Can we have too much stability?

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Heliano

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My view on this subject (obviously anyone is free to disagree - differences make a richer debate):
1 - There is no excess stability. The problem is: generally along with high stability comes low controllability. In other terms: it is more difficult to change the angle of attack of a very stable aircraft, requiring more elevator deflection. Basically it is a short blanket: you cover your feet and uncover your head, or vice versa.
2 - The longitudinal flight qualities depend not only on the tail volume but also on pitch damping. A short tail cone aircraft may have the same static stability of a longer tail cone aircraft (roughly the same tail volume), but the lower pitch damping (the so called Cmq, where q is the angular rate) makes it more "pitch sensitive".
 

Eugene

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The problem is: generally along with high stability comes low controllability.
This is exactly what I was trying to say with comparing flying to driving inside of a round tunnel. I just didn't know how to word it correctly. Smaller diameter tunnel = gets more stable and less controllable.

So Aircraft designer is trying to find this compromise in between. That is why we have different airplanes behaving differently. Because we have different aircraft designers. They did go to different schools and they had different books written in different languages. Some very successful and some not so much.

Thank you!

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BBerson

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The problem is: generally along with high stability comes low controllability
Precision aerobatic planes normally have relatively high pitch stability because of the straight lines required in competition.
For the control needed for those floppy air show maneuvers they use extreme elevator deflection and high power.
Sort of powered control.
 

TFF

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With aircraft design it is either a brain child or design by committee. Design is about getting an idea across. That you or others line up with that idea is a whole different question. The math is the math. Right and wrong is only about crashing.
 

Tiger Tim

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I think it could be argued that some ultralights of the 70s and 80s must have had too much stability as some seem to have gone from two axis to roll spoilers to ailerons with minimal alteration to the rest of the design.
 

Pops

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For an aircraft as small and light as a 601 HD to feel like a much bigger and heavier Archer means, to me, that it's overly stable. I have flown Archers for IFR training and they are ideal for that, because of their stability. Even a 172 isnt as stable as an Archer, especially for IFR work.
A C-172 has noticeably less roll stability at 12K.
 

Aesquire

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I think it could be argued that some ultralights of the 70s and 80s must have had too much stability as some seem to have gone from two axis to roll spoilers to ailerons with minimal alteration to the rest of the design.
The original Eipper-formance Quicksilver hang glider was a weight shift pitch, body shift rudder "1 axis?" design. The rudder was activated by lines to the harness, so control was ergonomically identical to flex wing Rogallos of the time ... And today.

Rudder yawed you, and fairly high dihedral rolled you into & out of the turn. Effective, simple, and the wing was a "ultralight ladder construction" with no moving parts. The horizontal stabilizer was fixed with no elevators. The Rudder was the Only control surface. And almost the only moving part.

Basically the most copied wing design in the past 3 decades of the 20th century.

So, yeah, excessive roll yaw coupling & stability was built in, on purpose, and the powered versions went through an evolutionary process, as did all the copies/knock offs/and homages, many with the advertising that they were better, though very few were.
 

Eugene

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In our little airplanes, it is hard to imagine taking an existing airplane that can fly its entire envelope, then enlarge the tail, and have it fly worse

Billski
But, sounds like this airplane will become less exciting for some guys.

I was under the impression that the minimum requirements always forcing engineers to design relatively stable airplane. Did they see during test flights that Bonanza not very stable aircraft? Why didn't they increase dihedral angle?

During my training in Cessna 172 I was told one time by instructor if something will go wrong and you wouldn't know what to do, don't do anything. Airplane is so stable, that will probably land it self. Try to do that with Bonanza in IFR situation.
 

User27

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As Billski alluded to stability and control can be a complex subject. In our aircraft pitch can usually be considered on its own roll and yaw have to be taken together. We all know that when cruising along most aircraft will not stay pointed in the same direction for very long without some intervention from the pilot, even if it is very carefully trimmed in pitch roll & yaw. The gusts and eddies in the atmosphere knock the aircraft off course. A highly stable aircraft means the forces (generated by the shape of the aircraft) retuning the aircraft to where the pilot left it are greater than the forces pushing it away - that implies large (heavy) stabilisers. It also means the pilot will have to overcome the stabilising forces every time the aircraft is manoeuvred - that gets tiresome. Nowadays electronic stability (autopilot) might be a better solution. In pitch I guess we all know the horizontal stab usually pushes downwards. For a highly pitch stable aircraft with a large HS the wing must generate more lift (be larger, heavier and create more drag) than is the HS was smaller.

The middle of the scale is an aircraft that is barely stable, the stabilisers can be minimum size, weight and drag. The stick forces are light, the aircraft manoeuvres readily. In the cruise the aircraft is tiring to fly as the pilot must continually correct the heading and altitude - for example flying on a very thermic day.

The unstable end of the scale will result in an aircraft that is unflyable for most pilots. Any turbulence in the air will make the aircraft pitch/roll/yaw away from where the pilot wants it. If left alone the aircraft will stall/exceed Vne/roll on its back/fly tail first or just drop out the sky. Fore and aft stick will control pitch rate rather than pitch attitude. Small amounts of instability result in large increases in pilot workload. Some aircraft can be mildly unstable when the flaps are lowered (RV-4, 6, 7) meaning the pilot has to work at holding a constant approach speed.

The level of stability/instability is dependent on the relation between the centre of gravity and the centre of pressure/aerodynamic centre. If the cg is too far forward in a C182 the force required to flare for landing can be very high. I would say that is too stable in a practical sense. It meets the FARs but many owners carry a case oil oil in the trunk when flying solo to shift the cg backwards (reduces the static margin) and makes the pitch forces lighter.

The shape of a control surface, how much area is ahead and behind the hinge line, can have a significant effect on the apparent stability if the pilot's hands/feet are removed from the stick/rudder (stick free stability). The friction in the control system can have a very large effect on the apparent stability - high friction controls do not allow a control surface to return to the 'trimmed' position and do not provide the pilot with very good feedback. That is why most stability and control engineers hate the thought of teleflex cables being used in primary flight controls, there's just too much friction in the cables.

An aircraft can be directionally unstable, but it is unusual, more likely the rudder is not returning to centre when the pilot stops pushing on a rudder pedal.
 

User27

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Precision aerobatic planes normally have relatively high pitch stability ...
I don't agree with you, usually low pitch stability is required to enable large control forces to be applied with managed stick pressures. The pilot must adjust the inputs all the time to maintain a precise flight path. Some owners of aerobatic aircraft will move the battery to the tailpost to move the cg aft to reduce stick pressures and improved stall/spin/snap performance, me included. My old aircraft was a bear to get to spin inverted with the cg forward of the midpoint.
 
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wsimpso1

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But, sounds like this airplane will become less exciting for some guys.
I think your mods will make it faster, more controllable and maneuverable, and a better airplane. It is not an aerobat, and will never be one. It will still be more interesting than a C172.

I was under the impression that the minimum requirements always forcing engineers to design relatively stable airplane. Did they see during test flights that Bonanza not very stable aircraft? Why didn't they increase dihedral angle?
I suspect that the original Bonanza was fine under the rules and certification environment of the immediate post-war environment. Walter Beech put more importance on maneuverablity than on platform solidity, so the Bonanza passed stability rules in that airspeed, heading, and altitude will stay put even as it Dutch Rolls and the yaw angle oscillates. As to roll stability - not required as long as the roll off is slow enough. Being slightly tending to roll off aids maneuverability and ability to precisely hit headings and line up for landings. Airplanes that are very stable in roll are unpleasant to fly and difficult to line up for landing. Vaughn Askue's book on flight test is a great resource on all of this stuff.

During my training in Cessna 172 I was told one time by instructor if something will go wrong and you wouldn't know what to do, don't do anything. Airplane is so stable, that will probably land it self. Try to do that with Bonanza in IFR situation.
The 172 is boring and the Bonanza great fun to maneuver. That is the trade...

Billski
 

Eugene

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I suspect that the original Bonanza was fine under the rules and certification environment of the immediate post-war environment.
Wondering if we can make similar statement about my Skyboy. Going through German certification in 1993 and having 20% larger wing with P3 airfoil, 20% larger horizontal tail on 120 x 2 mm tube and only 50 hp lighter engine behind the wing = aircraft was probably much more stable.

This harmony went out of the window after smaller and faster 4412 wing was used, smaller horizontal tail on same 120 mm tube and 2 times more power and heavier engine.

Maybe Tailwind could tolerate such dramatic changes without big penalties, but not aircraft with engine on top of the roof.

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TFF

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With all your threads, it is clear you have out grown your aircraft. That aircraft just happened to be designed with political and economic constraints of its home country. Those will churn out a certain type of airplane. In its context, it probably makes a lot of sense. Out of it, it lacks. That is the kind of choices designers make. The US has plenty of space, plenty of aircraft material, and rules that allow you to build anything. Supersonic if you want. Would Russia let you build a supersonic homebuilt? No. Cant show up the government. What you have been trying to do is Americanize a European budget airplane. Plain and simple as that.
 

BJC

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With all your threads, it is clear you have out grown your aircraft.
Eugene:

I think that Tom is correct. You have an engine, propeller, wheels and brakes, basic instruments and lots of hardware that could go a long way in another homebuilt, especially if you find an uncompleted kit. Alternative, you could sell your airplane as a flying model, and start new.


BJC
 

Eugene

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you could sell your airplane as a flying model, and start new.
BJC
OK, here is my proportional response:

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The way I translate his message to Russian, is that airplane is the puzzle of few major components. I don't see anything wrong with my wing, wheels, engine or fuselage. Only one thing that needs to be changed to bring this airplane into acceptable flying machine is the tail. I will be paying somebody to do needed calculations and after that for the next three years I will be going to Airport every Saturday to build it. And I will do that instead of going fishing every weekend. This way my wife wouldn't even feel the difference at all. What's wrong with this approach?

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Fuselage structure was shifted by me recently 2 inches to accomplish few things. Compensate CG movement with new tail and to change fuselage incidence at the same time. I lost ceiling height from 5 inches down to 2.5 from my headset.

Somehow I don't see anything wrong with what I am doing and I don't see how building brand new aircraft would be easier or smarter. When I'm all done this will be truly home build experimental and not home assembled. But it is typical for me, usually I don't see anything wrong with what I'm doing until I talk to my wife about it.
 

BJC

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Somehow I don't see anything wrong with what I am doing and I don't see how building brand new aircraft would be easier or smarter.
Certainly, you should do what you want to do.

I have seen people get deep into a project with cascading complications because they were so narrowly focused that they did not realistically assess their situation. My comment was intended to spur you to assess your situation.


BJC
 

Aesquire

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My eyes keep telling me the thrust line on that Skyboy is wrong. I've been told it isn't, but I keep thinking rotating the engine nose down a few degrees would reduce power change pitching.

Back to the OP... Stability causes drag in pitch, but you need some to be flyable. Roll stability isn't as much a drag issue as it is pilot workload. I've had hang gliders that had negative roll stability past a certain bank angle, requiring opposite control to maintain bank angle. That was a design choice to improve roll in to turns, as gliders spend a high percentage of time turning, and often turning into the wing that is pushed up by a thermal. Zero roll stability while level is... I wouldn't say "exciting" , more tiring. But low stability works for some applications. Not so good flying IFR...

And that's Another reason not to fly in clouds without instruments. The Death Spiral.
 

Eugene

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Certainly, you should do what you want to do.

I have seen people get deep into a project with cascading complications because they were so narrowly focused that they did not realistically assess their situation. My comment was intended to spur you to assess your situation.


BJC
This slowly becomes the way of life. For me it is more interesting kind of life, more productive. I am having a good time and I don't have any desire to end this as quick as possible.

Those people who go fishing all the time, they don't have any desire to be done with that and never return to it again. They probably feel just fine to keep doing it for rest of their life.

To be able to understand completely what's in front of me was real hard thing to do. But thank you to all of you guys I have a pretty clear picture by now, I hope. I will run this by my wife and see what she thinks.
 

Heliano

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As many things in life, the subject is not as simple as it seems at first. There are several stability modes. Namely:
Longitudinal:
1 - Static longitudinal stability, stick-fixed (when stick has to always be moved backward to increase angle of attack and vice versa)
2 - Static longitudinal stability, stick-free (when forces tend to move the stick forward when angle of attack is increased and vice versa)
One example technique to increase stick-free stability is the servo tab in all moving horizontal tails.
For each of the above cases there is a distinct neutral point.
CG aft of the neutral point does not mean that the aircraft is not flyable. If it is slightly aft, the aircraft generally can be flyable without a problem.
3 - Short period dynamic longitudinal stability (not a problem with conventional light aircraft. Typically affects aircraft with low aspect ratio wings - e.g. delta wings - or marginally stable aircraft with very small horizontal tail volume. In some cases pitch dampers are used)
4 - Long period dynamic longitudinal stability (phugoid). Phugoid instability normally is not that critical unless it is very pronounced. An example technique to increase phugoid stability is the use of pitch-down springs attached to the elevator control (example: Piper Navajo)

Lateral static stability (dihedral effect): Some is needed, but excess static lateral stability is a problem: aggravates the spiral instability, among other effects. The static lateral stability is obtained a) with geometrical dihedral angle; b) with a high wing; and c) With wing sweepback angle. To avoid excess static lateral stability, a high wing requires less geometrical dihedral, And a high wing with a sizeable sweep back angle usually requires a negative geometrical dihedral to avoid excess static lateral stability.
Directional static stability: this one is pretty simple: the aircraft should have a natural tendency to zero the sideslip angle.
Lateral/directional dynamics: there are basically two modes: spiral stability and dutch roll. Spiral instability is when, once in a turn the tendency is for the turn to get increasingly tighter and nose down.
Dutch roll: coupling between directional and lateral oscillations.

All the above is about stability, not about controllability or maneuverability.
About controlability:
Billski mentioned an interesting thing: "In the roll axis you can have too much stability". Perhaps he was referring to lateral controllability: An aircraft must have enough aileron authority to counteract sideslips, asymmetric gusts, etc. There is a certification paragraph (FAA CFR14 Part 23 or 25) which requires an aircraft, with full rudder deflection at VA, to still have lateral (aileron) control margin in both directions. I am not sure that all experimental aircraft meet this requirement. Another aspect of lateral controllability is: aerobatic aircraft require crisp aileron control. In other terms, the aircraft must be able to produce a high roll rate (flight test people call it pb/2v).

The subject goes on and on. I am way too wordy here already.
Those who can not resist curiosity, I recommend two books:
- Aircraft Performance, Stability and Control - Perkins & Hage (a classic)
- General Aviation Aircraft Design - Snorri Gudmundsson
 

BBerson

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Some owners of aerobatic aircraft will move the battery to the tailpost to move the cg aft to reduce stick pressures and improved stall/spin/snap performance, me included. My old aircraft was a bear to get to spin inverted with the cg forward of the midpoint.
Stick pressure can be reduced with large aero balance surfaces on the elevator or moving the pivot aft in the case of an all flying stabilator.
 
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