Eppler 1230 Trailing Edge

[lelievre12]

I am finishing my SQ2000 project which has molded (not foam core) wings.  The 'as molded' trailing edge on my main wing is a pretty fat radiused edge which is ~1/4" radius (1/2" top to bottom). Looks draggy indeed.

 

 

Looking at the Eppler 1230 design I see that the trailing edge is 'sharp' so the question is "what trailing edge are other folks using" on their Eppler main wings?

I am inclined to fair the rounded edge to make it sharper perhaps down to around a flat aft face/edge of around 1/8".  I could go even sharper perhaps by using a thin carbon undersheet with micro to fair the top edge. I would have thought the sharper the better but not sure.   I have seen a more blunt edge say squared to 3/8" like a Lancair?

Any advice appreciated.

 

[Kent Ashton]

I don't see what you're describing on this SQ2000 project.   http://www.n416.com/17.html  (pic)  I would guess the flow past the T.E. is already turbulent so it doesn't make much difference about the shape.  Couple of ideas:  You might bondo a strip to the top or bottom and make a flox T.E. that is a little sharper (then remove the strip).   Or make a V-shaped form on your work bench, use it to form a 2-layer BID "V" and flox the V to the T.E.

The Eppler T.E. is a little funky anyway the way the upper surface bends down to meet the bottom.  Maybe any mod would alter that.    Just curious, does your bottom profile employ the reflex at the last 1/3rd of the airfoil? 

Edited January 1, 2021 by Kent Ashton

[lelievre12]

Kent, thanks for the reply.

I will check the wing next time I am at the hangar for reflex on the underside.  

The N416 wing was foam core (like a Cozy/Eze etc) however my kit came with the alternative molded wing (Lancair style with strakes and no core) and the trailing edge is pretty fat. 

I think what I will do is bond a 0.5mm strip of carbon on the top edge then flox under to fill the edge. (whilst the wing is upside down). The idea is that the carbon will keep the edge resistant to nicks and hangar rash despite the thinner edge.   

I am expecting that a feathered edge will be less draggy but read that "Harry Riblett said something about cutting off the trailing edge at 99% of chord length is better than a point for the airflow." so this is confusing me a little. I know that the Lancair IVP I flew certainly had a squared trailing edge that was around 1/4". However my hunch is that this trailing edge is there to reduce high speed aileron actuation forces rather than to reduce drag.  

[Marc Zeitlin]

12 hours ago, lelievre12 said:

I am finishing my SQ2000 project which has molded (not foam core) wings.  The 'as molded' trailing edge on my main wing is a pretty fat radiused edge which is ~1/4" radius (1/2" top to bottom)...

Looking at the Eppler 1230 design I see that the trailing edge is 'sharp' so the question is "what trailing edge are other folks using" on their Eppler main wings?

Trailing edges should be neither rounded nor sharp. Both lead to vortex shedding, drag, and unstable airflow. You want the TE to be a flat squared-off surface, no less than 1/16" thick and no more than ~1/8" thick, ,although as you say many Lancairs have TE's with 1/4" thickness on the squared-off surface.

This is particularly important (not to have a sharp TE) on the ailerons, so as not to affect control forces.

[Voidhawk9]

[Marc Zeitlin]

Ha. Picture's worth (in this case) 75 words.

Interestingly, this (remove the section of the wing aft of the dotted red line):

is what the TE of my right wing looked like after the prop hit it on its way to Joshua Tree NP 9500 ft. below us in 2006. The prop took off the TE outboard of the aileron, plus a little bit of the TE of the aileron over the last 4 - 6 inches. Plus 2/3 of the lower winglet. Left me with this:

souvenir, stuck in what was left of the lower winglet.

While I was mostly concentrating on other stuff at that point, I did not notice any substantial difference in the flight characteristics of the airplane on the way to the landing at Desert Center.

When Mike Melvill flew me out to pick the plane up a few days later with a new prop and extension, as well as a bunch of AL tape, we taped up the TE of the wing to close out the exposed foam. It's called speed tape, right? He chased me home in the company Duchess, and I never went faster than 120 KIAS. Once again, there was no discernible difference in flight characteristics on the way home. After the flight, we ruminated on why anyone puts the last few inches of wing on the wing if it doesn't make any damn difference to how the airplane flies. But I fixed it back to the plans design anyway, not being an aerodynamicist :-).

 

 

[Voidhawk9]

That is very interesting. Obviously without a full test series we cannot glean too much from it, but it would be interesting to know what effect it has on top speed, min speed, etc.

Perhaps there is a balance somewhere between increased 'base drag'(what would be the proper term here?) from the 'trimmed' airfoil and decreased wetted area and weight.

[lelievre12]

Here is a shot of the Lancair IVP trailing edge with that flat cutoff. My recollection was that it was 1/4" to 3/8"

 

On reading some IP (Wainfan Patent) https://patents.google.com/patent/US4867396A/en?q=Wainfan+micro+flap&oq=Wainfan+micro+flap it seems as though the more aggressive cutoff on the trailing edge is more about transonic stability than low speed drag reduction. Something I guess a Lancair might worry about but less so for us!

Edited January 1, 2021 by lelievre12

[Voidhawk9]

That is a pretty airfoil. 🤤

 

I was looking at a Sting LSA recently, and it had surprisingly thick TEs too. Didn't seem to be a problem, apparently it performed very nicely (for an LSA).