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Dihedral

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Richard Acland02/07/2020 20:22:02
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120 forum posts
38 photos

Currently building an 80 inch parasol wing model from plans. the plans shows a small amount of dihedral which I have worked out will give about one a half inches dihedral at the wingtip. The wings are built in two sections and I am finding it a bit of a bind trying to get the centre sections spot on. I have built a lot of models from plans and had a quick look at my different models. Some have quite a large amount of dihedral. and some not much. My question is without getting too technical would it make a lot of difference to the flying performance if with such a small amount of dihedral if I saved a lot of bother and just built a straight wing

Denis Watkins02/07/2020 20:35:17
4544 forum posts
123 photos

Personally, I would choose to fly a flat wing, as Cubs are a favourite.

But you do lose the " self righting " properties that the dihedral gives, and the extra stability.

Flying without dihedra! comes naturally after a while that whenever you roll the wings, you need to roll back again for straight and level

 

Edited By Denis Watkins on 02/07/2020 20:38:56

J D 802/07/2020 21:11:11
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1525 forum posts
84 photos

With parasol type aircraft dihedral is not so important because you get pendulum stability effect from the weight of the fuselage being well below the wing. Do build in some washout though.

Tim Hooper02/07/2020 21:24:46
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2902 forum posts
2406 photos

In my experience, it's the appearance that suffers. Flat wings can look droopy...

Tim

Megawatt02/07/2020 21:44:15
22 forum posts

Assuming the model has ailerons all of the above is true if, however, you are relying on the rudder then dihedral is essential for the model to roll.

Martin Harris03/07/2020 01:02:40
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9411 forum posts
256 photos

Although they may give the impression of being flat, Piper Cubs do have between approximately 1/2 and 1 degree of dihedral.

Pendulum stability may be a small factor [there are arguments from people far more qualified than me that it doesn't actually exist] but the levelling effect comes from the differing angles of attack from sideslip induced by bank.

I think you'll find that all high winged aircraft with flat wings or anhedral have swept back wings and therefore need to reduce the high inherent stability which adds a lot of drag and reduces control effectiveness.

Edited By Martin Harris on 03/07/2020 01:04:57

Richard Clark 203/07/2020 05:48:11
424 forum posts

As long as you have ailerons dihedral is completely unnecessary on an RC aircraft.

So what's it for?

Mostly to prevent a droopy appearance, as Tim says. (That applies to full-size 'light' aircraft too which today are mostly 'consumer products' not purchased entirely for logical reasons. Even then few modern light aircraft have any, except for any thickness taper being entirely on the underside, again for appearance.)

Another reason is to keep low set 'podded' jet engines as far off the ground as possible.

perttime03/07/2020 08:42:34
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164 forum posts
11 photos

 

It would be nice if somebody could give us a brief lesson here. Or at least give an informed opinion.

Edited By David Ashby - Moderator on 03/07/2020 16:29:46

J D 803/07/2020 09:00:03
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1525 forum posts
84 photos

I did and see where you are coming from Jason, recon my idea of pendulum effect came from old Aeromodeler features of long ago when most flying models were free flight.

So would I be correct that only hang gliders and balloons/airships benefit from pendulum stability ?

Former Member03/07/2020 09:04:12

[This posting has been removed]

Jon - Laser Engines03/07/2020 09:19:19
5561 forum posts
271 photos

In general the more dihedral you have, the more stable the model is in roll. Its more complicated than that, but that is all we really need to know.

The pendulum effect is real, but aerodynamic forces will over power it. Most high performance aerobatic aircraft are mid wing and while there are other reasons that is a good thing its probably a contributory factor. Very few high wing aircraft have large dihedral angles and yet most low wing aircraft have a reasonable amount. High wing aircraft with anhedral are trying to make the aircraft less stable longitudinally for some reason. Often this is to stop dutch roll in high wing T tail aircraft. BAe 146's and most military transports are high wing T tail aircraft with swept wings and these all have anhedral to prevent dutch roll and other stability problems.

To say that dihedral angle means nothing if you have ailerons is flat out wrong. I took all the dihedral out of my little hurricane to make it more scale and the effect is clear. The wings do sometimes look droopy, and while stable wings level, it falls into turns more than my old one did with a few degrees on it. Admittedly this is partly due to other scale inaccuracies in the wing meaning the model does not behave like the full size, but compared to the same model with a dihedral wing there is a clear change in the handling.

In terms of the OP if you have a 1.5'' target at the tip +/- 1/4 inch will make very little difference and in truth +/- half inch would probably be ok. You would notice it, but only if you flew the two back to back. I would not be thinking of taking it all out but i would also not be loosing sleep about getting it totally accurate.

Bob Cotsford03/07/2020 09:19:45
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8646 forum posts
483 photos

Richard, before this thread goes too far off course, I agree with others that it will look as though it has anhedral if the wing is flat. It will fly just fine but a) look wrong, and b) you will always feel beaten by this lump of wood.

Take your time setting everything up in a dry run before committing with glue and remember it's only wood. If you cut too much off then glue a shim back on and try again. If it's in a critical area reinforce your joint. If you drill a hole in the wrong place, make a ply washer the right size and glue it on in the right place, or for smaller holes fill them with dowel and try again. It's the only way we get better at building! The only real rule is 'don't fill large cockups with glue and pretend they're not there'.

Richard Acland03/07/2020 10:15:05
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120 forum posts
38 photos

Thanks for all the replies. I think that on the whole I will persevere and build in the suggested dihedral. Even if as some say it is only to stop it looking droopy

Richard

Richard Clark 203/07/2020 14:05:52
424 forum posts
Posted by Jason Channing on 03/07/2020 09:04:12:

lesson 1 = You don't have dihedral on an airliner just to keep the pods clear of the ground or just to get the look correct although it is funny. Book wise I have read Martin Simons book many times and is a particularly good read.

Pods? Take a look at the front view of an A380.

Also note that I deliberately confined my 'looks' comment to light aircraft as they are often 'consumer' products, whereas airline passengers often don't even get to see the outside of the  airliner they are  about to fly in.  

As for 'looks' Cessna actually admitted that the swept fin of their light aircraft was introduced only for appearances' sake, while also admitting it is heavier and needed more area to be equally effective as the previous roughly rectangular one.

As it happens I have an HNC in aerodynamics/aircraft engineering, spent several years in the aircraft industry after the six year 'student apprenticeship' which led to my HNC, and am a 'full-size' pilot of everything from microlights to 'executive' jets.

A little exercise for you. Tell us (1) what happens when you use the rudder to induce a turn, (2) what happens when you use the ailerons to induce a turn, (3) what part does the fin play in a turn?

Edited By Richard Clark 2 on 03/07/2020 14:13:43

Richard Clark 203/07/2020 15:12:55
424 forum posts
Posted by Jon - Laser Engines on 03/07/2020 09:19:19:

In general the more dihedral you have, the more stable the model is in roll. Its more complicated than that, but that is all we really need to know.

The pendulum effect is real, but aerodynamic forces will over power it. Most high performance aerobatic aircraft are mid wing and while there are other reasons that is a good thing its probably a contributory factor. Very few high wing aircraft have large dihedral angles and yet most low wing aircraft have a reasonable amount. High wing aircraft with anhedral are trying to make the aircraft less stable longitudinally for some reason. Often this is to stop dutch roll in high wing T tail aircraft. BAe 146's and most military transports are high wing T tail aircraft with swept wings and these all have anhedral to prevent dutch roll and other stability problems.

To say that dihedral angle means nothing if you have ailerons is flat out wrong. I took all the dihedral out of my little hurricane to make it more scale and the effect is clear. The wings do sometimes look droopy, and while stable wings level, it falls into turns more than my old one did with a few degrees on it. Admittedly this is partly due to other scale inaccuracies in the wing meaning the model does not behave like the full size, but compared to the same model with a dihedral wing there is a clear change in the handling.

In terms of the OP if you have a 1.5'' target at the tip +/- 1/4 inch will make very little difference and in truth +/- half inch would probably be ok. You would notice it, but only if you flew the two back to back. I would not be thinking of taking it all out but i would also not be loosing sleep about getting it totally accurate.

Jon,

I am afraid I have to disagree. While dihedral gives a change in the handling (mostly making aerobatics more difficult - a Hurricane is MEANT to fall into turns easily and is often reported to be more aerobatic than a Spitfire) its presence or absence simply does not matter.

Most modern light aircraft, both low and high wing, have little or no dihedral - the Piper Cherokee is somewhat of an anomaly today.

As for models, I mostly fly 'classic pattern' ones, plus a few 'Auster' type planes. But the only model I have where the rudder, though large with a lot of movement, has zero effect on the overall flight path when used alone is the very semi-scale Graupner Bolkow 'Monsun'.

Dihedral on a parasol model? I wouldn't bother. It makes a Luton Minor look weird, but it does make a early Pfalz look cute.

John Stainforth03/07/2020 16:18:19
367 forum posts
64 photos

Martin Simons pretty book is marred by many typos and incorrect equations. (It is hard to believe that my copy is a fifth edition.)

Jon - Laser Engines03/07/2020 16:41:15
5561 forum posts
271 photos

Richard, to clarify my comments my Hurricane feels like its stood on top of a ball. Start to roll, wings go over a bit, Ok, over a bit more..and then it falls off the ball and flips over. That is not how its supposed to be and some dihedral would help it out.

As for the Hurricane being better to aerobat than the spitfire..depends on the metric you use. I can imagine the Hurricane being much better at tight aerobatics at low speed/altitude. Equally, a spitfire (especially a late Mk) would mug it if you wanted to do a larger 'pattern' routine due to its greater power and better overall aerodynamics.

If i ever win the lottery i will buy one of each and test it

Richard Clark 203/07/2020 19:41:57
424 forum posts
Posted by Jon - Laser Engines on 03/07/2020 16:41:15:

Richard, to clarify my comments my Hurricane feels like its stood on top of a ball. Start to roll, wings go over a bit, Ok, over a bit more..and then it falls off the ball and flips over. That is not how its supposed to be and some dihedral would help it out.

As for the Hurricane being better to aerobat than the spitfire..depends on the metric you use. I can imagine the Hurricane being much better at tight aerobatics at low speed/altitude. Equally, a spitfire (especially a late Mk) would mug it if you wanted to do a larger 'pattern' routine due to its greater power and better overall aerodynamics.

If i ever win the lottery i will buy one of each and test it

In the early 1970's a Spitfire Mk 9 with a low hours engine, a spare zero hours engine still in its crate, , a spare zero hours prop and a three year Certificate of Airworthiness was advertised for 5000 quid in Flight magazine. How times have changed

alex nicol03/07/2020 22:21:30
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389 forum posts
17 photos

Hi Richard,

For what it's worth hi wing aircraft will have an element of self righting due to the pendulum effect caused by the weight of the fuselage hung below the wing. A low wing aircraft will have less or no self righting as the weight is on top of the wing and as soon as you bank there is no self righting effect as the fuselage assists the bank. With regard to the wing looking droopy the model in the image has a span of 82" has no dihedral and no sign of any droop

img_20200628_201348.jpg

J D 803/07/2020 22:43:05
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1525 forum posts
84 photos

I still want to know if a hang glider pilot act's as a pendulum, after all he/she is swinging around like one.dont know

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