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#1 chiozza Sep 08, 2003 01:35 PM

CG and Glide Ratio
 
I am flying a foam park flyer with a 38" wing with a 6 inch chord. I am having trouble either getting the cg correct or somthing else is wrong. The plane flys very good with just 1/4 throttle. When I cut the throttle all the way the planes nose drops hard but it is clean. I also have to keep some up trim when flying at half or above. I tried moving the cg back but then it seems to be tail heavy. I am also having the same type issue to some degree with the GWS BN2. Any IDeas

#2 Ollie Sep 08, 2003 03:28 PM

The CG is for adjusting stability. The decalage is for adjusting glide pitch trim and the thrust line is for adjusting power on pitch trim. Control throws are for adjusting control response sensitivity. If you use the CG to adjust pitch trim in the glide, you may have stability problems. Use each adjustment for what it does best and not for things that complicate matters.

#3 chiozza Sep 08, 2003 03:56 PM

So is my case the decalage is negative or positive?

#4 Ollie Sep 08, 2003 06:36 PM

Assuming the pitch down is not a stall when the power is cut, the way to raise the nose in the glide is to shim up the leading edge of the wing or shim down the leading edge of the horizontal tail, which ever is sounder mechanically. Either change will increase the decalage. Too much of an angle change will result in a stall so, the changes should be made in small increments until the best glide is achieved.

Decalage is defined as the angle between the chord line of the wing and the chord line of the horizontal tail.

#5 chiozza Sep 08, 2003 07:02 PM

Ollie, Thanks very much.

#6 shaneyee Sep 08, 2003 08:55 PM

How do you tell if increasing the decalage hurts your glide or helps it? My Graupner Chip is a fast glider and the way she is trimmed now, the glide is fast - good for fun flying but not good for an All Up, Last one down competition I am taking part in. Should I increase the decalage until she almost stalls or I am hurting the glide more than I am helping. My aim to is keep it up in the air for as long as I can.

thanks

Shane

#7 vintage1 Sep 09, 2003 01:13 AM

In teh old days of Free Flight, I was told - and have no reaosn to doubt it - that forward CG with lots of decalage was not teh best way to get the glide right.

The lowest rate of sink is when the tailplane has no downforce, or even is actually generatung lift. That means a very rearward CG and usually a very long tail. for stability. CG might be as far aft on such a model as 50-70% of wing chord. Decalage was VERY small.

Moving the CG a bit forward, got the best glide ANGLE - i.e. the gretaest range from a given height.

Howe this would apply to a fly8ing wing, I don't know.
Nearly all my models need up elevator to glifde well compared with power on flight. Or if you like, down trim to avoid ballooning, under power.

Just use computer mixing to get it. And move the CG as far back as staibility will allow.

#8 shaneyee Sep 09, 2003 01:54 AM

Thanks. It all makes sense but translating it into the real world is a bit more difficult. I thought the way to get the CG was to dive the model and see if it flew straight or recovered on its own. The latter indicating a forward CG. The CG, once found, is left alone thereon. Its a new concept to be moving the CG back until just forward of the point of marginal instability. It does make sense though that you want the CG back and thus minimizing the need for decalage....just a shift in the paradigm.

Shane

#9 Ollie Sep 09, 2003 05:36 AM

Changing the decalage changes the trimmed airspeed, assuming the CG location provides some stability. There are two possible definitions of best glide. One is for lowest sinking speed (duration), the other is for best glide angle (distance). The best duration will be obtained with the trimmed glide speed close to stall. The best glide angle will be obtained with a bit less decalage to trim the glide speed a little faster than for lowest sinking speed.

See:http://www.polecataero.com/
Go to articles and select the article by Dr. Mark Drela on CG. This shows how to adjust the CG of a glider to suit your flying style. Some people want or need more stability than others.


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