Can we have too much stability?

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Pops

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My Falconar had large control surface gaps , ( built to plans). I had a slightly heavy left wing. Install a fixed trim tab on the aileron to fix. Few years later I added gap seals on all the control surfaces and it was a different airplane. The heavy ailerons went away, now almost as good as an RV-4. Also now the right wing was heavy. Took the trim tab off and it didn't have a heavy wing. Rudder and elevator seem about the same. The big surprise was it picked up 17 mph in cruise.
 

Eugene

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My Falconar had large control surface gaps , ( built to plans). I had a slightly heavy left wing. Install a fixed trim tab on the aileron to fix. Few years later I added gap seals on all the control surfaces and it was a different airplane. The heavy ailerons went away, now almost as good as an RV-4. Also now the right wing was heavy. Took the trim tab off and it didn't have a heavy wing. Rudder and elevator seem about the same. The big surprise was it picked up 17 mph in cruise.
Those changes you describing have exactly the same affect with passenger or not? Passenger adds weight and moves CG forward
 

Eugene

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Try adding some rudder to increase roll rate. The faster you push the stick side to side the faster to add rudder at same time coordinated.
Sure, I understand, but I can rock my wings pretty quickly solo without rudder. Why is it changing so dramatically if you add weight and move in CG forward? How is small tail affecting roll rate?
 

BBerson

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I think more weight requires more angle of attack so you have less reserve angle of attack to make a roll with the ailerons.
At some angle of attack the ailerons might not work at all. The forward cg adds trim drag and that drags down speed also requiring more angle of attack and less reserve for roll. It takes more force to get a heavier mass moving or stopping in roll.
 

Eugene

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I'm not too sure about the thrust line being better pointed higher/blowing on tail. You could be right but I wonder about thrust induced pitch changes.
Propeller downwash will be somewhat in line with wing down wash. New tail will be a little longer and a little bit higher. So, horizontal stabilizer will be somewhat in line with prop blast. Higher RPM will only increase velocity across stabilizer.

This way nobody is really will be blowing from up above onto something down below at any kind of crazy angles.

tempImageARlCg7.png
 

BBerson

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I think the biggest defect in the Skyboy is the tail boom angle makes it very high under the prop so the engine is very high. The tail boom and engine could be much lower if the tail boom was curved like the CGS Hawk
 

Aesquire

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Like the sketch!

I'm very confused, and apparently ignorant on the subject of thrust lines & stability & efficiency.

Logically the thrust line should be horizontal at cruise, right?

Possibly offset in 2 planes for torque correction, which would only be perfect at one rpm/loading combination.

So why the angle on the Flyboy? Can someone explain that please?
 

Eugene

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Like the sketch!

I'm very confused, and apparently ignorant on the subject of thrust lines & stability & efficiency.

Logically the thrust line should be horizontal at cruise, right?

Possibly offset in 2 planes for torque correction, which would only be perfect at one rpm/loading combination.

So why the angle on the Flyboy? Can someone explain that please?
IMG_5600 2.jpeg
What you see in this picture is how test pilots made conclusion for the best engine angle. Which is 3° down blowing into the tail. Apparently they describe this as self trimming affect. More power will get more down force on horizontal tail.

However manufacturer quickly discovered that they have all kinds of elevator vibrations with this installation and decided to flip engine up by 3° versus down by 3°. As a result vibrations partially went away and self trimming effect at the same time.

In cruise if you remove power abruptly your nose will go straight up because horizontal stabilizer has 5.7° incidence and naturally wanna be level at 90 miles an hour
 

wsimpso1

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Logically the thrust line should be horizontal at cruise, right?
Nope. You want to minimize losses in converting engine power to accelerating flow through the prop. Yes, ideally, having the thrust lined up with the direction of travel would be ideal, but it turns out that the losses due to making the air go skew wise through the prop is a far bigger loss.

Let's think about misaligned thrust first. If the prop makes 500 pounds aligned ideally it makes 500 pound shoving the airplane forward. Misalign it 5 degrees, the cosine of zero degrees is unity (1.0 with as many zeros as you want), cosine of 5 degrees is 0.9962, you now have 498.1 pounds of thrust. That is so small you will have difficulty measuring the difference...

Now let's look at the prop angle to the air. Imagine the prop blades as airfoils as they goes around with the rotation axis several degrees up (or down). Let's also imagine the blade average angle is at best L/D, which is also pretty far up on the lift vs AOA line. The blade angle of attack goes quite a bit flatter at one point, with lower L/D there. Opposite the first point, it can go into stall with awful L/D. In between these two extremes, the foils is approaching ideal AO. So, the blades spend a bunch of the rotation away from ideal L/D and that costs you thrust. But there's more. Because the misaligned prop is constantly and rapidly cycling through these extremes, the air flow over the blades is constantly in transition, and never gets to settle down around the ideal situation. The combination makes for big losses. 10% is easy to imagine, 25% is possible.

You want the prop aligned with the airflow that would happen through it if the prop were not there. At what speed? Depends on where you want the most effectiveness. For most of us it is cruise but for a STOL contest airplane it might be landing and takeoff airspeed and AOA, and for an air racer it might be a race speed.

Billski
 

Aesquire

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So, because the Skyboy prop is positioned in the down flow behind the wing, it's most efficient adding to the wing pitching moment, requiring a bigger tail as thrust increases?
 

wsimpso1

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Starting with Eugene's question and then Aesquire's, which is really a subset of Eugene's, I will try to give a satisfying answer, which can be difficult to achieve.

The first thing to remember is that when we pass an airplane through the air that is making lift, we mess with a big chunk of air. We need some concepts:
  • Frame of reference is based on the wing (we will add the fuselage and tail later);
  • The atmosphere is trying to travel straight aft along the airplane, and that air has momentum;
  • The passage of the air over the airplane has drag and is the net momentum change in the direction the airplane is pointed;
  • Lift is the net momentum change perpendicular to (normal to) drag;
  • The effect the airplane has on the passing air does become smaller and smaller as you get further away from the airplane;
  • Now imagine a way to make it simpler - a chunk of air that is all affected the same amount that produces the same lift, drag, etc. That chunk is a circular cylinder one wingspan in diameter. That is a bunch of air...
Got that last part? The equivalent disturbance is one wingspan in diameter. Yeah, and airplane with lousy tip design might be a little smaller, with good design a little bigger, with winglets a little bigger again, but all about one wing span in diameter. And that cylinder is based on the wing, starting a little below ahead of the wing sweeping up a little to the wing and then sweeping down quite a bit as downwash behind. A wingspan in diameter.

Now we can add the fuselage which also pushes this cylinder out a little bit, further adds drag, and usually does not add substantial lift. How little, well if the wing is 30 feet, it adds less than a foot. Then we stick in the horizontal tail - It is a little wing pulling down, so it takes a cylinder of air one tail span in diameter from within the big cylinder and tries to divert it upwards, slowing it too. And the vertical tail does the same thing laterally.

So now we have this huge amount of air changing directions once for the wing and again for the tailplanes, while getting slowed down a little by everything. This picture I just tried to put in your head is with a well shaped wing and tail planes and a well shaped fuselage that does not interfere with the wing and tail.

To make up for the drag of this arrangement we could angle our line of flight down, and take stored energy out as fast as the drag times airspeed uses it up. We eventually will reach the ground this way, so instead we stick an engine with a prop on it, to accelerate a small cylinder of air aft to make up for the drag. So now we have another chunk of air being diverted, this one sped up as it goes by. Cool. Now we have a nearly perfect airplane that some folks like to think we can approach. LOL.

A perfect fuselage would not be pushing of pulling on the air that is flowing over the wing. Straight vertical fuselage walls through the wing do pretty good at this. Do we have that here? Nope the fuselage expands from nothing to about 10 ft^2 just ahead of the wing and then tapers back down to far less than 1 ft^2 at the prop disc. So the wing is trying to shove down on all that air to make lift WHILE the fuselage is getting smaller and trying to pull air towards the vertical plane on the centerline ALSO WHILE the prop is trying to speed it up. This churns up the air some more and dumps another increment of slowing down to the air around the airplane - more drag.

What else is going on? Hmm, the very middle of the wing is sort of chopped out and the engine, its accessories, oil tank, HX's and other distinctly un-aerodynamic crap is plugged in. And this is on the centerline where the change in pressure from moving all this air is supposed to occur. This dumps another big dose of disturbance to the air trying to go aft and be shoved down by the wing, and the prop is in that chopped up flow trying to turn power into lift pointed forward. Less lift, more drag, etc.

As if all of the rotten deal being handed to the wing and prop were not bad enough, the Skyboy designer pointed the prop centerline up as it ran aft, so the prop is not just trying to pull air along the wing and fuselage, it is changing the direction that air is going, driving it up a little. Hmmm. Equal and opposite reactions, and all. For every 100 lbs of thrust the prop is making, a 3 degree nose down push angle will make 5.2 pounds more lift that the wing has to carry while it makes less thrust than it could have because the prop blade AOA is varying as it goes around. And this air being lifted by the angled up and back prop is being pulled off of the wing that is trying to divert air down.

Now let's go to the tailplanes. The air flows back and and has to flow around the horizontal tail plane, which is also diverting the flow upwards a bit through here. The vertical tailplane is doing a similar thing. And the Skyboy has them coinciding, so the flows from each are interfering with the other. Now let's make it worse. Let's make the horizontal tail small enough that the incidence is big and the elevator is almost stalled too. And the flow getting to most of the tail is chopped up and slowed by the mess ahead of it. These effects are reduced a bunch by offsetting the vertical and horizontal tails. Like on the A-36/P-51...It works.

As if that were not bad enough, the tail boom on this bird is moving up and down inches in flight, changing the incidence of the tail as it does so, further degrading the stabilizing effect of the tail, and probably forced the up angled engine and prop.

Now none of these many disturbances of the air flowing around the airplane happen in a vacuum. Nope, you can not actually just divert the prop flow from the wing downwash, they all are messing with each other, which is called interactions. Interactions frequently are more important to understand that primary effects. We might like to think that a situation looks like:

Y = A*X1 + B*X2 + C*X3

When in reality it looks more like:

Y = A*X1 + B*X2 + C*X3 + D*X1*X2 + E*X2*X3 + F*X1*X2*X3 + G*X1^2....

All of these nasties combine against you. So, pointing the prop along the flow off the wing will help, but it will help more if the engine is cowled, and will help more if the tail is enlarged and supported securely, and so on.

What do I think will make the Skyboy a much better airplane?
  • The tail does need to be bigger/further aft to control the airplane without stalling the elevator;
  • The engine needs to be adjusted to align the prop to run along the airflow of the wing;
  • The aft fuselage needs to be stiffer and stronger:
    • To support the more effective tail;
    • To stand the wash from the properly aligned prop on the tail planes;
  • The engine needs to be cowled;
    • So that the wing can operate over its full length instead of having the middle chopped up badly;
    • So that flow through the HX's and then over the engine is reduced for lower drag while the rest of the air flows cleanly over the outside;
  • The fuselage may need some vortex generators to make the flow follow into the prop.
Billski
 

Eugene

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Reading this explanation from Bill, reminds me of reading about interference drag. Sort of same logic. Two perfectly shaped pieces flying apart not generating much drag. But if you put them together drag could be enormous.
 

wsimpso1

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So, because the Skyboy prop is positioned in the down flow behind the wing, it's most efficient adding to the wing pitching moment, requiring a bigger tail as thrust increases?
You are doing one thing at a time instead of optimizing the whole airplane...

The prop IS most efficient when it does not change the direction of the flow going through it, and the effect of off axis is large.

Now how do we add or subtract from pitching moment, and how much does it add to tail load and lift? The short answer is not much...Let's grab some theoretical numbers. Let's say the prop is 12" above the CG and 20" behind it. 80 hp at 75% power and 85% efficiency and 90 mph is about 212 pounds of thrust. Let's say that is with the thrust line parallel to the direction of travel. At zero angle, that is 2544 in-lb of nose down pitching moment.

Now let's try out 3 degrees tilted up as it goes aft. First, the 212 pounds becomes about 190# as the prop efficiency drops. 190#*(12"*cosine(3) - 20"*sine(3)) = 190*(11.9836-1.0467) = -2086 in-lb.

Now 3 degrees tilted down and prop efficiency goes up. 230*(11.9936+1.0467) = -3041 in-lb. Yes it is bigger, but how much in the big scheme?

Typical Cm of -0.06, MAC is 4.62', S is 138 ft^2, Wing pitching moment is 790 foot-lb = -9500 in-lb.
Airplane static margin of 15% MAC is 8.3" times weight of 1320 lb is -11,000 in-lb

Total pitching moment the tail has to react is thus a high of -22586 in-lb and a low of -24541 in-lb. 8% more pitching moment to react with the tail to get 20% more real thrust to move the plane sounds like a pretty good trade to me. YMMV.

Then comes the second topic - the wing works with lowest drag for a given amount of lift if the wing's airflow is not messed with. Changing the direction of the air over the wing with the prop will rob the wing of some lift and add drag. So, you point the prop perpendicular to the air flow, get best thrust from the prop, get lowest drag from the wing, and ask for a few percent more balancing moment from the tail. If you also increase the tail area and arm, this comes at about the same or maybe even less drag than the current tail does, and it is a net win all the way around...

Billski
 

wsimpso1

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Reading this explanation from Bill, reminds me of reading about interference drag. Sort of same logic. Two perfectly shaped pieces flying apart not generating much drag. But if you put them together drag could be enormous.
Except that interference drag normally only talks about one thing messing with the wing. The Skyboy has about five things all combining to mess up the wing, three things messing up the prop and a couple of those also messing up the tail. There are so many things combining to make big drag and poor behavior.

Billski
 
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