you "blew it away". Maybe the problem was the "flyswatter effect".
The first flyswatters were broad, flat, "whatever" types of objects that didn't have holes in them. Then someone came up with the flyswatter design that all of us older folks still remember buying at the local hardware store that was made of window screen. It was made with a wire mesh that had all sorts of holes in it--and it worked!. The typical flyswatter we find today is made of plastic. But it is still full of all sorts of little holes.
Why, do you suppose, do they put all those holes in the flyswatter? No, it isn't to save on plastic. Plastic isn't that expensive. It's to let the air escape. Not only are you then able to swing the flyswatter faster, but, without the holes, the air would blow around the flyswatter, and actually blow the fly away before you hit it. Try swinging a flyswatter. try it first with all its holes open. Then tape some paper around it to cover all the holes and try it again. Once the holes are covered, the longer the handle and the faster you swing it, the more force it takes to swing. Or, put another way, the more pressure builds up as the air, which compresses ahead of it, seeks a way around it.
Now let's look at our airplane. Look especially at the tail. That stabilizer, attached to the fuselage, looks suspiciously like our flyswatter...only without all of the holes. And every time we "bat it around" and then neutralize our controls, it acts just like our flyswatter with the holes plugged up and slowing it down.
Now I am not suggesting that we punch holes in our stabilizer or create huge gaps between the stabilizer and elevator in order to allow the air to get by. That stabilizer is there to do just what it is called--to stabilize the flight of the airplane. And we need to fit the gap between the elevator and stabilizer as closely as possible or even to seal the hingeline with tape to make the elevator as efficient as possible. In some ways we actually want the stabilizer and elevator to act just like a flyswatter without the holes! The purpose of this whole article is to help us understand what is happening here.
Let's take that flyswatter with the holes plugged up and try something else. I want you to hold it out the window of your car as you go driving down the highway at 50 miles an hour. First, hold it as close to the "swatter" area as possible with the flat area to the "wind." You can now see and feel the effect of a short tail moment on your airplane. Now hold that same fly swatter by the very end of the handle and stick it out the window the same way. Suddenly it is hard to hold, isn't it? You can now see the advantage that "leverage" has on the fly swatter.
The stabilizer works because it is primarily a fly swatter. It is not a lifting body as much as it works to act like fins on a dart (although, in actual practice, for stability sake, we put the center of gravity ahead of the center of lift and then balance the plane by adding a slight down force to the stabilizer. Flaps down for lift, elevator up for stability.) If we wanted the airplane to be able to move as quickly as possible, then we should shorten the tail moment and decrease the stabilizer area--you know, that flat part that doesn't move. But if we did this, we would end up darting around like a combat plane. In CL stunt we want stability with our models as well as the ability to turn a quick corner. Stability comes with longer tail moments and larger stabilizer/elevator combinations. Quickness comes with shorter tail moments and smaller "flat" areas (the part that doesn't move). But each comes with its own compromise. Less "fly swatter" might help in increasing the turn, but it is the total area of the stabilizer and elevator that is needed (and used) for stability.
Even the wing can act like a flyswatter at times. The hardest corners to turn are the bottom corners of the squares and the triangles and the hourglass. With any corner that you turn, you have the flying moments and the thrust of the engine working together to turn the plane and send it off in another direction, and you have inertia trying to keep the airplane moving in the direction in which it was traveling in the first place. But in those bottom corners, we have both inertia and gravity combining to keep pulling the plane down, even though the flying forces have turned it and the enginen thrust is trying to move it in another direction. But once we have the airplane turned, we have the biggest flyswatter of all--we have got the wing reacting against the crush of the air, holding it back while the new forces take over.
So a certain amount of tail area is needed--not only to turn the plane, but to stop the turn. And a certain amount of wing area is needed--not only to provide "lift", but to counter the forces of inertia and gravity in a turn. This is why many designers will refer to the "numbers" that are being used in their design. The tail moment is computed as a percentage of wing span or area. The tail area is designed as a percentage of wing area. Nothing, however, is perfect. Everything is a compromise. And if you look at a number of designs you will see that one designer may prefer a certain "something" over something else. But compare the numbers and you will find that all well thought out modern designs are in pretty much the same ball park. Deviate from it too much and you will lose stability, or you will lose turn.
Remember the "flyswatter effect" when you design your airplane. It is both your enemy (in the turn) and your friend (countering the effect of inertia). Although they may hurt the rate of turn, longer tail moments and larger tail areas help the stability of the airplane, and smoothness in the pattern does count.