Horizontal tail construction

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Eugene

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I disagree with your premise.

If you extend the tail arm, you may not have to enlarge the horizontal tail at all. Your current boom is big enough at the supports, but way oversize as it goes aft. A new tailored tailboom will be right-sized everywhere and is thus likely to save some weight aft;

If you enlarge the tail but hold the tail length, the W&B result will be similar to the above. Your current boom is big enough at the supports, but way oversize as it goes aft. A new tailored tailboom will be right-sized everywhere and is thus likely to save some weight aft;

In between we have smaller changes in boom length and tail size, with similar effects;

In total, a well designed new tail boom, tailored appropriately will likely have little effect on Weight and Balance.

Remember that any increase in tail volume also moves your neutral point aft, giving you more margin on aft CG. Now as to adjusting wing sweep, it both moves your CG and moves your neutral point. It can change the stall behaviour too. This makes you the test pilot for finding your new aft CG limit while exploring your stall behaviour.

Billski
Please forgive me for any stupid statements I might make here, because I am operating at the very limit of my thinking abilities.

If I see it correctly, even if I manage to extend somehow magically my tail pipe by 2 feet without adding any weight. I will be increasing arm and as a result CG will move aft. I agree that it will be very little and no compensation with ballast up front will be necessary. But it will move for sure!!!.

And yes, I agree, that this will be better behaving Aircraft with bigger tail volume. And new carbon fiber tail boom without aluminum pipe inside will improve flow to horizontal tail and can be longer to get to Vh = 0.5-0.6 or so.

And if new carbon tail will end up little bit heavier then I hope. Can I change wing swept to positive 1° or so to get my CG where I needed? Nothing will be wrong with that? They build wings on my airplane perfectly square. They just attached them with crooked brackets to get to 1 or 2° negative swept. I made wings straight with zero swept using different brackets. I don't see nothing wrong to change to 1° positive swept if necessary for CG reasons after I am all done with working on new tail.

"In total, a well designed new tail boom, tailored appropriately will likely have little effect on Weight and Balance." - I really like the statement.
 

BBerson

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I think you meant buffeting. The buffeting is from the prop blast. Each prop blade sends a pulse. That can wear out the parts. Not as much concern as catastrophic flutter.
 

Eugene

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I think you meant buffeting. The buffeting is from the prop blast. Each prop blade sends a pulse. That can wear out the parts.
[/QUOTE

Yes, I remember, during my attempt to fly without elevator counter weights, airplane was doing good with engine idling. But as soon as you move power forward = control handle was starting shaking in your hand. You really need 100% elevator balancing on Skyboy. So, I didn't go very far as a result, with engine at idle.
 
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wsimpso1

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Time for a discussion of loads in beams. The beam in question is your tail boom. The loads come in from the vertical and horizontal tails. By far, the biggest loads are the vertical forces from the horizontal tail and the horizontal forces from the vertical tail. At Va, the elevator and rudder may be simultaneously deflected to their stops, and nothing should be in danger of breaking or even being bent permanently.

So, if the elevator and rudder are deflected fully and make the biggest loads we can expect from them, what happens to the tail boom? At the place where they are attached, the tube see shear load down and to the side, and torsion from the vertical tail being above the tube. This same shear and torsion is carried from the mounts for the tail surfaces all the way to the mountings of the tube at the fuselage.

Now let’s get to bending. At any spot along the boom, we also have bending. The tail load times the distance from the ¼ chord point of the tail to the spot we are interested in is the moment. The moment is zero at the ¼ point of the tails, and grows with the distance all the way to the boom mounts. At the mounts, these moments are the big. Usually, the tube size selected was chosen from among tube sizes commercially available and was most likely the lightest one that would carry the bending moment. That means it is way oversize over most of its length. Oversize means “weight save opportunity”. And with composites we can tailor the heck out of the tube, both in diameter and in wall thickness.

A smart designer will figure out how much tail load is possible using the old FAR part 23 rules on required tail and control surface loads, and scheme out a tail boom of min weight at each of several stations starting at the boom mounts and going aft through the tail mounts, control surface mounts, and controls. The most likely design will be min thickness of the chosen composite over a tube no bigger than is needed to carry the total loads at each station. The tube will then be pretty much one wall thickness, will start out somewhere near the original tube diameter and will grow in “diameter” as you go forward. There will be extra plies and perhaps internal bracing everywhere a fastener penetrates the boom. The new tail boom (if the designer is on his game) may be about the same weight per unit length at the fuselage end, but will be increasingly lighter per foot than the original as you go aft.

I suspect that a well done boom will be a net decrease in weight.

I just did a quick example calc in aluminum. Yeah, I stayed in aluminum and I stayed round. When make the tube diameter big enough to carry bending, shear load uses about 2% of available strength, and torsion uses less than 1%, while bending at the root end needs almost the whole section. When I compared that straight tube to a tapered tube of closer to optimal diameter everywhere along its length, the weight was halved, and the bending moment just due to the boom weight was about a third of the straight tube. While your specifics will matter and your mileage may vary, I expect similar results. Go composite and you can tailor the shape, make the tail boom an ellipse of a faired shape for other purposes, and make each section only as beefy as the section needs to be. A glass boom tailored to your loads is likely to save weight even if longer, and a carbon boom even more. It will save enough weight so that an enlarged tailplane might still not be a weight increase, and it will save so much moment that the more aft and/or enlarged tailplanes may still not shift the CG aft.

You really gotta do some more calcs, and then stuff them into a spreadsheet so you can play what if games until you are happy.

Billski
 

Dana

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On a largish diameter tail boom the critical loading is almost certainly bending, with failure due to local crippling... not easy to analyze. In bending, the highest loads will be right at the aft point where the tube attaches to the other fuselage structure.

So, some hillbilly engineering: If, for example, you made the tail boom twice as long, the bending loads would double. Therefore if the original tube was just strong enough, two tubes of the same size would be strong enough for the doubled moment. Or, double the wall thickness to make it twice as strong.

But you're not doubling the boom length. If, for example, you add two feet to a ten foot tube, that's an increase of only 20% to the bending loads. And (here's the good part) if you strengthen the tube, you only have to reinforce the front 2 feet; the loads on the tube at that 2' point on the extended boom will be exactly the same as the load at the root of the original tube.

I assume the boom attaches at the very front of the tube, buried in the fuselage somewhere, and some point farther back at the back of the fuselage. If you buy a new piece of tubing to make a longer tube, Buy another piece the same size equal to the length the attachment points plus two feet, slit it lengthwise and spread it a bit so it can slip into the front end of the new boom and rivet it in place, you should be stronger than the original. The slit should be at the top as the highest compressive loading is on the bottom of the tube under positive g.

That assumes, of course, that the fuselage attach structure is strong enough to handle the increased loading.
 

Eugene

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A smart designer will figure out how much tail load is possible using the old FAR part 23 rules on required tail and control surface loads, and scheme out a tail boom of min weight at each of several stations starting at the boom mounts and going aft through the tail mounts, control surface mounts, and controls.

I suspect that a well done boom will be a net decrease in weight.

A glass boom tailored to your loads is likely to save weight even if longer, and a carbon boom even more. It will save enough weight so that an enlarged tailplane might still not be a weight increase, and it will save so much moment that the more aft and/or enlarged tailplanes may still not shift the CG aft.

You really gotta do some more calcs, and then stuff them into a spreadsheet so you can play what if games until you are happy.

Billski
My wife is telling me that I look pretty smart from the side when I am reading complicated messages like this. She has no idea that deep inside I feel pretty stupid.

For everything I did before only really needed was time and money. To calculate new carbon fiber tail boom that will be perfect aerodynamically and strong enough to support new tail = I need to get 100 times smarter than I am right now. Otherwise I will look pretty stupid if my new tail breaks apart in flight.

On the serious note I do believe that I understand general concept of what you are trying to explain. Thank you very much for not giving up on me. I will do my best to get smarter. Since we're not allowed to go anywhere anyway, might as well to start reading some smart books.
 

BBerson

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Need to check deflection. Increased length brings increased deflection, both bending and torsion. Also changes the fuselage tube bending and torsion deflection flutter frequency. (based on rigidity)
Eugene has reported the tube already has about a 2° degree deflection at 1g. Eugene also reported concerns from the designer.
 

Eugene

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Yes I understand what needs to be done if 2 feet longer pipe will be used.

IMG_3987.jpg

And by now I think tail boom needs to be longer + horizontal tail needs to get larger. This is only way for me to get horizontal tail volume to 0.5 or larger.

IMG_3851.jpeg

To replace existing pipe with another 2 feet longer will do nothing to fix flow separation on back side of my fuselage. So, some kind of rear composite afterbody will be needed with additional weight on top of longer and heavier pipe.

IMG_1881.jpegIMG_1882.jpeg

Someone much smarter than I am was telling me, looking at this pictures, that he doesn't see any problems to attach new composite tail boom / rear cowling. He recommended to find someone local to help me with calculations. Not sure were I should look for that guy. Or maybe girl (will need permission for this one).

It will take for sure some time and money, but I will catch 2 birds with one stone going this way. And for sure will learn a lot in the process.
 

poormansairforce

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Anything in front of the wing will have destabilizing effect. I was told that long time ago.
So there are no canards flying currently? Its all in the implementation. The canard would move the COL forward taking pressure off the tail. A flying canard that has only a jackscrew for trimming to flight conditions.
 

BBerson

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It's just a tiny auxiliary canard. Makes it into a "three surface configuration".
Note that tail boom in the above photo. Many times deeper than yours, with the stiffness needed for a T-tail. I built a T-tail. It wasn't light weight.
 

Urquiola

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About the message from Russia, attached please find their images after 20 seconds under 'Microsoft Office Picture manager', you select 'Improve Color', then put the arrow in an image area you want changed into white, and click. Or does the gray bottom of page have some function I ignore?
I always liked the 'V' tail, as in Beechcraft Bonanza, Fouga, but I was told by an aeronatical engineer in Romania it has structural problems, yes, several Bonanza were destroyed from Tail failures.
Also the concept of an 'A' tail, as in Lazair, having the advantage of induced roll in the right sense when banking. Comments? Blessings +Plan_13_1.jpgJ-IB 'Don Quixote'.jpg
 

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