murrat

I have recently put together a spad delta. The span is 30inches with a 45 degree sweep, and length is 22 inches. I put a MVVS Q500 .40 with GRRT pipe on it. MVVS turns APC 8x7 at 20K static. Has one vertical stab. Tank/radio/servos all cowled in. Didn't build cowl for engine and pipe yet. Model weights about 3 lbs. I decided to put a flat bottom on this one to cut down on drag. The model balances at 11% MAC. No down thrust on engine mount.
No relfex built into airfoil.

Flying:
This model needs slight up elevator to hold level flight at about 140 to 150Mph. It need very little travel on ailerons, with 75% exponential.

Comparison to my Spad Diamond Dust Clones:
I had designed other fully symmetrical airfoil deltas, and they flew level at 110mph with no up elevator. This new model however has it maximum wing thickness of 1 inch well behind the balance point. ( A good 3 inches.)


Why do I need up elevator on this model? Could it be a balance problem? (Doesn't drop or climb in corners)
Is this maximum thickness point the aerodynamic center?
Is this aerodynamic center back so far as to push the nose down?(i.e. lift the ass end up)
Should delta AC be at %25 of the cord as other models?
Should the maximum thickness point/AC be at my balance point?

Can anyone offer some insight into these questions.

My ultimate goal is to break 200mph with a delta. I'm not going to do it with this setup however.

Thanks in advance for your help.
Todd Murray
Ottawa, Canada

S3HD

My Feedback: (1)

What is the chord at the very centerline of the plane, and what is the tip chord? Is the centerline chord 22"?
Give me the values in inches and decimal fractions of inches, and I may be able to get you your aero chord, and Mean Aerodynamic Chord.

murrat

The root cord (center cord) is 22 inches. The tip cord is 4 inches.
The sweep distance back is 15 inches, to give a nice 45 degree delta.

Used this web page for balancing model:
http://www.palosrc.com/instructors/cg.htm

I entered these parameters as follows:


Enter Root Chord (A): 22
Enter Tip Chord (B): 4
Enter Sweep Distance (S): 15
Enter Half Span (Y): 15
Enter %MAC Balance Point: 11

This gave me the following output
Sweep Distance @ MAC (C) = 5.77
Mean Aerodynamic Chord (MAC) = 15.08
MAC Distance from Root (d) = 5.77
Balance Point @ Root Chord (CG) = 7.43


The only number I have used here is the CG of 7.43 inches for balancing the model.

Do you think my maximum thickness point should be at 7.43 inches as well?

Thanks

banktoturn

murrat,

I'm not sure about where your AC is, although it does sound as if your CG may be further forward than necessary, and this could be why you need up elevator. One thing is pretty sure though, a flat bottom wing is not the lowest drag choice you could make. If the bottom is flat, you probably have quite a bit of camber. To reduce drag at high speed, you should use an airfoil section with just enough camber to give you enough lift to fly straight and level, at small angle of attack. A good approximation of this is a symmetric airfoil, which would require a small AoA at high speed.

banktoturn

S3HD

My Feedback: (1)

You got all the numbers that I was gonna get already - as far as the max airfoil thickness placement, well, I not sure about that.

I don't think flat bottom is what you want for speed.

I'd try adding weight to the tail, 1/4 oz at a time. Then re-arranging the innards as best you can if it flys better then remove the weight.

That flat bottom wing could be requiring the up elevator - in effect making you fly with "reflex".

~JD

York

Murrat, is your figure of 45 degrees sweep from the 25% chord lines (as by convention) or from the LE?

murrat

Hi York
My 45 Degree sweep is along my leading edge.
regards