KF Airfoil CG ?

[alan0043]

Hi Everyone,

I have a question on where the cg is on an KF airfoil ? I have been during some searching on line with no luck. The wing is going to be a standard shape (rectangular). No sweep design. I was thinking about 35% from the leading edge. How does that sound ?

All input is welcomed,
Al

 

[Merv]

....I was thinking about 35% from the leading edge. How does that sound ?...

It sounds a lot tail heavy.
The CG on Kf wing is in the same place as a regular wing. The CG should be 25-30% of the MAC, (Mean Aerodynamic Chord). On a rectangular wing with no sweep, the % MAC is same as the % of cord. In other words the CG at 2.5 inches from the LE on a wing with a10 inch cord is 25% of MAC.

The CG at 25% will give you a very stable flying plane. At 30% you need some advanced skills. At 35% your plane will be very much, a flying squirrel.

Remember the old saying, a nose heavy plane flys poorly, a tail heavy plane only flys once.

 

[alan0043]

Thank you Merv. Your example was great.

 

[telnar1236]

Only exceptions to what Merv said are for a flying wing/delta and for some T-tail designs. On most designs the horizontal stabilizer can take some of the slack of keeping the plane stable but on flying wings and deltas there is no horizontal stabilizer, and on some T-tails the wing can blank out the stabilizer which means it doesn't do much. For designs like that, 25% of the wing is the absolutely furthest back the CG can go. A flying wing with a 25% CG is neutrally stable which means it is wild but is at least not actively unstable, while any further back is unflyable. Generally, for flying wings and deltas, the CG should be 15%-25% of the MAC, so 1.5" to 2.5" in Merv's example.