Alternative airfoils

[Cookie the Dog's Owner]

I decided to build a clone of the old Astron Falcon. When it came time for the wings, I didn't feel like sanding airfoils into 3/32 balsa--too much like work!--so instead I built the wings with a Kline-Fogleman airfoil. (https://en.wikipedia.org/wiki/Kline–Fogleman_airfoil) It's pretty easy to build: cut a full-sized wing out of 1/32 balsa, cut another "half-width wing" that runs from the leading edge to 35% of chord. Glue a 1/32 x 1/32 strip to the "trailing" edge of the half-wing, then glue the leading edges of both wings together so that the smaller one sits on top of the larger with the 1/32 x 1/32 strip between them. Round off the leading edge and you're in business.

I haven't had the chance to fly it, or even go outdoors for a serious attempt at trimming yet, but on short indoor flights across the living room it seems to glide well.

Has anyone ever tried a K-F airfoil on a boost glider? If so, how did it work for you?

 

[Chuck]

Looks interesting. Can you post a picture or two of your work? Thanks

~Chuck

 

[BABAR]

Interesting. Comparing #s 2 and 3 to 1, they seem to be upside down, or at least counterintuitive. Sure looks like it would save a lot of sanding, a lot of mass, and be pretty darn strong as it would be laminated, especially if you use balsa laminated in orthogonal grain directions.

 

[fyrwrxz]

Yes, I built a shuttle derivative about '84 with cranked delta, winglets and canards.with the KF about 48% due to hollow aft body and solid nose cone. I did follow a NACA foil outline with the step in shadow on top. It did seem to penetrate a little better and whether by design or not, showed little of the classic delta wing rocking (no dihedral at all). It transistioned off the carrier quite cleanly and came to glide slope rather quickly. I think I just loaded the wings too much and it was your basic lead-sled with a low L/D and high mass. It was just brutally fast on recovery and was abandoned after a dozen launches. I found the carcass worse for wear in the garage the other day. sorry this 'puter don't do pics. It's interesting to note I lost the first Falcon I flew ('66-67?) due to perfect trimming and no built in rudder deflection. It was last seen at 500' headed to Florida. Sad 'cuz that was a nice bird. Se if you can find q copy of "The Perfect Paper Airplane by them, it's worth the read. It also has a few plans you might find interesting. Good Luck!

 

[powderburner]

When I was in college this "airfoil" hit the news with all sorts of miraculous claims (stall-proof, low drag, high lift performance, etc). For my senior project (back in 74-75) I made two-dimensional wind tunnel models and took actual measurements with pressure taps and wake rakes. I tested step location (as a percent of chord), wedge angle, and leading edge radius. My models had a nominal chord of 24 inches and were tested at q of around 300 psf.

I compared my results to reported data for the 23012 airfoil, which is quite literally from the era of the Wright brothers (and has been bested by many other subsequent airfoil designs).

The K-F has higher drag, far less stall resistance, and a lower lift coefficient than the 23012. The K-F generates more lift if you turn it upside down. The pitch moment coefficient is worse in both magnitude and slope of the trendline.

I can only attibute the claimed advantages of the K-F to three-dimensional effects of a highly swept wing planform. In other words, the airfoil really doesn't have any significant effect on the performance of a paper airplane.

If you have ever seen Monty Python and the Holy Grail, you should be familiar with the phrase "RUN AWAY! RUN AWAY!"

 

[powderburner]

For model rocket boost gliders you will do quite well with simple, flat plate style wings.

Boost gliders (and rocket gliders) have glide speeds that are very, very low and the consequent Reynold's numbers fall far below the range of RNs where airfoil data is tested and measured. Attempting to use airfoil data intended for "real" aircraft will have you extrapolating several orders of magnitude outside of their RN database.

Boost glider RNs are quite literally down in the range of bumble bees, mosquitos, and other flying bugs, which have flat plate wing "airfoils" and also have (relatively) extreme surface roughness.

 

[Cookie the Dog's Owner]

Here's a couple of photos. The plastic nose cone is what I had lying about. It's not glued in and I plan to replace it with good honest balsa before trying to fly it.



I'm aware of the critique of the K-F airfoil; I built this one this way mostly because it was easy to do. If/when I get this one flying properly, I plan to build another Falcon or two and try alternate wing designs, just to see if one flies any better than the other. It hit me you could make a pretty decent airfoil by cutting two wings out of 1/32, glue a 1/32 square "main spar" to one of them, then laminate the other one over top of them and sand down the trailing edge.

 

[BABAR]

I have built this and had several good flights, but also had problems with the main spar breaking. I am not sure, but I think it may have something to do with having a single vent hole, which "snaps" the rocket around at ejection time. When I put in TWO holes, one on the left and one on the right, it solved the broken spar issue.

Has anyone had any luck up scaling this? I have tried this and the CiCi as up scales and been less than successful on boosts. I am not sure is simply proportionally upscaling is the thing to do with front engine boost gliders.

Good luck on your flights

 

[burkefj]

I've used them on a few of my depron rocket gliders, not so much for any performance advantage as my wing loadings are so low that a pure flat plate works fine, but mainly to hide the spar that I glue to the surface of the 6mm depron wing and add a bit of stiffness and it is simple to do. I simply glue/tape the spar to the rear of the forward doubler plate. I have not seen any adverse effects and stall characteristics are similar to a pure flat plate on my designs...

Frank

 

[BEAR]

Your Falcon is a project, near and dear to my heart. I have built one also, but it is a 2.3X upscale. I fly it on an "E" with a motor adapter in the motor pod, and a streamer on the adapter so I can get it back. I have only flown it a couple of times, and was quite spectacular. It does tend to angle off from the launch rod at about 6 degrees on take-off and does a very impressive barrel roll across the sky while under power, but we are working to reduce that and get more elevation. It glides really well, though. Hear is a picture with a Nighthawk and a 1/2A called a Bumble Bee. Wingspan of the upscale is about 27"

 

[Cookie the Dog's Owner]

That upscale looks pretty!

 

[Cookie the Dog's Owner]

Finally got the chance to fly it last evening, and it flew magnificently on a 1/2A3-2T. A little too magnificently; it glided over the trees and out of the football field-sized park I was flying from and into the neighborhood, suffering a chip in one wing on landing.



I built a combination 13mm adapter/recovery system for it which worked perfectly.

 

[Cookie the Dog's Owner]

On its second flight, the Falcon caught a thermal and sailed out over the subdivision, never to be seen again. Must build another.

 

[BEAR]

Yeah, that's the Falcon I know. Yours looks petty good, and looking at your motor mount, which looks just like mine, I did not know for a moment what I was looking at until I read the comments. For all I knew, from your picture, that was an "E" motor mount for a bigger tube. As mine used to fly away also, I used to build two or three at the same time so that I had back-ups. I also made sure I wrote with a ball point pen my name, phone number, and e-mail address somewhere and on the eject-able motor mount, so that I could recover everything.