

Mar 19, 2012, 03:18 AM  

I did some searches on RCgroups. Should have done that earlier.. Sometimes I am just to enthousiastic and want to shear thinks I am doing to soon...
Page 13: http://www.rcsoaringdigest.com/pdfs/...SD200408.pdf 
Mar 19, 2012, 08:08 AM  

Berrie,
Wonderful project! I'm glad you're finding Sailplane Calc useful. Also make sure you're using the most current version which can be found at www.TailwindGliders.com The link you provided for the RCSD article by Dr. Drela's on Tail Sizing are the formulas I used in Sailplane Calc. I didn't add the radius of gyration because I wasn't sure how he came up with the numbers. If you find how he calculates radius of gyration (rg) or a good rule of thumb for different sized models I'd be happy to incorporate it into Sailplane Calc. Curtis Montana 
Mar 22, 2012, 09:11 AM  

For a good understanding of what affects turning control, read Don Stackhouse’s comments over on the Crimson discussion. He makes great points there. Start here, http://www.rcgroups.com/forums/showt...236486&page=57 , then go to the next page.
In pitch response, you have two competing requirements, static pitch stability and dynamic pitch stability. The static case is more critical, and easier to get right. Dynamic instability is harder to keep out of a design, but you can live with it more easily. Static pitch stability is the ability of your model to hold a speed without you having to stay busy on the stick. Dynamic stability is the ability of the model to return to its trimmed speed after a disturbance without you having to get on the stick. In order to have good static stability, you need a “big enough” horizontal stabilizer and your model has to balance comfortably ahead of the aerodynamic center, say 10% static margin for the test flights. Small tails and short tail booms are no problem as long as the CG is far enough forward. For dynamic stability, things get more complicated. Generally, the shorter the tail boom and the smaller the horizontal stabilizer, the worse the dynamic stability problems get. The tradeoff between boom length and tail size is not linear. (That is, you can’t fully compensate for a short tail boom by building a bigger tail.) My rule of thumb for 2 to 3 meter models is this. Measuring from wing quarter chord point to tail quarter chord point, the tail boom should be at least three times as long as the wing’s mean aerodynamic chord, and the horizontal tail volume coefficient should be at least 0.35. You can use average chord in a pinch, but it will be slightly shorter. Also, as the wing aspect ratio goes up, the tail boom needs to get longer. These minimum numbers are slightly unstable, but it takes a while for things to get out of hand. Static pitch stability requires that the CG be moved forward as horizontal tail volume gets smaller and the boom gets shorter, but that makes the dynamic response worse. At some point, the two requirements pass each other and you simply can’t make a short tail model both statically and dynamically stable. Then you have to decide if it’s easier to live with really twitchy elevator response, or stalldivezoom….stalldivezoom….. runaway. The choice usually winds up in favor of dynamic instability, since that mainly develops in turbulence, it develops slowly, and you are actively flying the bumps anyway. You just have to work a little harder to keep things from getting out of control. That said, dynamic instability can become a real problem at speck out altitudes. If things start to get out of hand, the only alternative can become slamming on the spoilers and waiting for everything to sort itself out. (You did build nice big spoilers, didn’t you?) 
Mar 25, 2012, 07:00 AM  

I am now trying to understand dynamics....
to say it short: static stablitly can be managed with a short tailboom. Dynamic stablity prefers a longer tailboom. (Am I right?)
The dynamic issue reminds me after an RCSD artikle: http://www.rcsoaringdigest.com/pdfs/...SD201111.pdf It started at page 21. Dutch rolling is:
Yesterday I did some testflying at my field. I compared my homebuild Supra with my own design TD glider that I build before I build the Supra. I recognize the tailwagling in more turbulant air of my own design glider. The Supra behaves more calm. (Both planes fly well btw, ony has my own design has some habits that I would like to understand) To be continued, with small steps. edit: these doc's shall be studied: http://www.xflr5.com/docs/XFLR5_and_...y_analysis.pdf http://xflr5.sourceforge.net/docs/XF...ts_Rev_0.1.pdf 

Mar 25, 2012, 10:11 AM  

Quote:
You can get static stability with any length tailboom, including one where the distance between the trailing edge of the wing and the leading edge of the stab is zero. That particular case is a plank style flying wing, where the last 20% or so of the chord in effect acts as the horizontal tail, and the reflex needed in the plank flying wing's airfoil trailing edge is analogous to the "decalage" between the wing and tail of a tailed aircraft. So, yes, as long as you have enough pitch authority to trim the plane, you can get a given amount of static pitch stability from essentially any length tail moment arm by finding the required C/G location. That said, the shorter the tail moment arm, the more down force you need at the tail for a given amount of static stability. In addition, it also takes more tail area to make that down force, and that means more whetted area in the tail. OTOH, if you make the tail moment longer, you have less whetted area and weight in the horizontal tail, but more whetted area and weight in the tail boom. This is the tradeoff Beech (and others using similar construction philosophies) faced in the design of the Bonanza and similar aircraft. That type of construction results in a lot of weight and whetted area in the tail cone, so making it longer carries some significant penalties. One approach to optimizing this is to choose a parameter; weight, whetted area, material and labor costs, whatever; and then look for the optimum combination. When I was designing a Spitfire Mk22 Quarter 40 pylon racer, I chose the tail moment arm based on what combination of tail cone and tail surfaces, for given tail volume coefficients, resulted in the lowest total whetted area, and therefore the lowest skin friction drag. The result actually came out surprisingly close to scale. The typical method for managing the static stability and control authority vs tail area and moment arm question is with tail volume coefficients. Basically we put everything that helps the tail do its job and put that in the numerator, and things that make the tail's job more difficult go in the denominator. There are some articles in the "Ask Joe and Don" section of our website www.djaerotech.com that go into the details. For example, the formula for the horizontal tail volume coefficient is: Vht = (tail area / wing area) * (tail moment arm / wing MAC) where MAC is the wing's Mean Aerodynamic Chord. Typical numbers in model sailplanes for this are around 0.45 to 0.55 . Now, if we want to look at the dynamic stability question, simply square that second factor, to create what I refer to as a "Dynamic Tail Volume Coefficient": Vhtd = (tail area / wing area) * (tail moment arm / wing MAC)^2 Volume coefficients are a way of attaching a number to the effectiveness of any particular airplane's tail design. Find examples of planes whose control response and static and dynamic stability you like, calculate their static and dynamic tail volume coefficients, and with enough examles you should see a pattern emerge, an typical range for those coefficients that result in the behavior you are looking for. There will be variations caused by things like the amount of inertia about the pitch and yaw axes due to masses in the extremities, additional tail area needed to counteract aerodynamic pitching moments due to flaps, extra fin area in multiengined planes due to the need for enough rudder authority to overcome asymmetric thrust in an engine failure situation, etc., but assuming your examples are sufficiently similar to your situation and to each other, there should be a pattern, a range of numbers that seem to work well. Use that as a guideline for your design. 


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