Trailing edges - Sharp or Square - RC Groups
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Oct 09, 2017, 03:24 PM
More Pile-it than Pilot
mdennis's Avatar
Discussion

Trailing edges - Sharp or Square


I have always had sharp trailing edges on the wings and tail surfaces of my planes. That is the way is on full-size planes, so that's the way I figured it should be on my models.

But I have read that the squared off back might be better for objects traveling less than full-size plane speeds. The trailing edge of most planes is actually squared off at about 1/4" thick. Not that cars are any great comparison, the squared off back end seems to be making kind of a comeback and cars like Corvette's, Ferrari's and Lamborgini's have it.

Any ideas? Is there any evidence to really support the efforts to use tapered or sharp trailing edges or does it really matter?
From a scale standpoint, they look closer to scale, but I won't be entering any of my planes in Scalemasters, (or the state fair).
Square trailing edges are easier to cover and probably more durable.

Mark
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Oct 09, 2017, 03:35 PM
More Pile-it than Pilot
mdennis's Avatar
Maybe it matters to keep the airfoil on the main wing, but does it matter on the tail surfaces?
Most of the time those just have rounded front edges and aren't really airfoil shaped at all.

Just thinking.
Oct 09, 2017, 07:35 PM
Faster is Better
Slopemaster's Avatar
Square is my vote. Makes it easier for a better covering job too. I'm not talking big square, just enough.

Most modern pattern planes have thick square trailing edges.
Oct 09, 2017, 08:29 PM
Registered User
Daemon's Avatar
Given a choice between square and rounded, choose square.
Rounded trailing edges tend to create oscillating vortexes.

But tapered to a sharp edge is still way less draggy than square.
It wouldn't surprise me if the pattern planes use square trailing edges to
actually help slow em down on the downline and such.

Many years ago tapered balsa elevon stock was less available and more expensive
and a number of foamie kits shipped with square stock which most folks left square.
Flown against comparable (or same models) with tapered stock, they were slower.
I still run into this a lot on my EPO foam FPV models. Sharp edges are tricky in
injection molds so they often leave em a bit thicker and square. On early planes I left
em square, and on later planes I pinch em down to a taper, or add covering that wraps
over the trailing edge and creates a sharp trailing edge, and they fly better
(faster/quieter on glide). Also since most FPV planes use pusher props we find that anything
you can do to taper the trailing edge of the wing in front of the prop makes them
much quieter and more efficient overall.
Oct 09, 2017, 11:54 PM
U down with EPP? Ya U know me!
Aerogance's Avatar
Agree with everything Ian said. My experience has shown that it is easier to build with a square trailing edge and perhaps it would do better than a poor attempt at a sharp trailing edge. The main advantage I see with a thicker trailing edge would be for balsa surfaces, since the extra thickness translates to extra stiffness. If you add carbon to your balsa surfaces, then sharp would be the way to go. This is just from my personal observations.
Oct 10, 2017, 08:25 AM
More Pile-it than Pilot
mdennis's Avatar
Good information guys.
So if speed is what we are after, then we definitely want to use sharp trailing edges.

On something like a combat wing, where durability is more important than speed, then maybe it makes sense to use a square trailing edge.
Oct 10, 2017, 12:47 PM
Registered User
Daemon's Avatar
Maybe, but I still use relatively sharp trailing edges on my combat wings because
performance and efficiency still matter. I cover em with good clear packing tape.
They sometimes get a little crushed under the tape, but I re-shape it by hand
and squirt a little thin CA into it, and it's good as new.
Oct 10, 2017, 01:05 PM
average user
Ive found a sharp/pointy trailing edge makes the aileron more torsionally rigid.
Oct 10, 2017, 03:40 PM
Registered User
Square trailing edges to control surfaces give a soft or exponential feel to control inputs, whereas sharp trailing edges give sharper responses and are better for most types of soarer, as well as more efficient. Phil Kraft was one of the first fliers to use blunt trailing edges back in the 70s to make his FAI power aerobatic models smoother to fly. This was way before exponential became commonly available...
Oct 10, 2017, 04:14 PM
More Pile-it than Pilot
mdennis's Avatar
Quote:
Originally Posted by Ole slope
Square trailing edges to control surfaces give a soft or exponential feel to control inputs, whereas sharp trailing edges give sharper responses and are better for most types of soarer, as well as more efficient. Phil Kraft was one of the first fliers to use blunt trailing edges back in the 70s to make his FAI power aerobatic models smoother to fly. This was way before exponential became commonly available...
Interesting information that I had never heard before. Thanks
Oct 10, 2017, 06:31 PM
It's not going to build itself
TRISME's Avatar
I’ve seen increased performance after sharpening trailing edges on an arf scale ship. Sharp gets my vote. Look at a database of airfoils, I bet most are drawn with a sharp trailing edge.

Tom
Oct 10, 2017, 07:04 PM
Faster is Better
Slopemaster's Avatar
Here's an example of a square trailing edge on the X-15.

Of course it had a rocket engine for thrust.

Interesting though.
Oct 10, 2017, 09:48 PM
Registered User
Daemon's Avatar
Ya, but that's for an entirely different flight regime.
At hypersonic airspeeds (Mach 5+), there can be no communication of pressure between
the free stream air and the airframe. The air molecules just smash into any surface
visible from the forward profile and are forced outward around them, but cannot
rejoin any part of the airfoil behind the widest part of the profile, so it doesn't matter
at all what the trailing edge looks like.
They said the vertical fin of the X-15 had 60% of the area of the main wing, and produced
as much drag as an entire F-104 Starfighter, but was necessary to maintain directional
stability at hypersonic speeds.
It had additional panels that folded out to increase drag to slow down for landing.
Oct 10, 2017, 10:03 PM
IT'S NOSE HEAVY!!!!
cityevader's Avatar
Sharp OR square?
Officially, I'd say DO NOT DO NEITHER.

From my quarterback-aeronautical-armchair, I'd say that if one cannot get it "truly sharp" due to materials/construction/covering techniques, then of course square it off.
Oct 11, 2017, 12:03 AM
U down with EPP? Ya U know me!
Aerogance's Avatar
On my lightweight slopers, I tend to go square just for the extra bit of stiffness without the added weight of strapping tape or carbon/glass. If bagging the surfaces, sharp preferred since the stiffness will be there.


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