What effect does the shape of the wing tips have? I'm fairly new to RC and have noticed some smoothly rounded, others follow the contour of the wing with the bottom flat and angled, and some are chopped off straight. Can anyone explain? TIA...

Bill Wheeler

The effects are too small to be noticed, except at the very high speed end, where racers go.

The idea of changing tip shape is to reduce induced drag - that is drag from the tip vortex generated in the production of lift.

in straight and level flight a racer is unlikely to be creating much tip vortex drag because lift co-efficients will be low - however during turns lift co-efficients will increase admittedly.

The variance in shape of tips in models outside the sphere of competition is governed by styling largely.

The likes of Andy Lennon claim an advantage in using diffuser (tip curving up) and retarder (tip curving down) wing tips as seen on certain full size aircraft.

I've heard a rounded tip is worse drag wise than a cut-off tip.

I had a trainer with the tip shape you mentioned with the bottom flat and angled up. Terrible tip stall into a spin at the drop of a hat.

Thanks for the responses. I was curious since the various tip shapes did not correspond to any particular type of plane.

Bill Wheeler

I suspect your tip stall was due to other factors. I've used angled up tips with great success on two of my designs and seen this method on many others over the years. The classic use of this being a flat bottom airfoil where the tip outline is a stretched version of the airfoil with the bottom beveled up to meet it.

OK enough nonsense go to this link on [link=http://www.zenithair.com/stolch801/design/design.html]wing tips[/link]. Go down a little and it shows and tells how different kinds drooped, raised, square, rounded, and the best for a trainer horner wing tips work. Check it out it also tells how STOL aircraft work.
later Zach

Oh yeah I have a dura-plane 20 trainer (more like a flying rock than a plane) with horner wing tips and there is a noticeable improvement in flight. Also before the new tips I rebuilt the PVC fuse with 1/8 inch marine ply wood and lost about 150g off the AUW. Ply is much more shock-resistant than PVC ecpicaly when it's cold out.
later Zach

nonsense?

which part?

however your link is interesting albeit relating to a full size aircraft design- where reynolds numbers are a lot higher than the realms of models.

my point still stands that for a model which is not involved in contests where reducing induced drag helps you win, tip shape is not important. I.e. in silent flight or free flight contests you need to reduce induced drag as much as possible.

If hoerner wing tips where best then all competition silent and free flight models would use them.

As a matter of interest what's the stall like on a 60 inch span piper cub? (i've been told they can have stall vices - but you wouldn't expect that with a flat bottom section and a rectangular planform).

Tips on this size model -- meaningless.
But drop the wing loading down on an aerobatic model and change from a flat /rouned tip whatever - to a slashed from bottom up at say 45 degrees.
this will change effective dihedral.
We have done this where more dihedral was needed to control adverse roll with rudder setups. It does work.

I agree with DickHanson and BMathews, some tip changes have a noticable effect on some models, especially when they are 40 size or larger.

They will make some difference even if it is not exactly noticeable. Hey building is 75% of the fun so why not go the extra yard and give more efficient wing tips instead of square tips (hey they look cooler to). Hey most RTF trainer have sorta crude horner tips (crude in the way they are flat not slightly curved ware they should).
later Zach

Thanks for all the replies. FYI, the question was prompted by the construction of my first kit - a CG Tiger 60. The wing tips are to be carved from triangle stock and I was looking for input on how different shape tips would effect the plane.
I have decided to take the lazy way out and extend the airfoil on top and smooth the existing 45* angle on the bottom of the wing tip. This leaves about the last 20% of the tip with a rounded curve from top to bottom while the rest is a 45* angle to the top of the wing.

Bill wheeler