Biplane incidences

[Erfolg]

With a biplane, is it usual to have the lower wing at a higher angle of attack than the upper? That is with a standard stagger arrangement.

If so by how much, logic suggests to me about 2 degrees?

I am assuming that if the lower wing stalls first, the upper will continue working, albeit not supporting all the weight (in equilibrium), the consequence will be a nose drop, where upon the lower wing starts working again.

Then a gain, is all this load of "cods wallop"?

[A.T.]

From previous articles in this magazine and others

Downthrust -2 to 3 degrees
Right Thrust 2 degrees
Stagger 25%
Top Wing 0 degree Incidence
Bottom Wing 0 degree incidence

More under sub sections:
"Calculators, Conversion, Center of Gravity, Electric Flight , MAC & Servo Torque calculation charts."

"FAQ = Answers for Beginners and Advanced RC'ers"

at

Alan's Hobby, Model & RC FAQ Web Links

Alan Tong

[Erfolg]

Although I can see that you can rig the incidences at 0 degree differential, it seems to me that the stall could be softened by a differential rigging, at the expense of an increase in overall drag.

I am also a little suspect at the application to a universal set of values for all models. Although for the model I envisage 2 degrees down thrust could be about right. The right thrust seems reasonable also.

As for the stagger, it is what it is, at about 50%.

I will look at the CG calculator, as this could be a useful tool, for validating my long hand calculation when I get around to doing it. or a warning to check again.Particularly as the model is a sequi winged plane.

[Martin Harris - Moderator]

I must say that I've always thought that the top wing should be at a more positive AoA so that there's a more positive nose drop at the stall.

Edited By Martin Harris on 24/04/2013 23:27:26

[John Olsen 1]

I beleive that it was quite usual to rig the upper wing at a slightly higher angle of attack, although I wouldn't know where to find any confirmation for this. I think part of the thinking is that the lower wing is affecting the flow of air at the upper wing more than the upper wing affects the flow at the lower wing, because a wing affects the air above it more than below.

I suspect for model sizes, it is not all that critical.

John

[Bob Cotsford]

There are some bipes where the top wing is set at -1 to the lower. I think the downdraft from the top wing can affect the effective incidence of the bottom wing. Look at smaller stuff ie Pitts, Jungmeister and the like.

[Charles Smitheman]

I have built and flown five biplanes now.

The first one (.20 sized) was a horror until I fitted the interplane struts (lazy) and made the incidences within 1 deg of each other. It snap rolled at the slightest provocation, how it survived its first flight remains a mystery to me.

The other plans called for various different incidences.

I think that so long as left and right are the same and everything is firmly rigged they all fly fine. One degree top or bottom does not make a difference that I have been able to notice.

What did make a difference was getting the cg wrong,and consequently doing a five point landing at the feet of the great Boddo! Blush! Oh well nothing hurt but my pride.

[Greybeard]

I doubt that a 1 or 2 deg difference will make the slightest difference in model terms as the aerofoils work differently at our scale speeds.

[Ruprect Spode]

Posted by Bob Cotsford on 25/04/2013 07:19:23:

There are some bipes where the top wing is set at -1 to the lower. I think the downdraft from the top wing can affect the effective incidence of the bottom wing. Look at smaller stuff ie Pitts, Jungmeister and the like.

Is this because the airfoil section has a positive incidence? Always made sense to me that if the lower wing is always flying around at a relatively positive angle, it's a good plan to have the upper reduced a tad as when you fly through the speed ranges it has beneficial effects on stall and drag characteristics. ie. When one wing is approaching the stall the other is flying quite happily. Dunnoe.

[Colin Leighfield]

Erfolg. Although it seems that there are varying views on this and different, perfectly valid experiences, I seem to recall that on the early biplanes the approach was typically 0 degrees on the top and 2 or 3 degrees on the bottom. It would be my natural approach with a scale type early design, although I can imagine that with a modern aerobatic type having symmetrical wing section, zero on both might be more typical. I claim no expertise, I'm just trawling my memory really!

[Biggles' Elder Brother - Moderator]

I don't think there is a hard and fast rule here. Biplanes have been designed and flown quite sucessfully with; no difference top and bottom, slightly open (ie top at higher incidence than the bottom) and slightly closed (top at a lower incidence than the bottom).

It all going to depend on lots variables - that's why I suspect there isn't a general case. For example what is the chord and span difference between the upper and lower wings? Do they both have the same aerofoil? What is the degree of stagger? Etc.

I'm afraid I disagree with John's comment above - the critical wing on a biplane is the upper wing in the vast majority of cases. The reason the reason for this is the as the lower wing effectively operates in the downwash off the trailing edge of the upper wing it is much less efficient. This is why biplanes don't have exactly half the wing area of an equivalent monoplane for example - the efficiency factor in having two wings is typically only about 0.7 of that of a single wing is area terms. So actually it the air below and behind the upper wing which is more influential - not the air above.

Does it make a lot of difference? Probably not for our models. Generally, and this is just a personal view, I would go with slightly open - 1 or 2 degrees. Why? Well equal incidence is a singularity that can't in practice be achieved - you will never get exactly the same incidence top and bottom - so you're better settling for knowing its either open of closed. Open offers the advantage of putting the most effective wing in a definite lift condition - althogh closed may mitigate slightly against the downwash effect on the lower wing - as I think someone suggested above. But it would be extremely marginal at the scale and the sort of differences in incidence we are talking about here.

BEB

[Martyn K]

IIRC, I was told that the upper wing should be marginally (< 2deg) higher AoA than the lower. The reason for this is at the approach of a stall, the airflow over the top of the lower wing becomes turbulent and upsets the airflow under the upper wing with unpredictable results.

It was deemed safer to allow the upper wing to stall completely while the lower wing was still generating some lift.

Martyn

[Simon Chaddock]

Erfolg

I must say I tend to agree with you and logically set the top plane at a slightly lower incidence. This is to actually delay the stall as due to the 'slot' effect the lower wing stalls at a significantly higher angle than the top wing.

As BEB says the interaction between the wings does depend on the chord, separation and section.

My own experiments with a scale Fokker Triplane confirmed a significant slot effect such that the stall became very ill defined and it was able to maintain a high angle of attack (20+ degrees), a "hanging on the prop" characterisitic that was noted on the full size as well.

Edited By Simon Chaddock on 25/04/2013 13:15:36

[Erfolg]

Hmmm!

I seems I have a little to consider, before I make any decision and probably more decisions to make.

I have designed and made a Bipe many years ago, probably about 32 years ago.

I do know I used differential AOA.

With my superb, well honed memory, I cannot remember what, the logic. Nor did I make any notes..

However it did fly well, with a gentle stall and still exists, somewhere at my mothers house.

Any way I will read the posts, and ponder, where did I get my original info? How does it relate to todays opinion.

[Tim Hooper]

For what it's worth (if anything).....

All my scale-type multi-wingers use a ClarkeY type aerofoil, with both wings set at the same +1° incidence, using the top longeron as a datum.

What happens in the air is anybody's guess, but all the models fly nicely.

I've yet to do a full-aerobat, so can't comment on using a fully symmetrical aerofoil.

Hope this helps!

tim

[Lindsay Todd]

Again for what its worth my latest design Skywriter has lower wing at +1 and top wing at zero degrees with a flat section wing top and bottom. 2 degrees right and down on the motor and tail at zero.

Adds to the confusion no doubt but fly's very nicely.

Linds

[Bob Cotsford]

My GP Skybolt has the top wing at -1 1/2 to the bottom and uses symetrical sections. Flies like it's on rails and slows ok for landing but it is designed for good flick manouveres.

[Erfolg]

It seems that the norm is to have the upper wing on a positive stagger bipe at a higher angle of attack, by a very small degree up to +2 degrees, or 0-0 degrees.

I will therefore follow this standard, mainly because the convention works, so it seems. I have now read of -1 degree in a few cases, this seems to be on aerobatic models to improve inverted flying.

I have tried to understand why. To date, I can only surmise that the two wings tend to straddle the CG, when staggered, the aerodynamic centres are therefore are exerting an up ward and pitch down couple at 25% chord, the tailplane providing a downwards force to balance. At the stall, it would appear that forward wing would stall first, the rearward wing still lifting (although, would the down wash disappear from the upper wing?) with the tailplane exerting a down force (due to taking/moving the wing to the stall). I guess that the arrangement softens the stall?