Bad Driver

Hi guys,

I was just wondering if anyone could comment on the differences between flight characteristics of a pattern/aerobat type plane with three different style wings:- 1] Swept-forward - straight L.E., 2] Straight - spar straight, and 3] Swept-back - straight T.E.

I have a design for a small pattern plane that has a straight T.E. (swept-back spar) and was wondering how it would fly if I built it with a straight spar, tip-to-tip?

Many thanks!
B.D.

wellss

Generally, you wouldn't want any sweep in a pattern plane as it makes for yaw/roll coupling

sweep back = similar to dihedral
sweep forward = similar to anhedral

And unless you plan on flying at transonic speeds, there is no benefit to sweep (the plane won't fly any faster).

Sweep forward does have the benefit of having very desirable spanwise stall characteristics. That is, the wing root stalls first. But, you will have reduced lift capacity of the wing as a whole with a mushy, gradual stall. Again, probably not the best choice for a pattern plane.

BMatthews

For the amount you're talking about it's not a big issue for sport flying. But because you're moving the wing area forward with this change there may be some minor CG issues. If you're being fussy then use one of the online MAC calculators (see the sticky FAQ's at the top of the listings) to find the current MAC and then modify the wing and move the root position so that the MAC is in the original place.

If this will be for contest pattern flying then I'd leave it alone. The slight amount of sweep may well be what helps the model track well in the various attitudes.

LouW

I agree with Bruce. Changing the sweep from what is designed will not likely improve it.

A straight trailing edge with a swept leading edge is pretty typical of pattern airplanes. The moderate sweep has some definite advantages. As the previous post noted, sweepback acts similar to dihedral but with an important difference. It only acts when the wing is at an angle of attack.

Normal dihedral acts to provide positive lateral stability when the wing is operating at a positive angle of attack but in inverted flight, where the angle of attack is negetive, it is unstable laterally. The dihedral also continues to operate in knife-edge flight where it is called roll coupling.

On the other hand a swept back wing provides positive lateral stability when operating at a positive angle of attack and it is also positive when the angle of attack is negative (inverted flight). When the wing is operating at zero angle of attack, such as in knife-edge flight, there is no dihedral effect, hence no roll coupling. Again, as Bruce said, this enables the ship to track well in various attitudes.

A straight flat wing has little or no roll coupling, but also has no lateral stability, which requires a little more active thumbs in all attitudes. The swept forward wing has little roll coupling but is somewhat unstable laterally in all attitudes, which requires even more thumbwork.

Obviously all three configurations can be made to fly in an acceptable manner. But for a smooth groovy flyer, your best bet is to build it like the plans.

Bad Driver

Gentlemen,
Many thanks for your very educational replies.

To be specific about the model it is a small, 48" 'pattern' plane called Chilli Breeze (there's a larger version called Chilli Wind too) which was a free-plan in a UK mag a few years ago. Originally designed for .25-.36 IC motor I intend to electrify mine.

The reason I asked about swept-forward/back, etc. wings is because I was thinking it would be easier for me to build this model if it had straight spars (i.e. no sweep) and I could therefore use a straight dihedral brace. Original design does include approx. 1.5 degrees of dihedral per wing panel; LE sweep-back is approx 3.5" to give a straight trailing-edge. FWIW, root chord = 10.8", tip chord = 7.25".

I am by no means skilled enough to be a pattern flyer so I am only looking for a good sport model. If the change in wing plan layout has only a minimal effect on the flight characteristics with the CofG checked & adjusted - as per Bruces post - then I think I will give it a go. It would be interesting to also build a set of original wings at some point and try them both

Thanks again!

Tall Paul

I have those plans also... I would build it just the way the plans lay it out.
Ease of construction should take third place to flying well, which it is designed to do.
Modifying the wing would mean extra effort into making the plane fly as good as it can

BMatthews

I don't know what the plan looks like but there's no problem using a swept spar. You just need to use the right sort of joiners. In my sketch here the spars are joined at the center with a flat delta shaped top and bottom joiner. Then standard webbing is used to box the whole works in. This is actually the proper way to do a spar join in that it joins the spar caps togehter with more material out at the surface rather than a relatively whimpy piece of plywood. where only the outer edges of the plywood are doing much.

There'll be some sanding of the points on the delta joiners needed to allow for the spars being tapered towards the tips but it's minor.

Bad Driver

Tall Paul,
O.k. maybe I should build it as designed to begin with and then try a modified wing at a later date.

Bruce,
Thank you kindly for the advice re. the spar joiner - I'll give it a go.

Cheers guys!

stek79

I don't agree with that statement. It is a common mistake to say that when the plane is in knife-edge, in a climb or in a dive, the wing is at zero AoA... the AoA is forced by elevator trim and hence is alwais the same, provided that the elevator trim is the same.

To prove that, one of the way to trim the airplane pitch behaviour during KE is changing the position of the CG, which let's you chenge the elevator trim.

BTW, interesting thread!

LouW

In knife-edge flight the wing must be producing no lift force or the plane would be moving sideways. It would have been more accurate for me to have said the wing is operating at the zero lift angle of attack which for a symmetrical airfoil happens to be zero. If the trim is set for level flight, knife-edge will require some down elevator to hold the line.

Angle of attack is set by trim and elevator position. Every deflection of the elevator changes angle of attack.