What are the advantages / disadvantages in wing planforms

[Owdlad]

Simple question in what is the difference in lift / aerodynamics on these wing sections......

1. Tapered wings
2. Parralel wings
3. Eliptical Wings
4. Sweapt back wings.
5. Delta

Intrigued that's all

[Ernie]

It's an interesting one Owdlad, A big problem with wing planform, is the airflow at the tips, so, the best form is a wing without tips

ernie

[Martyn K]

...or the wing tips separated as much as possible - i.e. high aspect ratio

I thought that BEB covered most of the design considerations of wing planforms in his series in RCM&E about 12 months ago.

The question is huge and not going to be answered in a simple box.

However tapered wings have 2 advantages over parallel chord wings

1. The mass distribution is better. ie the mass is centred inbound, not halfway out each wing - has less inertia where you don't want it

2. The load distribution is a better match to the shape of the wing, but not as good as an elliptical wing

3. Elliptical wings are harder to make and the very narrow tips (at model size) have negligible lift properties but they do have reduced vortex losses

 

Anyone want to expand on that?

M

edit ...and that was my 3000th post and I nearly missed it

Edited By Martyn K on 28/07/2015 15:05:57

[The Wright Stuff]

Yes, as Martyn says, to answer your question properly would fill a book. For the sake of a general answer, this is how I tend to think of it.

Simply, the flight envelope can be partly estimated from the wing loading, among other things, which then translates to wing area for any given weight of aircraft. For this reason, it is convenient to think of the differences in terms of what happens if you keep the wing area constant, and vary the shape and dimensions. So the simplest option is a parallel wing. If you taper it, you are reducing the area towards the tips and increasing the area inboard. A swept wing presents less cross section to the airflow and hence reduces drag. However, the net effect of all of these considerations is quite complex, and related to to different flow types in the air. A delta wing has lots of area - but also lots of drag - which I guess is why you never see a delta winged biplane!

Try looking at the fountain of knowledge.

Edit: Congratulations on your milestone, Martyn!

Edited By The Wright Stuff on 28/07/2015 15:20:12

[Simon Chaddock]

Owdlad

I am afraid you would need an aerodynamics book to explain all the differences between those wing shapes. Each has benefits and disadvantages.

Any wing creates lift as the result of aerodynamics so the answer really depends on what you are after.

If you are after high efficiency, lift for least drag, you can ignore swept and delta wings. They are inefficient at low speeds but have big advantages as you approach the speed of sound.

As Ernie says the shape of an un-swept wing is all about the tip. No matter how efficient the wing section is at creating lift the tip represents a point off loss as the higher pressure air under the wing spills out over the end of the wing.

On this basis an elliptical wing should be best as it has virtually no tip but it is hard to make and as the wing gets ever narrower towards the tip it gets less efficient and tends to stall at a lower angle of attack than the broader root..

A tapered wing is the next compromise as it is easier to make but the more the taper the more it suffers from the characteristics of an elliptical wing.

A parallel wing is the easiest to make and the most benign stall characterisityics but it has the largest tip losses.

Then there are lots of variations that can be applied to alter the way any wing behaves so ultimately the choice tends to be a compromise between the desirable aerodynamic characteristics, structural efficiency and ease of manufacture.

[Dave Hopkin]

Swept and delta wings were introduced as aircraft were approaching the sound barrier as they dont suffer the same loss of control surface authority around mach 0.9 like traditonal shaped wings do

[Martyn K]

I have tried a slight swept wing on a free flight glider (F1A) many years ago to see how it behaved in strong lift, the idea was that as the turned tightened as the model centred it would start to slip. The inboard wing presented a larger lifting surface and thus prevented the turn from tightening too much. It was lost in a huge thermal about 3 months after I built it..

M

[Pete B]

Guess you can call that a success then, Martyn!

Pete

[Mowerman]

Parallel wings are easier to build and cover.

Yes I know that probably makes me a Phillistine.

[Geoff S]

Doesn't a slightly swept wing have the same effect as dihedral? A Pitts biplane, for example, has some dihedral on the straight bottom wing and top wing is flat with a modest sweep.

Geoff

[Tim Hooper]

Posted by Owdlad on 28/07/2015 12:53:23:

Simple question in what is the difference in lift / aerodynamics on these wing sections......

1. Tapered wings
2. Parralel wings
3. Eliptical Wings
4. Sweapt back wings.
5. Delta

Intrigued that's all

You missed one.........

[Bob Cotsford]

Geoff, I think that with many, if not most, swept wing biplanes angling the top wings back is more to do with the relevant positions of the centre section and the cockpit - moving the centre section forward moves the cabanes in front of the cockpit and affords the pilot/passenger easier access and better visibility.

Edited By Bob Cotsford on 28/07/2015 21:10:10

[Griffin Emter]

On the airflow at wing tips- I may be wrong, but winglets are supposed to solve that, by providing a boundary, aren't they? And, although someone may have said this, tapering wings, there is less lift at the ends, so there is less of an upwards flexing force on the wing.

[Erfolg]

The issues for most modellers are not the same order as are a problem for full size aircraft.

For most power flyers, tip losses and efficiencies are not critical.

Many full size aircraft have tapered wings, in part for structural efficiencies. Most models do not have anything approaching a optimised spar structure. Particularly in that we generally do not taper the spar to the tip, nor have we any real knowledge of the load distribution. Who then sits down and calculates the second moment of area, or guesses the load distribution and calcs the shear force and bending moments.

Those who advocate elipitical wings often suggest that they are more efficient. The lift is in theory can be distributed evenly, right down to the tip. The downside is that the ideal ellipse will stall pretty much simultaneously. So the wing is often washed out, In dong so much of the theoretical benefit is compromised.

Tip losses for us are at best a guess, as are the solutions from end plates, winglets, Hoener tips, or your favourite curve. The tapered wing generally helps here. But does it matter for us power modellers

For most of us planforms do not matter that much, aesthetics perhaps is the driver for most.

That is until you start looking at gliders, then everything starts to matter to some extent (if max duration or strength to weight, is the goal), and you will also think about, not just plan dorm, but also consider aerofoil section, wing loading and so on. Much coming down still, to experience and seeing what works with similar models. For many gliders structural integrity is a good part of the package. Unfortunately (to this cash poor modeller), moulded wings etc., generally perform far better, being accurate, strong, exhibiting all the desirable attributes that a built up model cannot achieve,

[Nigel R]

Posted by Erfolg on 29/02/2016 20:15:16:

Those who advocate elipitical wings often suggest that they are more efficient. The lift is in theory can be distributed evenly, right down to the tip. The downside is that the ideal ellipse will stall pretty much simultaneously. So the wing is often washed out, In dong so much of the theoretical benefit is compromised.

I guess the thing that strikes me about elliptical wings is that very very few types have them (and no recent types that I can think of) - almost all full size has converged on the same solution, i.e. slightly tapered (ignoring anything flying close to speed of sound, which brings its own problems and solutions). A straight taper can be engineered to have almost identical lift distribution to an elliptical planform anyway.

Excellent post BTW, Erfolg. As you say, for power flyers the planform almost doesn't matter. In a model we can out-power any inefficiency a particular planform will exhibit.