Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

washout on a biplane wing ?.

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Old 09-26-2018, 02:16 PM
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r ward
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Default washout on a biplane wing ?.

is it done ??. I know that with a bipe the idea is to get the bottom wing to stall before the top wing, but I understand that is done with incidence,......so the question is,..... what about wash out ?......do I need it ?......top wing or bottom wing or both ??. I predict, if there would be any in either wing, it would be in the top wing,...is that correct ?.
I've found two plans that I like.......the 62 inch "Lazy Ace" and a 52 inch Pitts Special from R/C Modeler Magazine, that I will most likely enlarge to around 60-62 inch span. two essentially different planes as far as flight characteristics, I know....... I might build both,.....the Pitts a little later as I get more confident on the sticks. ( building veteran,....R/C flying rooky ! )
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Old 09-26-2018, 03:42 PM
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From experience with my bipes:
The engine is pointing directly forward.
The wings have no incidence.
The wings have no washout.
The airfoil is NACA 0017.
The horizontal stab is set at zero.
The wheels have no toe in or out.
A click here or there required for hands off flying.
.
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Old 09-26-2018, 03:49 PM
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Old 09-27-2018, 02:50 AM
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I would venture to say you can find a number of combinations of thrust and incidence that will work just fine. The thing to do is have a starting point and work from there for desired performance and characteristics. For me, I'd start everything at zero. Then once familiar with the plane, I'd do a lot of power off (idle) gliding to check the CG and adjust the stab. Then I'd go to the stall to see which wing goes first and if that's what I want/like. And adjust the wings accordingly. More than 2-3 degrees adjustment should be suspect. Not saying it's wrong as it may well be what that model needs. But just double check to be sure.

And once you think you have things set, do all the tests over again. Yes, this could take a summer of flying to work out.
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Old 09-27-2018, 08:19 AM
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If both wings are set to zero how would you know which one is stalling first?
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Old 09-27-2018, 11:19 AM
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speed, does it matter which wing is stalling first? Dan
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Old 09-27-2018, 12:32 PM
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Dan, I would think yes. The result I imagine is the top wing stalling first is a less docile stall with more likelihood of dropping a tip. Although two completely different airplanes, my Reed falcon has a much more abrupt stall then the Divergent. Falcon is 0-0 Divergent is not.
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Old 09-27-2018, 12:35 PM
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Originally Posted by All Day Dan View Post
speed, does it matter which wing is stalling first? Dan
Only if the characteristics for one are more erratic than the other. Start both at zero and see how the plane stalls. Then change the incidence of one of the wings and see if you can see any difference in the stall. You might have to get extreme (5 degrees) to see a real difference. Then return the first to zero and change the other wing and repeat the test.
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Old 09-27-2018, 06:09 PM
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I think that we are getting a little too focused on a single aspect. An airplane is a symphony of comprise manipulated to get the desired result. Concentrating on just the stall may end up with great stall characteristics but crappy upline and downline tracking or knife edge tracking. The goal at least for me is to setup an airplane that does everything well. Not always an easy task. At the end of the day if your airplane flies up to your expectations and you are having fun then you are succeeding. If you are looking for certain qualities then it is time to investigate and test.
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Old 10-02-2018, 05:51 AM
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Yes, I agree, trimming a bipe is not just about stability.

Back to the original question, during the 20s/30s, according to an old timer, it was common to rig bipes with some washout. Some of them had fairly high aspect ratio wings even though they were bipes.

Itís common to set the top wing a little negative to the bottom wing (pos. stagger). The idea is that down wash from the top wing causes the bottom wing to fly at effectively a lower angle of attack, so setting the top wing a little negative evens it out. One pretty experienced bipe flyer on this forum says that by testing he has found it works well on all his aerobatic bipes. I set up my own that way.

Itís also common to set the top wing a little positive to the bottom (pos. stagger). The top wing stalls first and the bottom wing, further back, is still flying. I donít do that because thatís making the lower wing do the job of the stab. I want the lower wing to carry itís weight. It would make it hard to do snaps too.

Gordon Whitehead, who has published many scale designs, explains these theories and uses no difference between top and bottom at all, and he never mentions washout. Hard to argue with his success as a scale bipe designer. Jim

Last edited by buzzard bait; 10-02-2018 at 05:54 AM.
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Old 10-02-2018, 10:04 AM
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if one wing is going to stall before the other,...I would think you'd want the bottom wing to stall first. it carries more weight per foot because it is usually smaller and when stalled, you still have the secure geometry of a parasol wing arrangement with the top wing still flying. I guess i look at the importance of the plane being more snap-roll friendly with the top wing not flying as insignificant in the scope of overall handling. I would much rather have good landing approach manners than snap-roll readiness.
I wonder if Mr. Whitehead has the right idea,,,, both wings should be contributing their proportion to flight equally and the bottom wing will quit all by itself when the time is right, simply because it is usually smaller and more heavily loaded.
wash out or not, probably depends on how you look at the "which wing stalls first" aspect. if the bottom wing gives up first, then I'd want some wash-out in the top wing. I can't see an instance where I would want the top wing to stall first,....to me,.... that just doesn't make any sense in the scope of stability.
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Old 10-03-2018, 05:38 AM
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Mr. Whitehead did not make the incidences the same because both wings would contribute equally. He knew they would NOT contribute equally and explained the reason. This has been known since NACA days decades ago. He just made them equal as a sort of compromise between the two ideas. The lower wing will tend to stall a bit later than the top wing if both wings are set to the same angle because it flies in the down wash from the top wing. For the same reason, it contributes less lift.
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Old 10-03-2018, 09:34 AM
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with the amount of stagger that is typical and the distance of separation between the two wings, I just can't see the bottom wing "flying in the downwash of the top wing". with the forward movement of the pane in flight,...I can see the horizontal stab flying in the down wash,....but not the bottom wing. I guess i'd need more conclusive evidence of that, than just one man's thoughts.
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Old 10-03-2018, 10:28 AM
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I canít buy into the down wash idea either. My Divergent has the wing spacing at 10.5Ē with a cord of 13Ē at the root and a 2Ē stagger. Basically all the rules were thrown out the window. My reasoning is that we have much lower wing loading and fly the wing at less AOA so much less down wash is generated. Theory is somewhat proven as pitch trim does not change between level flight and vertical where the wings are mostly unloaded.
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Old 10-03-2018, 02:58 PM
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makes sense,....never thought about the vertical flight and trim,......I just can't see any downwash being produced in an airstream that is negative pressure under the upper wing's airfoil. that and the stagger isn't great enough to allow the lower wing to be in any part of the upper wing's wake. many books have been written on theories that are just a bunch of sentences to fill the pages between the covers.
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Old 10-03-2018, 03:13 PM
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Agreed, I have a nasty habit of being a bit overly aggressive with guys who want to dispute something I have written on a forum by quoting out of a book. Iím not infallible but usually when Iím answering a thread itís because I have been there done that and had some success. IMO we require the most out of our pattern airplanes and in most cases what is learned there has great potential to make sport airplanes better as well.
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Old 10-04-2018, 03:55 AM
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Originally Posted by r ward View Post
makes sense,....never thought about the vertical flight and trim,......I just can't see any downwash being produced in an airstream that is negative pressure under the upper wing's airfoil. that and the stagger isn't great enough to allow the lower wing to be in any part of the upper wing's wake. many books have been written on theories that are just a bunch of sentences to fill the pages between the covers.
Of course, the lower wing isn't in the upper wing's wake, it's in its downwash (two different terms and concepts). As to theories, this is all knowledge from the 1920s and 1930s, well tried and proven in practice. The two pictures from a 1950s Swiss aerodynamics textbook may suffice to explain the concepts, the pictures are in English after all.

If you are familiar with the concept of circulation, the upper part of the first picture tells why the upper wing's lift is bigger than the lower wing's. Any wing retards the airflow beneath and accelerates the airflow above. Superimpose both airflows and you get the airflow (streamlines) in the picture's lower part.
That is to say that on a biplane with two equal wings at the same angle of incidence the upper wing has always more lift than the lower one. That is the case even without stagger (second picture), but positive stagger has the same effect in addition.

If the lower one of the two equal wings has a certain amount of more incidence than the upper one ("decalage"), both wings have equal lift what minimizes overall induced drag. If there is positive stagger, there has to be even more "decalage" between both wings. Up to 3 degrees of decalage (angle delta on the left side of the second picture) are common. Total lift and total induced drag are independent of stagger. An equal distribution of lift on both wings just makes maximum total lift bigger (delays stall).
As to aerobats, if both wings have the same incidence, the upper wing has more lift so it stalls first. If there is also positive stagger, stall on the upper wing lets the center of lift shift back what makes for a down-pitching moment and helps recovering from stall. If the lower, rear wing would stall first, that would boost the stall and probably make for violent snap rolls.

Look here for a 3-view of the BŁcker 133 Jungmeister. It has two equal wings, cambered airfoil, and a bit positive stagger. The lower wing seems to have a bit more incidence. There's also a bit dihedral (more on the lower wing) and some sweep, but no washout. (A swept wing with dihedral may look like washout in the side view.) The German Wikipedia says it had a "clean stall behavior" - now who here can explain why? (I can't.)
I just presume that this configuration makes the airplane's stall not violent, not self-enforcing what it probably could be due to the thin airfoil. Washout may be not needed because there is no taper. On the other hand, there must still be a decent stall if asked for. The stall maneuvers in question are spin and snap, or better flick roll - both are autorotations which are commenced and maintained by a yawing angle in turn maintained with rudder. The wing sweep should help asymmetric stall in this slip motion.



Last edited by UStik; 10-04-2018 at 04:12 AM. Reason: typos
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Old 10-04-2018, 06:27 AM
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Thank you, I couldn't find my reference. Aerodynamics is often not intuitive at all. People studied these things intensively years ago, it makes good sense to benefit from what they learned.

That said, when it comes to model airplanes, we can easily experiment and find out what we like. A guy who seems to have done this a lot who often posts here as 'Rodney' says that all his aerobatic bipes, Aeromasters and others, fly noticeably better with what aeronautical engineers call "positive decalage", which means that lines drawn through the zero lift lines of the airfoils of two wings of a biplane meet in front of the plane. That is, the top wing is negative to the bottom wing. That's what I do too. And, by the way, that fits with what the engineers found.

On many scale types that aren't intended so much for aerobatics, I don't think it matters much. And back to the original question, I've never heard of any biplane models needing washout.

Jim
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Old 10-04-2018, 06:30 AM
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Useful information however we are back to discussing full scale aerodynamics again. As I said previously, our models have a much lighter wing loading which is going to reduce the AOA at which the wings fly. Would this not greatly reduce the amount of downwash from the upper wing? My well proven pattern Biplane runs 1 degree positive incidence on the lower wing and zero on the upper wing. Obviously it is not possible to know at what AOA they are actually at while flying except to say that the fuselage appears to be level with the flight path. According to my understanding of what you wrote this should makenthe bottom wing stall first and in fact during the design phase it was planned that I did want the lower wing to stall first to make for gentle stall characteristics. In fact the airplane does possess a very well mannered stall. Again the full scale requirements for a snap roll or flick roll is that they are stalled maneuvers, not so much with modern pattern. Truth is that the airplanes and required sequences have evolved faster then rules updates leaving us with a slight gray area where we are no longer performing snap rolls in a stalled condition. To make most modern designs " appear " to perform a snap roll we have to set up a condition switch on the TX that results in about 7 degrees of elevator and rudder but 30 degrees of aileron. The result is simply a fast roll with a little bit of pitch and yaw displacement. This is one of these cases where setup and requirements are vastly different from full scale so we need to somewhat step away from full scale theory a bit. Spins are the same, most modern aerobatic models unless you use a massive amount of throw or have a wing loading exceeding 26-28 oz per sq ft are still flying through a spin. I myself have not done a stalled spin entry in years. Nose is held up with elevator until the airplane has lost almost all forward movement, ease off the elevator and let the nose drop, when nose hits level give full rudder to direction desired which will drop a wing tip, come back in with with up elevator and aileron. Aileron is required because without stall there is no autorotation. One huge clue about all this is that the snap roll or spin will stop instantly when sticks are neutralized which to me does indicate the airplane is flying and not stalled. Hope this helps clarify what I mean when I say that our models are somewhat different then their full scale counterparts.
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Old 10-04-2018, 06:35 AM
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If you are using 1 degree positive incidence on the lower wing and zero on the upper wing then you have positive decalage. There is no disagreement.
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Old 10-04-2018, 06:54 AM
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Originally Posted by buzzard bait View Post
If you are using 1 degree positive incidence on the lower wing and zero on the upper wing then you have positive decalage. There is no disagreement.
Last sentence of USticks third paragraph. I think we would also disagree on how much if at all downwash from the upper wing affects the lower wing. Not nessearily debating the principles here, more like how they affect full scale and models differently.
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Old 10-04-2018, 08:40 AM
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speed, I discussed the full-scale BŁcker because it had been mentioned before and I find interesting that its special snap-ability seems to come just not from a violent stall. Sorry for the digression. Your explanation of model stall maneuvers is very good, and I didn't mean the BŁcker's behavior would translate to model size.

Of course you are right in that a biplane's decalage has to be chosen for a certain design point, a certain angle of attack that is. That is done to minimize drag and/or minimize stall speed, but for an aerobat there are different purposes. I just presume that your model has a good-natured stall because, if the lower wing stalls first, it produces much drag making for a pitching-down moment. With symmetric airfoils, that holds even upright and inverted. 1 degree decalage is not much, though, so it could be the airfoil as well that makes for a good stall.

Only in one point I disagree: Wing loading alone is not enough to compare models and full-size. You'd have to consider cubic (and for some movements even quad) wing loading because the airflow is the same if angles of attack are the same. Most models are rather heavy (seems to me) but 2-meter aerobats could be an exception since they are quite lightweight and are flown rather fast. That would indeed mean that biplane decalage has to be small compared to full-size. It's also true that models (at their low Re numbers) stall at lower angles aof attack and lift coefficients than full-size. But otherwise: same angle-of-attack, same airflow.

And yes, the original question has been not explicitely answered: Washout has nothing to do with biplane. It's just used to make a wing stall gentler (or reduce it's induced drag), independent of biplane or monoplane configuration. It could even complicate things on a biplane because it could make the lift coefficients vary over span. It may be even good for a non-tapered wing if it makes lift distribution elliptic and stall even more gentle. But that's not good for an aerobat since it works only upright and would be outright bad inverted.
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Old 10-05-2018, 09:33 PM
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Originally Posted by r ward View Post
if one wing is going to stall before the other,...I would think you'd want the bottom wing to stall first......
For pronounced positive stagger (upper wing positioned well forward of the lower wing), losing a big portion of the lift provided by the bottom wing may introduce a tail-heavy problem.
The location of the CG remains the same, but the center of lift moves forward some.

Last edited by Lnewqban; 10-06-2018 at 06:26 AM.
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Old 10-06-2018, 05:36 AM
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so true !,...never thought about that ! learn something here everyday !
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Old 10-06-2018, 11:06 AM
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Originally Posted by r ward View Post
,......so the question is,..... what about wash out ?......do I need it ?......top wing or bottom wing or both ??.
I've found two plans that I like.......the 62 inch "Lazy Ace" and a 52 inch Pitts Special from R/C Modeler Magazine
I would recommend you borrowing or buying a copy of the following book: "RC model aircraft design" by Andy Lennon.
According to chapter 5 of that book, for the rectangular wings of the Lazy Ace, the stall always starts by the central area and progresses toward the tips.
Also the normal low aspect ratio of the wings of biplanes, makes the wingtips very ineffective regarding lift: no much lift = no much lift stall.
That would make any wingtip wash out unnecessary, it seems.

The sweptback pattern of the top wing of the Pitts Special tends to induce a stall that starts by the wingtips and progresses toward the fuselage.
Since we want to delay any stall for the aft wing regarding the forward wing (for the reason discussed in above posts and hoping for a nose down-pitch in the event of a stall), washout of that lower wing only may be advisable.

I would follow the specifications of both drawings, setting all decalages and incidences using an angle meter, if you have one.
According to this article and its link, most full scale biplanes have no difference in the incidence angles of both wings:

https://en.wikipedia.org/wiki/Decalage



Last edited by Lnewqban; 10-06-2018 at 11:40 AM.
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