Can we have too much stability?

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TFF

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One thing that is different is how the CG shifts as fuel burns. The front engine planes as they burn fuel actually get easier to fly as the CG moves rearward. Easier to trim, actually can gain speed as the trim drag lessens. Your plane starts to loose it’s agility as you burn fuel. Your plane at landing feels like everyone else’s taking off. You gain trim drag, the less fuel you have. This is the price of your configuration. What you are doing is lessening the impact as much as possible with your tail change. Aft CG can be dangerous if too far aft, but aft CG in a good spot makes a nice flying airplane. Forward CG is safer to fly in that it is more stable; it is stable because it is in the “ heavy “ damping range of the CG.
 

flitzerpilot

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Regarding lateral stability, which is not directly the subject of this thread, but in relation to earlier posts about high wing aeroplanes and their 'natural' stability (due largely to 'pendulum effect'), I seem to recall that Duane Cole's aerobatic clipped Taylorcraft had very slight anhedral to enliven its roll performance. Whether or not I am remembering this correctly, an old friend fo mine built an exact replica of the Cole aeroplane in the UK which also had maybe 1/2 degree of anhedral. It performed well and I believe was stable when flown 'conventionally'.
 

Eugene

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One thing that is different is how the CG shifts as fuel burns. The front engine planes as they burn fuel actually get easier to fly as the CG moves rearward. Easier to trim, actually can gain speed as the trim drag lessens. Your plane starts to loose it’s agility as you burn fuel. Your plane at landing feels like everyone else’s taking off. You gain trim drag, the less fuel you have. This is the price of your configuration. What you are doing is lessening the impact as much as possible with your tail change. Aft CG can be dangerous if too far aft, but aft CG in a good spot makes a nice flying airplane. Forward CG is safer to fly in that it is more stable; it is stable because it is in the “ heavy “ damping range of the CG.
Theoretically this is absolutely correct logic. But in real life in my airplane with 15 gallons of fuel CG moves from full to empty from 34.7% to 33.8% MAC. Nobody that I know recognized any difference on airplane performance in Skyboy because of that. Taking passengers is a different story. Some guys flying only solo because of that.

IMG_1844.jpeg
This weight and balance situation is real improvement after 2.3% CG movement forward as a result of removing 1° negative wing swept.
 
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TFF

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You add a passenger, your CG goes forward. Add a passenger to the others and it goes aft. Forget empty CG for now. Loaded only counts when you are flying. Empty is about how to achieve flying CG with what is left of capacity. The way airplanes handle, the CG range is not linear. 1-2 % in the right area is the difference between dog and delight.
 

Eugene

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You add a passenger, your CG goes forward. Add a passenger to the others and it goes aft. Forget empty CG for now. Loaded only counts when you are flying. Empty is about how to achieve flying CG with what is left of capacity. The way airplanes handle, the CG range is not linear. 1-2 % in the right area is the difference between dog and delight.
Yes, we don’t care about aircraft Empty. They usually don’t fly anywhere.

I was trying to make a statement that normally loaded with me and my wife this airplane is typically at 25% or so. To get to 23% you have to have two people 220 pounds each. when I hear explanation for the reason that Aircraft is like field with lead at 25% because of forward CG. I wanna stand up and scream from the bottom of my lungs that 25% is not forward CG. It should be normal and should be right in the middle, should be right where you want it to be. Am I really so wrong on this?
 

BBerson

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I remember my Chief in cruise looking back at the elevator several degrees below the horizontal because of the 34% aft cg. The Chief doesn't need as much tail volume for several reasons: The fuselage is rigid unlike Skyboy. The thrustline is centerline unlike Skyboy. The fuselage is straight with the airflow, Skyboy in side view has obvious nose down angle. All those down moments are a problem for Skyboy.
 

TFF

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25% is a lazy rule of thumb CG number. I use it with my RC planes. The minute I take off, I know if I got it right; usually not, but hopefully good enough to get a second or third chance. Every inch of the plane matters to have a good flyer at 25% vs not. Every spec. Design a plane, build it, and then test it. Did it fly like you wanted? No? Then you start fixing. Changing incidence, changing CG range, changing power. You pick a configuration and you are stuck with the deficiencies. You are trying to say you want 25% to be your magic number, you probably want 28-30% not 23. Range is just the range that doesn’t kill you. Best flying CG has nothing to do with seats. Seat placement can be totally wrong for the aerodynamic profile of the plane to be a pleasure to fly. 25% CG can be forward, aft, or perfect. Each design is different.
 

Aesquire

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Hang gliders flown tandem handle different & it's not CG. Aeroelestic changes and more mass to wing ratio, a weird combination of sluggish & power steering. With some models it really changes trim by more bending of the leading edge spars.

And paragliders handle quite different at different wing loadings. I've flown the same model in different sizes, back to back, not as big a wing loading change as tandem, but radical change in response. Some manufacturers have models that change pilot rating with size.

I was solo test flying a "tandem" glider at the training hill, ( on a demo day where the dealer brought several ) and the instructor told me after, that he was initially nervous, since the " too light loading" would slow recovery from collapses, then noticed I was flying very conservatively. ( old, bold....) Then other flyers started complaining they couldn't stay up like I was, which went away after I top landed, unhooked, and told them it was like flying a truck with manual steering.
 

Eugene

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I am trying so hard for such a long time to squeeze correct answer out of you guys and it's not working so far. Let me try philosophical approach!

They give us for Skyboy approved CG range from 21% - 36.6% which is 9 inches on 59" chord. I have a feeling that they simply copied this numbers from Aeronca Chief because they use exactly same wing with the same cord. By the way horizontal tail volume is exactly the same as well = 0.33. Maybe they figure if it was working for Americans for such a long time it will work for them as well. We don't know. Maybe they forgot that engine on Skyboy is sitting on top of the wing and generating additional moment. And additional moment needs additional horizontal tail area to balance.

But what we do know that flying Solo with CG at 36% you are sitting on top of neutral point and Aircraft is ballooning and impossible to trim. We also know that many sky boy owners flying with actual 50 pound lead plate under passenger seat. Airplane is flying much better and they swearing up-and-down that it's actually faster this way. I did some experiments in the past with additional weight and found that window I really like is between 27% - 33%. And 6% on 59 inch chord is 3.5 inches that is exactly like on V-tail Bonanza. Maybe if they would install bigger tail for bonanza we wouldn't hear this exciting stories about landing tail heavy airplane.

Bonanza then would it be better airplane with bigger tail? But not exciting anymore? But may be more user-friendly?

So correct answer to my question is: yes bigger tail will give me a wider CG range. Neutral point will move aft and my "pleasant to fly zone" will extend to 25 - 35% about. And feeling of "field with lead" will go away. And mission will be accomplished. This is what I wanted to hear! Why was it so hard to tell me this?

So bigger tail will transform my Aircraft into a completely different machine!!! CG range bigger than Bonanza!
 

Eugene

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Two days ago I was convinced that I have all my answers. But today I was thinking again and found some questions inside of my head.

So, every aircraft have "safe to fly" CG range. Inside of this range somewhat smaller "pleasant to fly" CG range. If we make horizontal tail bigger, we will have improvements in both ranges. Aircraft will become more stable because neutral point will move aft. I think I understand this correctly.

Also, every aircraft has maximum "safe" gross weight. I am assuming that we can say that before maximum weight there is "pleasant to fly weight" that is smaller.

And my question is, what do we need to do to this aircraft to improve this gross weight number? Do we increase engine power? Or we need to increase wing area?

I do understand that there is structural concerns, but we will skip that for now. This is purely theoretical question and I'm not going to do anything stupid. I promise!
 

Eugene

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Reduce empty weight.
I was afraid of that recommendation. I understand that I can lose weight, my wife can lose weight, we can leave baggage at home and so on. But what if losing weight it's not an option. What do we need to do to Aircraft, theoretically of course, so this machine can carry more weight?
 

TFF

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More weight means more wing area or more speed or both along with the strength to pull it off.
 

TFF

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Probably not enough on your plane. You will not clean it up and go 20 kts faster. You are not going to add 50 hp either. You will also loose your slower stall speed if you want to increase gross with speed and not increase wing area.
 

Eugene

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Probably not enough on your plane. You will not clean it up and go 20 kts faster. You are not going to add 50 hp either. You will also loose your slower stall speed if you want to increase gross with speed and not increase wing area.

Got it. Need more lift = wing area, wing efficiency with engine cowling or winglets.
 

BBerson

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What do we need to do to Aircraft, theoretically of course, so this machine can carry more weight?
Look at every part to see if weight can be reduced. Such as lighter custom fixed pitch prop. Lighter battery or no electric system at all. Lighter wheels, less instruments, use handheld radio, lighter seats, lighter fabric, lighter floor boards, lighter exhaust, less fuel tanks. It's your choice.
 
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