Re: Re: 40% Stab size or 20%?

From: Don Slusarczyk <don_at_slusarczyk.com>
Date: Sat, 23 Oct 2010 01:10:09 -0400

Here is what I have found having use the spreadsheet heavily for years
back in the day.

Tail volume:
For most events I designed for a value or 1 or greater. You can go down
to .8 but only if necessary (due to rules) and going below .8 would mean
I would have to run a much higher SSM to be flyable in the real world.
Even so these models were tricky to fly. Models with length restriction
can make it tough. For instance a large chord (8") monoplane pennyplane.
The length is restricted and to get the tail volume up to a flyable
level you need a big tail. With the spreadsheet I was able to get the
setup correct for my dads monoplane pennyplane and he did 19+ at Akron
with it but it was a tricky model to get to repeat. It did not like high
torque needed to get to the top of Akron (180 feet), it was more of a
100 foot model, trying to go higher was inconsistent. Now going the
other way on tail volume, like over 2 to me had no noticeable flight
improvement in performance for the extra length. Models were just too
long and floppy so I flew many "short EZBs" as they were easier to build
and keep the weight down.

As far as tail Cl, you can see most models will be in the .3 to .4
range. By going smaller tail the CL is less as the tail is lifting less,
and in the negative case actually is producing negative lift. The tail
is no longer lifting up to push the nose down but making neg lift,
pushing the tail down to raise the nose up. I believe Bernie was testing
~15% stabs on his EZBs as I recall his tail CLs were essentially zero.

Don

> Don S,
>
> You're on the money. Here are the tail volume and CL for 37,30,20,10% stabs:
>
> tail volume coeff: tail moment * tail area / wing chord / wing area
> 1.6, 1.25, 0.80, 0.39
>
> CL: .354, .324, .226, -.018
>
> Can you explain the significance of tail volume?
>
> Thank you!
>
> -Kang
>
>
>
Received on Fri Oct 22 2010 - 22:23:08 CEST