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

From: Don Slusarczyk <don_at_slusarczyk.com>
Date: Sat, 23 Oct 2010 02:06:54 -0400

Kang,

One thing to keep in mind is using a spreadsheet program like that can
lead you down a wrong path. The math used in it can "break down" when
taken to extremes. The spreadsheet was created on a sound aero basis, if
you crack open an aero text book you can go through the cell formulas,
then you can back out all the equations. In fact I did do that at one
time for my own sake to check for any typos. When used correctly I found
the spreadsheet to be very accurate, the CGs were spot on and duration
predicted (at least on classes like F1D and Int Stick) were within one
minute. In fact the first model I every built using the program to
assist me was a unbraced 65cm old ruled F1D. The spreadsheet told me 42
minutes and some odd seconds and at Akron I flew a little over 42
minutes, basically within 10 seconds of the predicted time. I remember I
laughed when I saw the watch as it was cool to get so close to the
predicted time .

One thing you may want to do is take existing successful models from
INAV etc and enter them in the program. Do one sheet for EZB and one for
F1D, F1L, LPP etc. You will then see where models of a certain class
fall as far as SSM, tail volume, etc. Then you can make small changes to
see how it effects things. The hard part is knowing when you get
someting unreasonable. Most indoor models do not seem to go past a cl of
about .90 but if you did not know that and get a Cl of 3.0 you get bad
results. In reality you can never get a Cl of 3 on indoor model. So you
have to learn the limits. On the spreadsheet a wing Cl of .8 was typical
and worked well for me for cambered indoor airfoils. For flat plate
indoor wings 0.7 for wing Cl worked well in the spreadsheet. I guess my
point is the spreadsheet can be very helpful but also do not be a slave
to it especially when making an unusual configuration. If you find a new
configuration, try a half scale balsa glider and do some glide tests,
then if seems promising build a model and try it out.


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
>
>
>
>
> --- In Indoor_Construction_at_yahoogroups.com, Don Slusarczyk<don@...> wrote:
>> On the simulations you are running what are the CL value on the stab you
>> are getting? Are they close to zero or negative in value by chance? Also
>> what is your tail volume value? below 1.0?
>>
>> What was happening in the real world is the model would fly trimmed but
>> when disturbed that is when the trouble started they simply did not want
>> to recover. Bernie did some full size EZB tests as he saw the same thing
>> with tiny tails running low CL values on the spreadsheet. It just did
>> not work. My recollection on the flight tests is from emails we
>> exchanged back and forth about 8-10 years ago but the end result was it
>> did not work. I think it was really due to low tail volume as the stab
>> was so small and as you say the wing has to go back to move the CG
>> forward so small tail and small moment arm = low tail volume = model
>> that does not like to recover when disturbed.
>>
>> Don
>>
>>> Hi, Don,
>>>
>>> Thanks for your post. I'm surprised that stability is an issue. I assume that when using a much smaller stab, like 10 or 20%, while maintaining the same boom length, you will have to move the wing back an appropriate amount to gain pitch equilibrium and stability. In the design program, this is exactly what I had to do. I decreased stab size and moved the wing back to get SSM back to about 10% (10% is roughly what I target for in my EZBs).
>>>
>>> Some clarification would be appreciated. Thank you.
>>>
>
>
>
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Received on Fri Oct 22 2010 - 23:07:03 CEST