Good Morning to all

Could anyone please explain the difference in flying performance of the two types of wings showed below?

I 've seen many combat planes with the upper wing, but the GREMLINS COMBAT PLANES have all the lower shape.

Is there any article on the Net explaining the theory of the two types of wings?

Thanks

George

I've seen plenty of planes with both, as well as the third, where both the leading edge (LE) and trailing edge (TE) aren't straight, instead the 25% of the wing chord point is a straight line.

In theory, the straight leading edge will have a softer, more predictable stall, and may allow a plane to turn a little tighter as a result. However, in the real word, the effect is not a large one. Instead, the straightness of the LE or TE often has more to do with construction techniques, Center of Gravity (CG), and so on.

Aerodynamically speaking the straight leading edge actually results in a wing that is "foward swept", since wing sweep is measured at roughly the 25% point of the airfoil. When the LE is straight, the 25% point moves forward as you get to the tip. If a wing has much forward sweep, then you can get in to trouble if the wing twists in flight. A wing with a straight TE and swept back LE will tend to resist this twisting problem. I'm seriously over-simplifying here, and I don't think it's enough of a problem to worry about in planes the size of combat planes.

In full-scale aviation, sweeping the leading edge back helps in high speed flight, but our models don't go fast enough for that to matter.

If its important to get your balance point as far forward as possible, then the straight LE is better.

As Kirk mentioned, the sweep of a wing does dramatically change both the CG and the aerodynamics of the specific design of an aircraft. The forward swept wing usually suffers more aerodynamically speaking due to increases of drag and compensation for aft CG's. There are a couple different aerodynamic factors that contribute to the decrease in performance which may cause a slight difference for an aircraft.
First and most obviously, there is more LE surface perpendicular to relative wind with a forward swept wing resulting in more parasite drag, and therefore a decrease in performance. Second, the forward swept wing even though more efficient at producing lift while flying at lower speeds, suffers once again to a different type of drag called induced drag. Induced drag is a direct bi-product of lift. The more lift a wing produces, the more drag it will also produce. This is one of the main reasons why trainers with high lift producing wings fly slower and require much more power to maintain a certain speed when compared to a symmetrical airfoil. Their highly unsymmetrical wing produces large amounts of lift therefore higher amounts of drag. This is why you see higher performance aircraft with thin symmetrical swept back wing which leads me to my third and final aerodynamic point. Each specific wing produces lift on one focused portions of the wing called the center of pressure. The center of pressure is usually aft of the center of gravity on most aircraft (25% Mean Aerodynamic Chord or approximately 25% of the way back from the LE). When the wing travels through the air at a faster speed, the center of pressure will begin to shift rearward resulting in less pitch stability and eventually less yaw then less roll stability at higher speeds. The aft swept wing allows for this shift and compensates for the center of pressure movement making high speed aerodynamics possible. However as Kirk mentioned above, these factors will not take affect until much higher speeds (Mach range).
To sum it up however, wing shape will probably not affect your combat bird as much as you think when compared to other issues like center of gravity and aircraft weight. Each aircraft has a specific envelope or range where the CG can exist safely. Paying very close attention to these will prove to be much more valuable when flying. Overall, I prefer a solid lift producing and stable wing. Forward swept wings or rectangular types will generally produce these favorable qualities. It requires less aerodynamic input, therefore resulting in less drag making the aircraft faster, and overall ease of flying. Aft swept wings will be more difficult to fly at slower speeds and probably less of an advantage throughout all aspects of RC flying.

Thanks all guys for your help. Though I was absent for a long time and just returned, your information will be very helpfull to me.

It seems that it is impossible to find combat planes in Europe (apart the planes from "GREAT PLANES" that are rather expensive), so I decided to experiment with the "SICKLE" project, which I found in SPAD site. It has also a forward swept wing, which - I understand - you consider better for combat use.

Since I do not like wings made from "COROPLAST" I ordered the wings to a greek firma - it will be done from blue, heavy, strong foam.

The wing will have a smaller chord at the tips (as it is forward swept), but the airfoil will also be thinner at the tips, hope this will not be a problem.

Another change - the pipe tube can not be found here, so the fuselage will be constructed from 3 mm ply.

I will keep you informed on the results ! I hope that this construction will fly, at least!

Regards

George