Using Ted Fancher's Imitation numbers as base it has a Tail Volume Coefficient of 0.45. Question - what is a good range for this? What happens as you move lower and higher? What is better to change - tail moment length or tail area to adjust to be closer to the 0.45?

Best,    DennisT

Good questions.  Tail volume coefficient is kinda rule-of-thumby and leaves some stuff out even for static stability.   Here's a calculator, so you can measure some airplanes and make a chart.  Include the Max Bee.  It probably has a lot of it, and it's not excessive. 

As you increase tail volume, CG moves aft for the same static stability. 

For a given tail volume coefficient, making the tail longer and smaller:
1) will increase static stability a little bit because the rate of change of downwash with angle of attack is less
2) will give more maneuvering stability.  You'll need to increase elevator throw for a given loop radius. 
3) may reduce the maximum turn rate. 
4) will require more elevator/flap ratio as air density increases.

I've been doing some experiments to try to figure out what the tail does and to see if some sort of stabilator can do it better, although everybody knows it can't.

Quote from: Howard Rush on January 29, 2022, 06:07:53 PM
Good questions.  Tail volume coefficient is kinda rule-of-thumby and leaves some stuff out even for static stability.   Here's a calculator, so you can measure some airplanes and make a chart.  Include the Max Bee.  It probably has a lot of it, and it's not excessive. 

As you increase tail volume, CG moves aft for the same static stability. 

For a given tail volume coefficient, making the tail longer and smaller:
1) will increase static stability a little bit because the rate of change of downwash with angle of attack is less
2) will give more maneuvering stability.  You'll need to increase elevator throw for a given loop radius. 
3) may reduce the maximum turn rate. 
4) will require more elevator/flap ratio as air density increases.

I've been doing some experiments to try to figure out what the tail does and to see if some sort of stabilator can do it better, although everybody knows it can't.

Cool spreadsheet Howard..

Quote from: Howard Rush on January 29, 2022, 06:07:53 PM


I've been doing some experiments to try to figure out what the tail does and to see if some sort of stabilator can do it better, although everybody knows it can't.

Hi Howard,

Some of the top combat guys seem to know a lot about stabilators.  Maybe they can help you.

Keith

Quote from: Trostle on January 29, 2022, 06:57:24 PM
Hi Howard,

Some of the top combat guys seem to know a lot about stabilators.  Maybe they can help you.

Keith

Oh my, Kernel.  That was rude!

Quote from: Ted Fancher on January 29, 2022, 09:49:19 PM
Oh my, Kernel.  That was rude!

My one chance to get one off on Howard and I get called for it.  Maybe it was not as clever as I thought. 

Quote from: Trostle on January 30, 2022, 12:47:19 AM
My one chance to get one off on Howard and I get called for it.  Maybe it was not as clever as I thought. 

I think the intended humor was missed. That's my trick.

Keith, i thought it was great.take em when u can get um

 This thread exemplifies one of the things I love about Control Line: the relationships that get built to the point that friends can harmlessly poke fun at one another.

And I love that contests are more like family reunions than anything else.

Joe Ed Pederson
Cuba, MO

Quote from: Trostle on January 29, 2022, 06:57:24 PM
Hi Howard,

Some of the top combat guys seem to know a lot about stabilators.  Maybe they can help you.

Keith

Good point. Those guys really know their stuff. They get all the girls, too.

Edited to fix spelling

I knew i should have stayed with Combat

Quote from: bob whitney on January 30, 2022, 12:02:16 PM
I knew i should have stayed with Combat

Well, Bob, you should have stayed with Something!  😜

Quote from: Dennis Toth on January 28, 2022, 04:04:35 PM
Using Ted Fancher's Imitation numbers as base it has a Tail Volume Coefficient of 0.45. Question - what is a good range for this? What happens as you move lower and higher? What is better to change - tail moment length or tail area to adjust to be closer to the 0.45?

Best,    DennisT

I don't know what a good range is, but to answer that I'd take a look at the current machines that are winning and calculate their TVCs.  You run the risk of fulfilling the adage that if you take the average of the "numbers" on a bunch of stunt planes and build your own you probably won't hit an optimum -- instead, you'll have a plane no better than average!

(I'm reminded of a review in the 1990's of a 1970's book on sailplane design.  If you followed the authors rules on how to make the bestest sailplane ever, you ended up with a sailplane that flies like it's 1970.)

TVC probably has the strongest effect on the acceptable range of center of gravity that you can get away with.  So flying a slimer, you'd be motivated to go to higher TVC, so the change in CG doesn't affect the flying qualities as much.  I don't know if anyone has yet experimented with an electric that has a smaller tail -- it would be interesting.  Higher TVC also gives the tail more authority in turns, so if you went smaller you might need more elevator throw.

Making the tail moment arm longer and the tail smaller should give you roughly the same static stability, but because of circular airflow you'll need more elevator in the corners, and you may end up with a plane that's hard to get started rotating and hard to get stopped.

I suspect that evolution has given us airplanes that are close to optimal -- but people thought that in 1965, and look where we are now!  If anyone wants to experiment, a test dog that lets you bolt various tails on at various lengths could be very educational indeed -- but you'd need a really good pilot willing to put a lot of effort into the exercise.

Quote from: Tim Wescott on January 30, 2022, 04:53:35 PM
TVC probably has the strongest effect on the acceptable range of center of gravity that you can get away with.

Good point. If one was to design a plane for the masses, it should have a big tail to accommodate different engines and paint quantities.

Quote from: Trostle on January 30, 2022, 12:47:19 AM
My one chance to get one off on Howard and I get called for it.  Maybe it was not as clever as I thought. 

Actually, Kernal.  That remark was supposed to be a witty needle at a dear friend.  Please accept my apology.

Ted