Airfoil thickness vs. stalling angle.

[Jim Newberry - Nuviation Ltd.]

Hi folks,

I am just starting to look at my next semi scale OD job - Junkers D1 if anyone is interested. Now, the full size had a more or less parallel chord wing reducing in thickness from around 20% at the root to around 12% at the tip and what I am trying to figure out is whether this reduction in thickness has the effect of reducing or increasing any washout in the wing - in other words do I need to design in more washout than I would for a wing of constant chord and constant thickness to account for the thinning of the section? I am planning on using the same section from root to tip with just the thickness changing (OK so I know some of the pedants will say that a reduction in thickness means it is NOT the same section, but you know what I mean).

I keep trying to picture in my mind's eye what would happen to the lift distribution along the span as the wing I've described approaches the stall and each time I try I find that I can convince myself to land on either side of the fence, so if anyone has any knowledge they would care to share, I'd be very happy to hear it!

Thanks,

Jim.

[PatMc]

The air striking the LE doesn't necessarily break on the line drawn between LE & TE. When the AoA is raised with a blunt LE the point that the air breaks be at a lower point down thus causing the increase in AoA not to be as much as the increase in the angle that the nose is raised.
What you intend to do would effectively sharpen the LE at the root which would have a similar effect to wash-in. However you could maintain the same blunt LE at the tip but otherwise still thin the section as you intend. You could,  of course, also sharpen the root LE a little at the same time.

Edited By PatMc on 07/04/2014 13:18:53

[Jim Newberry - Nuviation Ltd.]

Thanks Pat, I was leaning towards feeling that a thinner tip section would be the equivalent of wash-in although I couldn't put my finger on why I felt like that so your explanation is very helpful.

Jim.

[Simon Chaddock]

Jim

I would agree that with the same chord a thinner wing section is more likely to stall at a lower angle than a thicker one however airflow does not scale so at small sizes and lower speeds the effects of its behaviour tend to be a bit different . In some respects it will depend on the physical size of your D1.

On a model the stall can be much less well defined than full size and the wing can continue to give significant lift without a dramatic increase in drag at angles beyond that for the initial separation. This sort of 'progressive' stall tends to be more pronounced with thinner wing sections.

Thus it is quite possible to have a wing with a thinner section towards the tip that still exhibits relatively benign stall characteristics without resorting to washout. The problem is the number of variables involved which is why the constant chord/constant section and/or washout options are often chosen.

Just as an aside the wing thickness taper on the original was more for structural than any aerodynamic reason.

[Jim Newberry - Nuviation Ltd.]

Thanks Simon, I had guessed the taper was structural and I am considering having a constant thickness wing - if only to make rib cutting easier - it will only be a semi scale model after all. I'm just thinking it will look like too much of an obvious deviation from the scale outline. Then again, the full size also had a corrugated skin and I'm not about to try and replicate that, so maybe I shouldn't worry about ignoring the taper as well!

For the record, I'm looking at a span of around 60" and some sort of semi symmetrical aerosol although I haven't decided which yet. (Still only in the very early stages of thinking about the project at the moment.)

Jim.

[Jim Newberry - Nuviation Ltd.]

*that should have said "aerofoil", not "aerosol"... obviously!!

[Biggles' Elder Brother - Moderator]

hi Jim,

there are a lot of competing factors at work here! All other things being equal the thinner section will have a slightly lower critical angle - but how big the effect is depends very much on the exact aerofoil section being used. With some basically symmetrical (or close to symmetrical) sections the difference is measured in tenths of a degree! And to be honest I'm not sure it would be discernible on a model at all! However with more asymmetric sections the difference can be as large as 2 degrees or so.

Actually I suspect that what might actually be a bigger problem is that thinner sections tend to have a "sharper", more well defined, stall; whereas thicker sections tend to go into a more "mushing" type of stall. Now this could lead you into difficulties because although the whole wing may well stall at, or about, the same AoA, the outer wings shorter "stall event" may lead to happening in a staggered fashion - giving you what is in effect a single sided tip stall. I am not saying this is certain to happen - but it is a definite possibility.

I would be tempted to think in terms of building some washout into this wing. The only real potential disadvantages are the slight decrease in lift efficiency and the extra work! On the other hand the major advantage is an insurance policy against an uncertain stall behaviour otherwise.

BEB

[John Olsen 1]

Since the ailerons are at the outer end of the wings, could the effect of washout be acheived by setting both ailerons slightly in the up direction when at neutral? This would be simpler than building it in to the whole wing, and would also be adjustable. Eg if the stall proved very benign, try reducing a little while if it drops a wing too readily try increasing. (I am assuming stall tests at altitude of course!)

John

[Jim Newberry - Nuviation Ltd.]

Thanks for that BEB, must admit, I was hoping you'd spot the thread title and get involved!

John - I've done that in the past. The problem is that raising the ailerons as you described is very visible - even from quite a distance - so I'd like to avoid it if at all possible.

Looks like I'll be designing it with washout and a semi symmetrical section.

Jim.