Can we have too much stability?

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wsimpso1

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Back to the original topic of this whole thread... Can we have too much stability?

Yes, it can happen in the Roll Axis. You can have too much stability in the roll axis, making the airplane difficult to fly precisely and land, with things like Dutch Roll becoming prominent. And we have found that we can fly airplanes with modest instability in roll just fine.

Not really in Pitch and Yaw Axes - Lets consider the four corners of this problem, and then the middle:
  • If tail volumes and static margins are small, we can have poor stability while control could go from inadequate to huge, but modest aft loading errors can render the airplane unflyable by a human pilot;
  • If tail volumes are small and static margins are big, we can have a lot of stability but have inadequate control, limiting maneuverability and CG ranges;
  • If tail volumes are large and static margins are small, we won't have much stability combined with lots of control, giving good maneuverability, but modest loading errors can render the airplane unflyable;
  • If tail volumes and static margins are both big, we can have a lot of stability, we can run out of control before getting to stall AOA;
  • If tail volumes and static margins are both adequately large, we can have plenty of stability and yet still get all of the control we want for lifting the nose to flare and stall while in ground effect (land the airplane under control) and otherwise manuever the airplane to stall;
The missing parts in the prior 100 posts is linking these issues together. Static margin when expressed in %MAC shows you how stiff the airplane is that axis. More static margin increases the moment that must be added to the airplane (via control surface deflection) to make it change pitch or yaw angle by a certain amount. Static margin comes from two things - tail volume and cg placement relative to neutral point.

Plenty of static margin and a too small tail will be distinctly unpleasant to fly. You might have not enough tail force available to lift the nose in ground effect. Sounds like the Skyboy at forward CG where Eugene is complaining about it being "filled with lead" (please, not "field with lead").

Make the tail volume somewhat bigger with more tail area and tail arm, and the neutral points shift aft. Prior aft limits will now be solidly stable, AND control moments available from elevator and rudder movements will be substantially increased. This comes at the price of somewhat increased drag from these tail planes and increased control gradients in elevator and rudder - in little airplanes that do not fly above 100 knots we rarely have too high forces in these axes, but we sure do in ailerons. Control harmony becomes the reason to think about aero balance in slow small airplanes. Grow to fast four seaters (think Bonanza/Navion/Mooney/Cirrus/RV10) and you need to think aero balance of all three control surface sets to keep forces in bounds and then tune for reasonable harmony.

One other thing is that if tail volume is increased, you tend to get more damping in that axis too. More damping may or may not be useful. Eugene has complained of the airplane doing a pitch oscillation - the airplane never really settles down in pitch. Some airplane designs already have pretty good damping, so adding more is not particularly beneficial in them.

Pitch damping goes with tail area times tail arm squared. More tail volume will give more damping too as long as you do not shorten the tail arm, and does appear to be advantageous in most airplanes.

Billski
 

BBerson

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The Skyboy has a larger than average cg shift with a passenger because because both seats are ahead of the cg. It needs a larger than average tail volume.
It would be less cg shift as a tandem, with the pilot out front and the passenger close to the cg.
Need to determine if "filled with lead" is overweight or low tail volume?
 

flitzerpilot

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That's a brilliant and comprehensive reply from Billski. I feel that Eugene is seeking a solution to a problem which may not be resolved by locating both pilot and passenger ahead of the CG and expecting similar and acceptable characteristics in both situations in an aeroplane of this size.

Unless I have been radian this thread wrongly.
 

wsimpso1

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The Skyboy has a larger than average cg shift with a passenger because because both seats are ahead of the cg. It needs a larger than average tail volume.
Completely agreed. More tail volume is the way to a better behaving airplane.

Need to determine if "filled with lead" is overweight or low tail volume?
I firmly believe that he has much more nose down moment than most, too small tail volume, very soft tail structure. You can improve the first correcting the other two.

I also believe that the uncowled engine and engine/prop angled up combine to make for badly messed up airflow over the wings and large trim changes with power adjustment. I firmly believe that a cowling over the engine combined with a stouter tail boom will allow pointing the prop along the wing's downwash, greatly improving prop efficiency while reducing drag. I thought we got over uncowled engines in the 1930's...

Between both of these improvements, the EugeneMachine (because it sure won't be a Skyboy anymore) should behave nicely, fly faster, and maybe even become fun to maneuver. And since Eugene is having so much fun doing it, more power to him. Many of us would take a different path, but his appears to be best for him.

Billski
 

BBerson

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Could stiffen the boom with two struts and a king post. Can't use a wire because the tail down load is in compression.
 

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Eugene

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Could stiffen the boom with two struts and a king post. Can't use a wire because the tail down load is in compression.
This kind of approach will maybe address tail tube deflection under balancing load, but there is so many more issues with this tail that will be still there.

I know that most of you guys feel that I am wasting my time with this airplane and I should started different project, better project long time ago. That is exactly how I feel about this tail. This whole tail needs to go and I need to start from scratch. I did enough experiments to prove to myself that there is nothing to be fixed anymore. Maybe this tail was OK for 50 hp ultralight at some point (otherwise they wouldn't be able to get German certification), but not anymore for 100 hp LSA.

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TFF

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I think keeping looking for reassurance is wasting time. Build it back and see. That is the only way to see if it works better than before. We want to see it built.
 

Eugene

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I think keeping looking for reassurance is wasting time. Build it back and see. That is the only way to see if it works better than before. We want to see it built.
Don't forget that 4 years ago I didn't know difference between angle of attack and angle of incidence. So, I needed to spend some time of convincing myself that people who build this airplane, educated aeronautical engineer's are wrong and I am just average heating and cooling guy, right.

Yes I want to see it build as well. All I need is time. Not love or money. Just time. Why did he give us only 24 hours in a day and make us to sleep 30% of that? That was terrible miscalculations on his part!
 

Tiger Tim

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Nobody asked me, but if I was going to make any change I would start with weight and balance. I would ask myself how close I was to the aft limit with just myself on board and then figure out what I could do to get it all the way there (like move the battery, change to lighter avionics, lighter nose wheel, or just start removing unnecessary stuff). Could I go really crazy and even move the passenger seat back a few inches? All of this I think would go a long way to helping to keep the CG in a better place with two on board.

If after test flying solo and two-up like that I was still not satisfied I would start looking at keeping the same tail volume but using a longer boom and smaller tail. The theory here is that the tail needs to create a moment which is force x distance, not simply a force. In fact, the longer tail would require less force which in turn would have your wing carrying slightly less load. Between that and the smaller wetted area of the new tail you could conceivably go a bit faster and get a better climb as a bonus.
 

Eugene

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I would start with weight and balance.
I did all this easy stuff already many times!!! I was convinced for a long time that something is wrong with me and not with how it's designed.

This tail is simply too small for this kind of arrangement. Vh = 0.34 and tail arm 2.2.

Real CG range when airplane behaves acceptably is very small. Yes you can do something with additional ballast and move it exactly to the right spot. And airplane will fly OK if you are right about at 30% MAC. But passenger seat is 6 inches in front of Leading Edge and absolutely nothing you can do to prevent your CG from sliding forward to 26 - 24% position. And that is already too much for this small tail to handle. Don't forget that this tail is operating outside of propeller discharge and needs to balance thrust moment from engine as well.
 

Eugene

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Need to determine if "filled with lead" is overweight or low tail volume?
I was thinking about it and I came up with more questions unfortunately.

- Will low tail volume make your airplane feel overweight, or should make airplane feel like only nose heavy?

- What makes airplane feel overweight? Isn't that supposed to be high wing loading only?

- Why in Cessna 172 if you upgrade to more powerful engine they let you carry more weight? I understand that with more power available, you will go faster and your wing will develop more lift. But during landing engine is idling anyway and not helping you at all. Your stall speed will go up with the weight.

- When I am flying Cessna with three big guys and I am overweight, and I certainly feel that way on my controls. The plane behaves completely different versus flying solo. If I magically in-flight upgrade my engine from 160 hp to 200 hp, will I instantly lose that overweight feeling?

- If feeling overweight comes from high wing loading, what is high and what is low? Skyboy solo 7.6 pounds per square foot. At LSA max 1320 lb wing loading is 9.5 pounds per square foot. But of course if this wing is not working efficiently, then you don't have your 138 ft.² anymore.

- How do we improve wing efficiency? I have hard leading edge installed but only first 10% or so. Will extending it to 20% make a big difference?

- Will replacing wing tips with winglets act as additional wing area?

- Ribs on my wings installed approximately 22 inches apart. Comparing to Aeronca ribs installed much closer about 14" or so. Does it make a big difference?
 

BBerson

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I don't think your Skyboy is overweight if it weighs less than the Chief. I think it has too much drag.
The way an airplane feels is about many things including the wing loading and power loading and drag ratio or L/D. The Skyboy L/D is 7 according the wiki.
 

wsimpso1

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- Will low tail volume make your airplane feel overweight, or should make airplane feel like only nose heavy?

- What makes airplane feel overweight? Isn't that supposed to be high wing loading only?
Sounds like you are combining effects. Low thrust to weight ratio makes for slow accels on the runway and slow response to power changes when aloft. You are not changing that.

If you get comparatively small response from a control input force, the controls are said to "feel heavy". Lots of reasons that might happen:
  • High friction in the controls, although this tends to make the controls feel numb;
  • High mass moment of inertia of the control surface and linkage in the axis, this tends to makes them feel heavy on quick movement, but OK on slow movement;
  • High mass moment of inertia in the airframe axis, which makes it slow to respond to quick control inputs, but OK on slow movement;
  • High aerodynamic hinge moments on the control surface - not usually a problem on tail planes in slow light airplanes, but can be big on ailerons;
  • Combinations of the above.
Want a great example? Stout Airplane Company (Ford) Trimotors. Slow airplane, high inertia airframe and control surfaces, big aero hinge moments, no aero balance. Folks describe it as an airplane with heavy controls but makes up for it by their also being ineffective...

Let's remember that the response to control surface movement is mostly due to control volume - area times arm. So if you have a big enough tail plane and enough elevator or rudder to change air pressure on the whole thing, it will be effective. Not enough arm or area, and it will take big control surface movement to get much response, and that feels heavy. So, controls at the back end tend to feel light enough if you have enough tailplane area. Those ailerons though, they have a tendency to feel heavy. Lots of inertia due to the wings being long, no prop blast on them, wing unit loading gets low out towards the tips, the list of whys goes on and on.

What to do about a control feeling heavy? If the control feels heavy when we are not moving, who cares? But if it is mass balanced that goes away. Then there is aero balance and effective trim tabs.

Aero balances. Now there is the tool for ailerons. You see Frise ailerons, area forward of the hinge, balance horns and spades. All to lighten the control feel of ailerons. Works on rudders and elevators too, but we rarely see Frise balance or spades at the back of the ship.

Once you have a trim tab on a surface, you can tune feel. Too light feeling? Rig the tab with anti-servo linkage. Too heavy feeling? Rig with servo linkage. Then tune the amount to suit overall control harmony.

- Why in Cessna 172 if you upgrade to more powerful engine they let you carry more weight? I understand that with more power available, you will go faster and your wing will develop more lift. But during landing engine is idling anyway and not helping you at all. Your stall speed will go up with the weight.
The more powerful engine added weight, so they preserved useful load by recertifying the airplane at the higher weight. Sometimes they have margin in the airframe strength, and previous max weight was set by takeoff performance and climb gradient at max weight and forward CG. Add power and it can still make performance at higher weight. If instead, they need to bump gage or add some doublers for the airframe, well, it makes the re-cert more complicated. And yeah, it has to raise Vs, Vso, 1.3*Vso, and most likely changes Va, Vx, and Vy. Maybe even Vne.

- When I am flying Cessna with three big guys and I am overweight, and I certainly feel that way on my controls. The plane behaves completely different versus flying solo. If I magically in-flight upgrade my engine from 160 hp to 200 hp, will I instantly lose that overweight feeling?
Not likely. That C-172 inherently has big nose down pitching moments from big tail volume coefficient and large static margins. The tail has to balance that moment by making downforce. And that tail downforce adds to the lift the wing has to make too. Looking at the C-172, I bet the CG is furthest forward two up, as the front seaters have their CG forward of the leading edge of the wing. Add back seaters, and their CG is around the drag spar, so they do not move the CG much, but they add a bunch more weight. I am guessing that the pitching moment is high with two, about the same with four, but just heavier.

Now the Cherokees, with all the seats shoved back a little relative to the wing, they feel nose heavy and a bit sluggish with two on board, but fill all the seats, the CG moves aft and the nose heaviness gets noticeably smaller. It takes a lot less pull on the yoke to achieve stall AOA with four onboard.

- If feeling overweight comes from high wing loading, what is high and what is low? Skyboy solo 7.6 pounds per square foot. At LSA max 1320 lb wing loading is 9.5 pounds per square foot. But of course if this wing is not working efficiently, then you don't have your 138 ft.² anymore.
Feeling overweight? The Fairchild Republic A-10 Thunderbolt II (nee Warthog) with a 50,000 pound max takeoff weight feels light and maneuverable to those who fly it. Aero/mass balanced controls, decent but not huge static margins, and hydraulic boost are all used in that marvelous bird. Manual reversion has been safely demonstrated many times including after battle damage has bled the poor thing dry, so it must not be too bad without hydraulics.

- How do we improve wing efficiency? I have hard leading edge installed but only first 10% or so. Will extending it to 20% make a big difference?
Doubtful. You would need a laminar flow foil and near perfect shape to at least 35% to make a measurable difference.

- Will replacing wing tips with winglets act as additional wing area?
Wingtip sails effectively add aspect ratio to the wings, but unless they are tuned carefully using either CFD or wind tunnel or both, they can just as easily add more drag than they reduce. Then there is the little issue of it increasing wing bending moments everywhere along the span - maybe you have enough strength in reserve for that, but do you want to analyze the whole wing and strut system, maybe design and build a new one?

- Ribs on my wings installed approximately 22 inches apart. Comparing to Aeronca ribs installed much closer about 14" or so. Does it make a big difference?
Rib spacing is just one part of the design of the wing structure. Have you heard of any Skyboy wings coming apart in flight? If not, I suspect that the structural design is OK. If the Champ has a smaller D-tube or the main spar is further forward or the aft spar is further aft or the Vne is higher, that could have driven the designers to their rib spacing. Or the Skyboy designer could have chosen stronger ribs and less of them in the Skyboy. Got any history of wing break-up in flight in either design? In no, then it sounds like both are adequate.

There are many design variables in an airplane. The Skyboy with a Rotax 912 appears to have too little tail hung on a too flimsy tail boom, has the prop wash aimed away from the tail, monster drag AND messed up wing airflow from that uncowled recovery parachute and engine. That is a lot of big design mistakes in one bird. A bigger tail hung further aft on a much stiffer tail boom, aim the engine down the wing downwash angle, and cowl all that crap on top of the wing while forcing all the air that does go through the cowl to go through the HX's, and it will be a better bird to fly and will look better too.

After that you can go searching for things to fair, clean up, etc.

Billski
 
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BJC

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If you get comparatively small response from a control input force, the controls are said to "feel heavy".
I don’t know if there is a universally accepted definition of “heavy controls”, but I use the term to mean that the force required to move the control stick or yoke is unpleasantly high. I do not equate it to control response.

Example: cusped aileron (early) Glasairs have heavy roll forces above about 160 MPH, yet they retain a favorable roll rate per stick deflection.

BJC
 

BBerson

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If all the design improvements add weight it might not improve much. That's the way it is.
The experienced designers focus on reducing weight and drag. Not easy.
 

Eugene

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Sounds like you are combining effects. Low thrust to weight ratio makes for slow accels on the runway and slow response to power changes when aloft. You are not changing that.

If you get comparatively small response from a control input force, the controls are said to "feel heavy". Lots of reasons that might happen:
  • High friction in the controls, although this tends to make the controls feel numb;
  • High mass moment of inertia of the control surface and linkage in the axis, this tends to makes them feel heavy on quick movement, but OK on slow movement;
  • High mass moment of inertia in the airframe axis, which makes it slow to respond to quick control inputs, but OK on slow movement;
  • High aerodynamic hinge moments on the control surface - not usually a problem on tail planes in slow light airplanes, but can be big on ailerons;
  • Combinations of the above.
Want a great example? Stout Airplane Company (Ford) Trimotors. Slow airplane, high inertia airframe and control surfaces, big aero hinge moments, no aero balance. Folks describe it as an airplane with heavy controls but makes up for it by their also being ineffective...

Let's remember that the response to control surface movement is mostly due to control volume - area times arm. So if you have a big enough tail plane and enough elevator or rudder to change air pressure on the whole thing, it will be effective. Not enough arm or area, and it will take big control surface movement to get much response, and that feels heavy. So, controls at the back end tend to feel light enough if you have enough tailplane area. Those ailerons though, they have a tendency to feel heavy. Lots of inertia due to the wings being long, no prop blast on them, wing unit loading gets low out towards the tips, the list of whys goes on and on.

What to do about a control feeling heavy? If the control feels heavy when we are not moving, who cares? But if it is mass balanced that goes away. Then there is aero balance and effective trim tabs.

Aero balances. Now there is the tool for ailerons. You see Frise ailerons, area forward of the hinge, balance horns and spades. All to lighten the control feel of ailerons. Works on rudders and elevators too, but we rarely see Frise balance or spades at the back of the ship.

Once you have a trim tab on a surface, you can tune feel. Too light feeling? Rig the tab with anti-servo linkage. Too heavy feeling? Rig with servo linkage. Then tune the amount to suit overall control harmony.



The more powerful engine added weight, so they preserved useful load by recertifying the airplane at the higher weight. Sometimes they have margin in the airframe strength, and previous max weight was set by takeoff performance and climb gradient at max weight and forward CG. Add power and it can still make performance at higher weight. If instead, they need to bump gage or add some doublers for the airframe, well, it makes the re-cert more complicated. And yeah, it has to raise Vs, Vso, 1.3*Vso, and most likely changes Va, Vx, and Vy. Maybe even Vne.



Not likely. That C-172 inherently has big nose down pitching moments from big tail volume coefficient and large static margins. The tail has to balance that moment by making downforce. And that tail downforce adds to the lift the wing has to make too. Looking at the C-172, I bet the CG is furthest forward two up, as the front seaters have their CG forward of the leading edge of the wing. Add back seaters, and their CG is around the drag spar, so they do not move the CG much, but they add a bunch more weight. I am guessing that the pitching moment is high with two, about the same with four, but just heavier.

Now the Cherokees, with all the seats shoved back a little relative to the wing, they feel nose heavy and a bit sluggish with two on board, but fill all the seats, the CG moves aft and the nose heaviness gets noticeably smaller. It takes a lot less pull on the yoke to achieve stall AOA with four onboard.



Feeling overweight? The Fairchild Republic A-10 Thunderbolt II (nee Warthog) with a 50,000 pound max takeoff weight feels light and maneuverable to those who fly it. Aero/mass balanced controls, decent but not huge static margins, and hydraulic boost are all used in that marvelous bird. Manual reversion has been safely demonstrated many times including after battle damage has bled the poor thing dry, so it must not be too bad without hydraulics.



Doubtful. You would need a laminar flow foil and near perfect shape to at least 35% to make a measurable difference.



Wingtip sails effectively add aspect ratio to the wings, but unless they are tuned carefully using either CFD or wind tunnel or both, they can just as easily add more drag than they reduce. Then there is the little issue of it increasing wing bending moments everywhere along the span - maybe you have enough strength in reserve for that, but do you want to analyze the whole wing and strut system, maybe design and build a new one?



Rib spacing is just one part of the design of the wing structure. Have you heard of any Skyboy wings coming apart in flight? If not, I suspect that the structural design is OK. If the Champ has a smaller D-tube or the main spar is further forward or the aft spar is further aft or the Vne is higher, that could have driven the designers to their rib spacing. Or the Skyboy designer could have chosen stronger ribs and less of them in the Skyboy. Got any history of wing break-up in flight in either design? In no, then it sounds like both are adequate.

There are many design variables in an airplane. The Skyboy with a Rotax 912 appears to have too little tail hung on a too flimsy tail boom, has the prop wash aimed away from the tail, monster drag AND messed up wing airflow from that uncowled recovery parachute and engine. That is a lot of big design mistakes in one bird. A bigger tail hung further aft on a much stiffer tail boom, aim the engine down the wing downwash angle, and cowl all that crap on top of the wing while forcing all the air that does go through the cowl to go through the HX's, and it will be a better bird to fly and will look better too.

After that you can go searching for things to fair, clean up, etc.

Billski
As always, thank you very much for the detailed response. I will have to take a day off tomorrow to study this message to make sure I understand everything. But there’s no guarantee that I will
 

Eugene

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I don’t know if there is a universally accepted definition of “heavy controls”
BJC
Any Skyboy owners will tell you, that if you take passenger airplane will become unbelievably nose heavy during takeoff. We know why, tail is too small to generate sufficient down force to lift the nose. And I don't have any questions about this one.

When I am flying to Oshkosh they ask me to "rock my wings". And I can do that pretty quickly and I usually get a complement " good rock, welcome to Oshkosh!". Airplane can do that pretty quickly and pretty responsively to my inputs. If I ever try to do that with passenger this will look like very slow and lazy motions side-to-side despite how quick I will rock my stick side-to-side. This is when you really feel again that you are overloaded.

Couple years ago one Russian engineer was trying to explain to me that this kind of behavior is direct result of small horizontal tail as well. But back then I wasn't smart enough to understand it. There is a hope that I am a little bit smarter today and I would like to try to get clear picture of how small tail will have such effect.
 

Aesquire

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A bigger tail hung further aft on a much stiffer tail boom, aim the engine down the wing downwash angle, and cowl all that crap on top of the wing while forcing all the air that does go through the cowl to go through the HX's, and it will be a better bird to fly and will look better too.
I'm totally agreeing on the bigger tail, at least a new Horizontal Stabilizer and Elevators, And the wing center section needs a smartly designed cowl, since both turbulent parasitic drag and that big root gap are real speed & lift killers.

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.
 
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