"Blunt" trailing edge
 

I've been looking at a few plans for pattern plans. This will be my first "modern" pattern plane when I decide on which one I want to build. My only other was a Phoenix 5 (I think) in a .40 size. It had retracts, a K&B 6.5 front intake rear exhaust with a pipe.

I've noticed that the trailing edges of the wings, stab, and rudder on many of these plans are very blunt. This is something I've not noticed before and was wondering what the advantage is. In the past when I've built from scratch I've always tried to get the trailing edge as sharp as possible. Have I been doing it wrong all these years? Thanks!

Ken

Very simple,
The blunt trailing edges are a design factor to add drag. Todays pattern planes are drag buckets. To over simplify the concept, the more drag at the rear, the more the back of the plane will want to stay behind the front. Modern aircraft have plenty of power, so the added drag is a plus. Directional stability while under power, and easier to maintain speed on the down lines.

I would think that the blunt edge would make the controls mushy.

Ken

Not mushy, just less sensitive around neutral

Correct on all accounts. When repairing/rebuilding my 40% Carf Extra one of the things I did was to chop off 1/2" from the trailing edges of both the rudder and elevator then cap with new fiberglass. The result was a 1/4"-3/8" blunt trailing edge. A noticeable improvement in tracking resulted and the tail wag that these airplanes are famous for was eliminated.

Actually, many recent designs have the trailing edges flared to increase thickness, especially the rudder. Check out the designs on the F3A Unlimited website or BJ Craft or Oxai.

Well, I guess it must work 'cause there are a bunch of airframes with the blunt/flared trailing edges. I guess function leads form in this case. I wonder what the desigh might be if function was the primary concern and not a "pretty" airplane was to be considered????????

Thanks for the info gentleman. "I've been schooled". :):)

Ken

The actual reason is it creates a turbulent area behind the control surface which helps keep airflow attached on the surface itself, especially as deflection increases.

Fascinating! Any further info or reading on this?

The thick trailing edge has two properties,(just my thoughts)

As stated above, it reduces the sensitivity of the rudder around neutral but does not reduces the effectiveness, this comes from full scale aviation, Cessna Citations have this, but they added a "T" to the trailing edge on the lower portion of it, or in some Bombardier jets the trialing edge is about 3/4".

The other is there is a small cavitation produced when the T.E. is thick enough, and this small vacuum tends to try to suck the airplane back thus reducing its speed.

Regards