I like the look of radiused leading and trailing edges on my planes but notice that most foamies are left square. Is there a difference in flying or finishing qualities? I round mine over but it seems to hurt the paint jobs, having a darker hue where the sanded foam soaks up more paint.

Thanks
Dave

The rounded LE and TEs results in minimally less drag, however, more drag can be a good thing when flying in small spaces. In addition, many planes have carbon spars along the LEs and TEs, making rounding either impossible or severly weakening the wing in the process of doing so.

Check out www.rcxplanes.com

They have rounded edges. They seem to fly more predictably than other foamys partially because of the design and partially because of the rounded edges. The planes with flat leading edges seem be less predictable in pitch and KE. The rcx planes are very smooth in flight characteristics.

This is an area with which I have some personal experience and study.
The first thing the rounding does is strengthen the foam edges by removing or greatly reducing the stress riser focus points there for both 6mm and 3mm Depron.
If you think hard about airflow, the rounding has many benefits. Among them are increased lift at any AOA and reduced unwanted darting in gusty conditions.
Using 3mm Depron for wings leaves little choice but to cap the LE and TE with carbon.

Bud

Rounding the trailing edge is a bad thing. It actually reduces control effectiveness as the airflow has more trouble staying attached to the surface.

Rounding the leading edge should be fine.

At low speed, and especially when the wing is stalled in a 3D manover, the LE shape is going to make virtually no difference.

As dcronkhite says, rounding the trailing edge will reduce attachment of the airflow - the full size Extra 230 (at least) has square trailing edges to help to maintain airflow attachment.

Mike

Originally Posted by RobinBennett

At low speed, and especially when the wing is stalled in a 3D manover, the LE shape is going to make virtually no difference.

In a perfect world and for those of you who only fly indoors this may be close to correct. When airflow is slowed the affects are minimized but the affect is still present. We don't live in a perfect world. When you step outside and encounter any type of breeze this is absolutely not correct.

Anytime airflow encounters a 90 degree step on a surface or when it is being split between two sides of a surface as with a wing, it must move the same dirrection and distance as the step plus half the distance of that step farther in the same dirrection before it can BEGIN to follow its original path. This is true regardless of the airflow being generated by airspeed of the craft or wind speed and dirrection. Changes in angle of the step change how far and at what angle opposed the air must travel before airflow can follow its original path and what distance of the chord it must be detached from the wing surface before becoming laminar again. These vorticies between the airflow and the wing surface at the LE that are created by the airflow being sheared in an opposite direction from its normal flow are what cause the darty behavior.
Now add in a slight gusting breeze and start thinking dynamically. The 90 degree step will have greater changes with gusts of "airspeed" in the size and shape of this created vortex causing larger changes in lift and drag than that of a rounded LE. Now think about this in a fully stalled 3D hover. When the wing tips encounter this wind speed generated airflow from the side, airflow is split in a similar manor even if it is 90 degrees to that of a flying wing and all of the same rules apply. So now you have these vorticies causing unpredictable lift and drag on the tips of the wing causing more unpredictable changes in the crafts rotational position around the prop center than the rounded wing tips. Every edge in every instance that encounters airflow will be affected the same way every time. It is simple physics that can't be escaped.

Bud

Originally Posted by dcronkhite

Rounding the trailing edge is a bad thing. It actually reduces control effectiveness as the airflow has more trouble staying attached to the surface.

Rounding the TE still adds needed edge strength and promotes laminar or attached airflow and is definitely not a bad thing.

Lets think dynamically here again for a minute. the most effecient aerodynamic shape is a teardrop. part of this is due to the airflow being properly split on the front with minimal vorticies being created.The other part of this is due to the rejoining of the split airflow with minimal disturbance and vorticies being created at the rear of the teardrop. This disturbs the air the very least as the teardrop shape passes through the air.

When airflow tries to rejoin at the TE of a wing or control surface in straight and level flight, the teardrop not rounded or square TE shape aids this most and creates the least drag and vorticies promoting laminar airflow. A square TE creates a vortex dirrectly behind the TE that changes size and shape and amount of drag with airspeed. A rounded TE creates a much smaller vortex.
This vortex actually creates a small area that makes the surface less effective at minimal control input. This is used and even exagerated on surfaces that display too much control with minimal input like linkage or servo play to stop a craft from unwanted wandering or line hunting. Some use a split piece of fuel line or halves of carbon tube along the TE of an IMAC planes rudder to stabilize and remove some of the control authority at very small increments of surface angle change. This is due to the very efficent and usually thiner airfoil designs used that promote laminar airflow and increase control authority.

Physics are physics and overall, the 10 minutes of work rounding your edges has many benefits over a square LE and TE and will result in a much stronger, efficent and predictable aircraft in all flight conditions.

Bud

Rounding flat foamieTEs screws things up, whether physics agrees or not. I have flown one with and one with out back to back on two otherwise identical planes, and the rounded one just flew much worse (tracking and control authority sucked).

We will dissagree on this point then.
I have done the same back to back testing with absolutely the opposite result. I would investigate something else in the planes design that made it require the vorticies to stabilize the plane.

Bud

On the small planes we make out of 3mm foam I think the value of rounding leading or trailing edges is a moot point. The Re is so low and the flat edge is so small that I don't see how there could be a dramatic difference one way or the other.
I'm of the opinion that leading edges should be rounded on the heavier foamy planes and those where the thickness of the foam is more than 3mm, and trailing edges should be left square regardless of the thickness or size of model.
The flat or square trailing edge has become the solution on large aerobatic aircraft as it improves control authority, and can reduce flutter. Rounded trailing edges have been known to cause all sorts of trouble.

The planes I flew were 6mm, so that may have made a differance.

My planes flying surfaces are also 6mm. I was always taught that radiused leading edges promoted airflow attachment, thus more control and more predictable stall breaks. In the old days we used to make leading edge radiias large as possible and adjust stall characteristicswith stall stripsin the LE as on full scale planes.