When I test flew my last prototype, I dialed it in without KFs and then with 40% KF4s. The designers of the KF4s specified 50% but the community seems to have adopted 40% from the KF2. When I added the KF4s I had to move the CG forward. After reading extensively, the center of pressure (COP) therefore moved forward with the addition of the 40% KF4s. The CG I was using was calculated based on mean area chord and theoretically the optimal CG. I honestly preferred the feel of the balance better at the MAC CG rather than the forward CG with the 40% KF4s.
For the present build I am going to make 50% KF4s which should move the COP back and into alignment with the MAC CG. Theoretically at least, I'll soon see and report back.
Stephan, agreed. I've found the 50% recommendation in the serious literature as well. What interests me is the step shape at the back of the KF airfoil. And, I've not seen much on chord thickness. I've built and flown a Morris Hobbies TopCap that uses a very thick airfoil. From what I've read the KF does improve separation at high angles of attack. I'll send anything I find that looks to be well grounded.
ReplyDeleteDave,
DeleteThe KF is truly fascinating yet totally perplexing. It is especially difficult to understand KF design on a swept wing with the central chord behind the CG. I wish I had a wind tunnel to try the various KF sizes on these parkjet swept wings. My previous testing was done on planes that weren't trimmed properly. Early on I believed the "no trim is the right CG Kool-Aid". Since learning how to properly trim these plans it has been an interesting experience with the effect of the KF on the COP. Or is it just the KF preventing air separation? On the latest prototype build, I am going to cut my KF into strips so I can add on and hopefully figure out the best KF to keep the COP in line with CG.
Sometimes I feel like I'm trying to figure out aerodynamics that don't apply to foam.
Stephan
Stephan, I think the KF airfoil is useful for flat plate wings if I'm absorbing the information that is correct. I think your method of adding strips is a great idea. As the KF appears to show benefit at high angle of attack I'd suggest looking for differences when you're climbing. Telemetry would help so we could time the rate of climb. This is where I have to admit I did pick up Spektrum's telemetry module and altimeter sensor. I'd be worried about weight. I'll weigh the airborne bits.
DeleteDaveM
Dave,
DeleteTelemetry is very cool stuff. Unfortunately it is all windows based and I use a Mac. On of these days I'll run Windows on my Mac, oh the dark side!
I watched videos last night, reviewed KFs on RCP planes, my planes, and felt like it was time to move forward. I wanted to get started on my Mig PAC FA hybrid so I went with 41.5% KFs at the root which line up with the CG point at the root not the center and they taper to 35%. They are basically the standard RCP KF2 and 4 from v3 and 4s. I'll get to testing one day when I have run out of other projects. Someone really does need to study various KFs and their effect on center of pressure.
Stephan