Cosmic Wind CG

[Allen Eshleman]

I have a hand-me-down Cosmic Wind.  I recently put a B-40 on it for break in.  The motor starts and runs fine. It's already had the 20 or so short runs.  I took it up for the motor's first flight yesterday.  However the plane was really touchy on the controls going up and down. I just hung on and flew, not trying to do anything.  I suspect that it is tail heavy.  The current CG is just forward of the the front line.   Any helpful ideas will be appreciated!   Thanks!

[Dennis Adamisin]

Hi Allen
Congrats on getting the new (old) bird airborne.  Here'sa couple things to check.

* Control throws: too fast an elevator can cause even a well balanced bird to be a little squirrelly.  If it is not there already, move the pushrod to the OUTERmost hole of the control horn.  If it is already there, I suggest you visit your LHS and lookever the selection of control horns and get one taller than what you have.

* Next up, if you can, reduce the spacing on your handle.  I don't like this one as much because you can lose control feel, but it can help a lot.

* Hard to imagine a B.40 not being enough to balance the bird, but there is always the option of adding nose weight.  If you are using a TOngue style muffler switch to the larger (heavier) muffler.  Alternatively, find one of the large brass Harry Higley "Heavy Hub"  That will put a big slug of noseweight as far forward as possible having a BIG influence on the CG.

GOOD LUCK!  and keep us posted about your progress.
 

[Allen Eshleman]

Thanks Dennis,

Mentioning the muffler,  I realized I didn't have it on yet.  I put it on and will try that.  I have some weight on the back that I can remove.

B-40 stock muffler - does anyone know how to set it up for muffler pressure?

[Ted Fancher]

Hi Allan,

It is very possible the Cosmic Wind is tail heavy.  The airplane is a little unique in that all the taper (and there's quite a bit of it) is in the trailing edge.  This has a very real aerodynamic impact.  It in essence makes the nose shorter by virtue of moving the center of lift forward.  

But don't get bogged down in the aero details because the solution is much simpler.  The Center of Gravity has to be located in the correct position based on the Mean Aerodynamic Chord (MAC).  Again, don't worry about the terminology. This is simply the way the engineer designers looks at the wing shape and area to determine where the lift will be generated with respect to the length of the overall airplane.  Again, don't worry about the technicalities.

The solution is to locate your CG in the correct position on the MAC.  It's simple and here's how to do it. (By the way, this explanation will be simplified because we're going to use the "AVerage" chord instead of the slightly more difficult to determine MAC.  For a simple tapered wing and stunt aerodynamics this is more than adequate.

First measure from the center of the fuselage to either wing tip.  Lets just say that is 22 inches.  Divide that in half and you come up with 11 inches.  Locate that 11 inch location and mark it.  

Now mark a temporary straight line, parallel to the fuselage, from the leading edge to the trailing edge at that point.

Measure that distance.   Just for example purposes let's say that distance is eight inches.

Now, because the Cosmic Wind is a flapless design I'm going to suggest that you locate your Center of Gravity at a point 15% aft of the Leading Edge at that "halfspan" location.  So you multiply the chord (8") times 15% (.15) and the result is 1.2".  This is where the CG should be located, 1.2" aft of the leading edge. (NOTE: Because your wing has a straight leading edge, with all the taper in the trailing edge, you can measure that 1.2" anywhere on the wing.  This is very convenient because you can go all the way out to the wingtips, make a mark on both wingtips 1.2" aft of the leading edge.  

Now add or subtract weight at the nose or tail until the airplane balances with a finger tip held at each of those marks at the tips.  I guarantee you that this will be an excellent starting place for the CG on an unflapped ship like your C.W.

Set the CG at that point and fly the airplane a few times and then, if necessary, make the adjustments to the handle and control system that Denny suggested above.  Doing so will result in a ship trimmed within a fraction of inch of the optimum center of gravity.

ONE BIG CAUTION.  THE ABOVE NUMBERS (15% PRIMARILY) ARE NOT NECESSARILY APPROPRIATE FOR FLAPPED DESIGNS ALTHOUGH USING 15% WILL ALWAYS RESULT IN A FLYABLE RESULT.  The reasons why are a little complex but not germane to Al's predicament.

Ted Fancher

p.s.  just noted your later comments on the muffler.  Yes, definitely take the weight off the tail and most likely go ahead and mount the muffler.  Then check the CG as described above and adjust accordingly.

[Allen Eshleman]

Thanks Ted,

        Actually,  I went and flew with the modifications I had made which were the placement of the muffler and the removal of the weight I had on the back which wasn't very much.   Before I even took off, a small piece of the back part of the fuselage fell off which also had some weight on it.  Well, it flew much, much better.  I did loops and figure eights and a bit of inverted which is my accomplishment level so far. 

       Then I went home and took the measurements you suggested.  I came up with 1.4 inches aft the leading edge.  Actually, that is where it currently balances.   It still seems a bit tail heavy when near the end of the run.  I may add a quarter of an ounce under the motor and see what that does.   

        Anyway,  thanks for the good advice.  I'm flying this plane happy now, not scared.  Actually,  I've been flying on 52 foot lines.  I think I'm ready to graduate to 60.  I always do that till I'm sure the plane will stay out there.

Thanks again!