Wing loading query

[fly boy3]

Hi all, been using the old grey matter working out wing loadings recently. Having arrived at a figure I just wondered if the answer was a high w/l or a low w/l. What do I compare my answers to. Obviously smaller models will have a smaller w/l but there again is it an optimum wing loading for that particular model ? Got a few maidens soon, so it would be interesting to know what sort of flight to expect. Cheers FB3

[john stones 1 - Moderator]

That's an answer for someone other than me fb3 but the all up weight on the plan or kit is usually a fairly reliable guide, so if your somewhere near she'll be right.

John

[Biggles' Elder Brother - Moderator]

Hi FB3,

well first of all small models do not necessarily have a low wing loading. It depends on the size of their wings and their weight!

As a general guide - for typical size models:

10-15oz/ft2 would be very light indeed - real floater country. But on a nice calm day it'll turn on a sixpence and not stall easily.

20-25oz/ft2 would be a nice flying sports model - this is a light enough wing loading to be agile but heavy enough to not be at the mercy of every puff of wind.

30-35oz/ft2 would be a heavish wing-loading - typically scale war birds (Spitfires, P51 etc) would be around here. When the wing loading gets this high it starts to limit the model's ability to perform tight turns without threatening to stall and flick in. Remember a model turns by directing some of the wing's lift inwards to the inside of the turn by banking and then increasing the angle of attack. But wings like these are working pretty hard and near their limit in ordinary flight - which means there is not as much scope for increasing the lift to make the surplus that is required to both hold the plane in the air and turn it. Any attempt to make a big surplus in lift to achieve a tight turn therefore runs the risk of exceeding the maximum angle of attack and stalling the wing.

40-45oz/ft2 this is basically a brick with small wings!

Now you have to interpret these numbers carefully. What I have said there is good for models up to around 60-70" span or so. But as you go much bigger then all those figures can be increased. So 40oz/ft2 would not be so heavy a loading on a model with a 10 foot wing span for example.

For typical models, of the size most of us fly, the figures above are a reasonable guide to what to expect.

BEB

Edited By Biggles' Elder Brother - Moderator on 18/03/2015 00:30:03

[Simon Chaddock]

fb3

A typical guide looks like this:-

Gliders 10 oz/sq.ft

Trainers 15 oz/sq.ft

Sport planes 20 oz/sq.ft

Warbirds 25 oz/sq.ft

There is no magic figure. Each type will have a range that may well overlap the other categories.

Physically small planes are likely to have lower wing loadings than an larger but otherwise identical one.

Wing loading is only one of many characteristics that determine how a plane flies but generally the higher the wing loading the faster is has to fly.

I hope this helps.

[cymaz]

For scale planes, a wing cubic loading would be more accurate.

Have a go with it here

[Biggles' Elder Brother - Moderator]

Well, "accurate" isn't quite the right word in this context in my opinion. Afterall, there is nothing "inaccurate" about a conventional wing loading calculation. The mass divided by the wing area can be perfectly accurately calcualted.

Some are of the view that the cubic wing loading is perhaps more a "representative" and meaningful figure in some circumstances. This is largely because it goes someway to removing the scale-ablility effect I described above where bigger models feel the effect of higher wing loadings less - so, with cubic wing loading you can more meaningfully compare wing loadings over bigger size range of models. This is due to the fact that the wing area goes up with the square of the scale factor where as the weight is more closely linked to the cube of the scale factor (I say "closely linked" because it not a perfect cube relationship because weight does not in practice go up proportional to volume - because if we double the size of model we don't generally double the cross-sectional area of all the structural elements).

So you can see it's a bit more complicated than it might appear at first sight. By all means have a play with cubic wing-loading (its good fun) but to be honest, I feel that unless you are working with models that cover a very large size range there won't be much benefit over the much simpler rule of thumb that Simon and I have outlined above.

[The Wright Stuff]

Interesting BEB, a good summary.

Forgive me for barging in on this, but would I be right to assume that at least part of the difference in usage between wing load and cubic load is an attempt to compensate for scale speed?

In an ideal world where gas molecules scale down, and the air flow behaves exactly as per the full size aircraft, then a model and a full size aircraft of the same wing loading would have exactly the same flying speeds. This is essentially because the lift-per-unit-speed would then be directly proportional to the wing area, and the gravitational force to be overcome is proportional to the weight, so it exactly cancels.

But this is very different from discussing whether the aircraft flies at the same scale flying speed, which of course is multiplied by the scale of the model.

So assuming this interpretation is correct, wing load would be more useful for discussing how easy a model is to fly, and cubic load more appropriate for discussing how scale-like a model flies.

[Dai Fledermaus]

Can I recommend Peter Miller's book Designing Model Aircraft. Chapter 2 Essential Aerodynamics, covers the subject in a way that even my simple brain can understand.

[Bill_B]

You might like to print one of these off, I've found it extremely useful over the years. **LINK**

[fly boy3]

Thanks Beb, and all who answered my query. all is now revealed. very interesting too. Will have a look at Peter Millars book too. Bill_B, very good link, has now been down loaded for the future. Cheers

[Peter S B]

This is a good link which could help the old grey matter

**LINK**

[fly boy3]

That really helped the old grey matter as you say Peter. Cheers

[fly boy3]

Hi all, is there any particuler reason why model warbirds have such a high wing loading. ? Thanks

[Dave Hopkin]

Posted by fly boy3 on 23/03/2015 19:48:37:

Hi all, is there any particuler reason why model warbirds have such a high wing loading. ? Thanks

Smaller thinner wings for less drag, smaller wings for faster roll characteristics

[fly boy3]

Thanks Dave, but I would have thought that thinner and smaller would equate to a lighter model ?, shows I know very little of aerodynamics. LoL

[Dave Hopkin]

The bulk of a wing (real or model) is air/fuel, so reducing the size and/or section doesnt reduce the weight that much (as opposed to drag) compared the the reduction in wing area (if span/chord is reduced) so the wing loading increases faster than it would appear

[PatMc]

Posted by fly boy3 on 23/03/2015 19:48:37:

Hi all, is there any particuler reason why model warbirds have such a high wing loading. ? Thanks

Not all, model or indeed full size, warbirds do have particularly high wing loading.

Edited By PatMc on 23/03/2015 20:36:37

[Biggles' Elder Brother - Moderator]

Full size warbirds were starting to push the envelop where speed range was concerned.

Think WWI: take off speed 70mph, cruise speed 120mph, flat out maybe 140mph. Maximum speed ratio then 140/70 - ie 2:1.

Now think WWII: take off speed maybe 80mph, cruise speed 250mph, flat out maybe 400mph. Maximum speed ratio then 400/80 - a whopping 5:1! And all that has to be done on the same wing. It has to fly at 80mph and it has to fly at 400mph. That's a bit of a challenge. One of the challenges is to stop a wing that works at 80mph from producing too much lift at 400mph. Answer - well somewhat simplified! - make it thinner and make it smaller area. Then add flaps to give it a hand at the low speed end.

The result of the small area is high wing loading. Messerschmidt made the wing very small on the 109 and the so the wing loading very high. Mitchell was a bit smarter, with the elliptical wing on the Spitfire he managed to get a bigger area than he would have done, without paying a big drag penalty. And so the Spitfire has a lower wing loading than a 109 - and hence at most altitudes could turn inside it.

[John Olsen 1]

I would also add that speed was important because the faster aircraft can choose to initiate and break off the action. So they have the choice of whether to dogfight or not...hence sayings like "Never try to out turn a Zero".

Deigning an aircraft for a wide range of speeds is what makes it really hard. Supersonic aircraft would probably look quite different if they never had to slow down to land.

I always enjoy watching the flaps deploy on a passenger jet when landing. What looked like a monolithic wing a few minutes before suddenly turns out to be mostly moveble bits.

John

[Delta Foxtrot]

It is likely that the planform of the Spitfire wing was not the idea of R J Mitchell, but rather down to Beverley Shenstone a Canadian born aerodynamicist who worked at supermarine. Shenstone worked in Germany at the beginning of the 1930's and was a friend of Lippisch and met Ludwig Prandtl.

Prandtl recognised the benefits of an elliptical planform constructed of two half ellipses with semi minor axis in the ratio of 2:1. There are significant structural benefits as well as minimising induced drag as shown by his elegant lifting line theory.

Shenstone quotes Mitchell as saying he didn't care what shape the wing was as long as it covered the b***** guns.

There is a very good paper on this by J A D Ackroyd: paper no 2013/02 published in the journal of Aeronautical History. Can be found through google

 

 

 

Edited By david fillingham 1 on 23/03/2015 21:29:22

[fly boy3]

Thanks all, the info. recieved to my wing loading query is mind boggling. Such a lot of aero nautical brains on this forum, great. Thanks