Gurney flaps on tow planes.

[edfmaniac]

I have an AMR Trainer 50 that I bolted a big outrunner on and it seems to lose lift right at touchdown sometimes, maybe due to lack of thrust at zero throttle? I'm thinking Gurney flaps will help, and prevent me from having to leave a little power in, so I bolted on some as an experiment for the next time I get the tug out to the field. Seems like most of the wind tunnel experiments that I read used flaps in the 2% of chord range so I went with 1/2" flaps on my 24" chord wing and they extend from the root area to about 2/3's span. I had reduced ailerons to 1/3 span on a previous modification. Hopefully it makes the plane fairly fool proof. It already flies great but I'd like it to fly a few mph slower before full stall. Maybe a CG change would help too but I always wanted to experiment with Gurney flaps so we'll find out soon.

I'd love to hear about any experiences by others. Conventional flaps are certainly a better option for glide path control but I'd like to hear if anyone has used them and what their opinion was.

[lenb]

sounds nose heavy to me, when the speed drops off the nose wants to drop in and pound the ground. Shifting the cg to a perfect or slightly tail heavy will help with the landing flair, however if you go too far the plane will float forever.


LEN

[edfmaniac]

Thanks. I'm right on the spar but I also put a lifting horizontal stab on her at some point so no telling where the CG should be. I'll move some batteries around after I try this experiment. Found some real time photos of GF effects on an airfoil. Pretty neat visualization.

[edfmaniac]

If the 2% Gurney flaps slowed the stall down, it wasn't by enough to offset the side effect of increased apparent incidence. I had to dial in quite a bit of down elevator to compensate, and the plane needed more and more down trim with increased power. Might be time to build some new hinges and put the big fowler flaps back on. I removed them before converting the plane to electric because the hinges weren't strong enough for the vibration of an 80cc ZDZ single. Landings were so fun with those things at full droop.

I should probably remove current flaps and then play with CG first though.

[ARUP]

How about putting fixed slats on the LE?

[edfmaniac]

That seems like a whole different experiment I don't have motivation to conduct. LOL Hopefully just a return to a normal wing and a CG adjustment will do.

[edfmaniac]

Balance was definitely too far forward. After removing the GF's and shifting the weight backward, it's a completely different airplane on approach. Even with no throttle and a windmilling prop, the glide slope is very good at low speed and I have plenty of elevator authority all the way to touchdown. And I tried to get the plane to drop a wing in a prolonged stall, but it just rocks back and forth a few degrees while floating down with full up elevator. I could probably go back a little more but plane seems happy where it is. I feel comfortable handing the controls off to someone else if I need a tow pilot on short notice.

[edfmaniac]

Here's some video of the stall test, a few landings and better video of the surround area. Clouds were begging me to break out a glider but I wanted to get this tow plane dialed in. Needs a few more things before our Big Bird event at the end of the month. Test starts at around 2:40 and lasts about half a minute. https://www.youtube.com/watch?v=Hji0...ature=youtu.be