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

biplane wing incidence.

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Old 09-10-2018, 07:25 AM
  #1  
r ward
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Default biplane wing incidence.

when setting incidence on a biplane with an swept upper wing, is the incidence set by adjusting the lower wing ?. I ask this because with the upper wing being swept, and built flat, as usual practice, if you set the incidence for that wing at the wing root the sweep will produce a drooping wing for positive incidence and a rising wing for negative incidence in association to the bottom wing. is the incidence set by the unswept bottom wing only, keeping the top wing parallel with the thrust line ?.
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Old 09-10-2018, 11:07 AM
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All Day Dan
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No incidences on this one. Thrust at zero. Dan.
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Old 09-10-2018, 01:34 PM
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r ward
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dan,.....if the wings on that plane are straight, as it looks they are,..... your reply is not pertinent to my post. I know biplanes look like,....I don't need to see yours . I didn't post the question to give you an opportunity to put another one of your planes on line. please be respectful enough to consider answering the question instead of wasting people's time showing them things they don't need or want or asked to see.
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Old 09-10-2018, 03:10 PM
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All Day Dan
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Just trying to help. Dan.
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Old 09-10-2018, 05:27 PM
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Not sure if this will answer your question either but what the hell. On my Divergent design the wing platform is a bit unique. The inboard panels both sweep but the outboard panels have a straight TE. To be honest I did not take any of that into consideration when setting the incidence of both wings. The upper wing was set to zero using the motor thrust line and stab incidence as reference points. Initially the bottom wing was set to .75 degree positive. After initial test flights it was determined that the lower wing needed a bit more incidence and it was increased to 1 degree positive. The theory behind this is that in level flight the bottom wing does most of the work until the AOA is changed and then the benifit of more wing area kicks in. The combination is working well as the airplane has what I call " true control " which simply means it has virtually zero control cross coupling.
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Old 09-11-2018, 05:07 AM
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r ward
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showing pics of planes does nothing to answer the question I posted. I cannot see the incidence on the picture and the picture does not help you explain an answer you are giving because we cannot see the incidence on the picture. further more neither of the planes shown have swept top wings, so all you have done is waste space and time posting a picture of a plane you apparently think is something special, regardless of whether the question posted has been answered or discussed.
I got absolutely nothing out of all your pictures and nothing out of the answers that were given because the answers were not related to the question posted. I can go anywhere, the library, any one of the other sites any magazine and see pictures of biplanes,....I don't need to see yours to get an answer to my question. all you have done is demonstrate that you have no respect for anyone's post unless they look at your planes and compliment you on them.,.....answering someone's question is the farthest thing from your intentions.
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Old 09-11-2018, 05:21 AM
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what non of you seem to comprehend or realize, is that you have to build a swept wing ( such as a swept upper wing on a biplane) with the incidence built into the frame work of the wing unless the incidence is established at the bottom wing, which usually has a straight leading and trailing edge. if it is not built into the wing's structure, giving the wing incidence in relation to the bottom wing will create either dihedral or andihedral effect both of which changes the planes response to the incidence spec'd by the plans.
showing pics of a plane you built does nothing to discuss the possible answers as no-one can see the amount of incidence the plane has and no-one can possible measure it, as presented in a picture.
you see,......your posting pictures were not for the benefit of the question, what-so-ever,....were they ............
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Old 09-11-2018, 05:38 AM
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I wonder how many newcomers to r/c flying and building get discouraged and give up on our hobby, because people have to post pictures of their planes instead of plainly and directly answering the posted' questions.
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Old 09-11-2018, 06:37 AM
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He stated the wings are both at zero. IMHO that indeed applies to your first post.
But, I will also agree it does not apply to the rants that are your responses to his post.

Now if you want a more specific response to your original question, you need to provide more information. For starters:
Are the wings the same area/size?
The top being swept, is the bottom then straight or swept also?
Which wing do you want to stall first?
Do the wings use the same airfoil?
Are the wings staggered?

Regardless of the wing planform, zero zero is always a good place to start. Alternative would be to add a degree or two to the top wing.

Information flow is a two way street! And like a computer, Garbage in! Garbage out!
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Old 09-11-2018, 08:09 AM
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Personally I would think the sweep angle on a bipe has very little affect on the incidence requirements. If you were to refer to the pattern forum there is a thread on this subject that will demonstrate there is no one size fits all on the subject. You could also pose your question on the modeling science forum on RCG and end up with pages of theory. I doubt that given the tone of your responses that anyone here would be interested in offering you any further advise.
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Old 09-11-2018, 08:49 AM
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Two things I'm going to say for this thread, one as a participant and the other as a moderator:

1. Sweep doesn't matter when setting incidence. The designer of the plane has specified an incidence that both wings need. If this is your own design or you are tweaking a plane to change the performance, each wing's incidence setting will affect the plane's flight. There is no rule that the top wing needs to be at zero or 1 or any other number.

2. Niceness gets you more help and makes you more friends than rudeness. Some people just really like to post pictures. Let it go.
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Old 09-11-2018, 10:33 AM
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Sweep should have more affect as observed dihedral effect. Although both wings on my pattern design pictured above are perfectly flat the airplane exhibits a slight dihedral effect while in flight. As a result the next one built will have 2-3 degrees of anhedral to eliminate this unwanted trait.

My pictures posted IMO is simply an effort to illustrate that there was considerable consideration on a great many details while in the design phase and not some simple hap hazard design. I stand by the idea that pictures can add to the credibility of the information presented.
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Old 09-11-2018, 12:32 PM
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Thinking about this a bit more, this picture of my 54" Reed Falcon illustrates the wing sweep well. Both wings and stab set to zero as per plans however the engine is set to 2 degrees upthrust also as per plans. Both the Carl Goldberg and Dave Patrick Ultimates ( both designed by Dave Patrick ) have the engine and both wings set to zero but have the stab set to positive 2 degrees. No set way to skin this cat.

Last edited by speedracerntrixie; 09-11-2018 at 04:56 PM.
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Old 09-12-2018, 05:27 AM
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r ward
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I guess to understand my posts you have to know a little bit about 9th grade geometry........
try to visualize a swept wing being set at a bit of positive incidence. the mid section of the wing will simply be pointed up in comparison to the bottom wing,....but as the wing travels out ward to the tip, it's swept geometry moves downward towards the lower wing. the chord of the wing still has the same positive incidence as the mid section , but because of the sweep angle, the leading and trailing edge have a downward pitch produce less distance between upper and lower tips compared to the distance at the mid section. .
if you have trouble visualizing this. cut out a small swept wing from a piece of cardboard and give it a decent amount of sweep. now,... holding the wing at eye level . rotate the wing to produce some positive incidence, and watch how the wing moves. you will see that incidence does affect dihedral.
as for my attitude, there seems to be a reluctance to accept someone with a different point of view on this forum by all the "regulars here". I have seen this on many forums and it is because someone who thinks they know it all has brainwashed everyone to think that if he says it is one way, they all support that idea and the response to a contrary post is refuted before anyone even gives it any thought. this is borne out by the responses in this thread, that claim "sweep doesn't matter when setting incidence".
you may think what you want,.......but math provides the truth and math dictates that sweep does infact, affect both incidence and dihedral effects.
speed racer,...by your post I assume your plane has negative incidence in the upper wing, if then, yes some amount of anhedral will correct the geometry.
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Old 09-12-2018, 04:24 PM
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So why start a post with an insult?

I can visualize what you are talking about yet I still fail to see what difference it makes in regards to setting the incidence. With your project you will most likely set the incidence to what you think it should be at and if you are proficient enough at flight trimming an airplane you will most likely make an adjustment the same as I did. As far as making assumptions concerning my airplane, please don't. I previously stated that my airplane has zero incidence on the upper wing. This has been verified by securing an aluminum bar onto the motor and then the airplane was leveled on a marble inspection plate at my place of employment and the upper wing was measured to have zero incidence. The stabs were also measured and adjusted at that time. Only the bottom wing has positive incidence.

as far as " regulars" are concerned I will agree that in these forums a certain breed of Cyber Bully do exist. What I have experienced are guys with some full scale education tend to be the blow hards. For some reason they don't see the value in experience accumulated by us modelers. They also tend to not back up anything they say with results. For example, I have explained the way I have my airplane set up and reasons why. It has competed in several pattern contests and has proven competitive. I have had a pattern pilot whom has been on the US F3A world championships team multiple times as well as being a former TOC pilot fly my airplane. At the end of the flight he had some very positive comments along with asking if he could be the first person to call should I produce a kit. In my world results is what counts. In others worlds, if it wasn't published it can't be true. So be it.
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Old 09-13-2018, 03:24 AM
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Rward- You last post is the first one where you talk about the dihedral effect being made stronger by adding incidence to the wing. Yes, there is an effect there. I think most of us understand 9th grade geometry well enough and would have verified that for you if that had been what you asked.
My only point, since you decided to quote me, was that there isn't a rule about wing incidence on biplanes. As speedracerntrixie has taken significant time and effort to show you, there are much bigger considerations when setting wing incidence, and there are successful biplane designs out there that do it all sorts of ways. Your original post asked if a swept upper wing has to bet set at zero incidence. The answer is no.
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Old 09-13-2018, 04:07 AM
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nowhere in my original post,.....or in any other post in this thread, for that matter,..... did I ask "if the upper wing has to be set at zero incidence".
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Old 09-13-2018, 04:24 AM
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" Keeping the top wing parallel to the thrust line " wouldn't that be the same thing? Even so, I did a little experiment last nite on my Reed Falcon wing. One degree of positive incidence equals about 1/16" wing droop at the tip TE. Just how much difference in flight performance would you expect from that on a sport model?
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Old 09-13-2018, 05:25 AM
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the point is,..... that it is there,....my question would be addressed as "would it be better to build that out by shimming the leading and trailing edges when building the wing ?.. it seems to me that this forum has a tendency to provide double standards as answers to many simple questions. a question is posed as a discussion catalyzer, and it is criticized,....then it is established that the question is legitimate and the replies suggest it doesn't matter.
again it's no wonder people get disenchanted and discouraged with the hobby if they come to these forums as a source of initial information.
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Old 09-13-2018, 08:01 AM
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Especially when the question asked is rather vague and the OP is quite rude when he gets answers other then what he wants. This thread is starting to smell like a trolling effort. Best of luck with your project.
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Old 09-13-2018, 04:29 PM
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Originally Posted by jester_s1 View Post
Two things I'm going to say for this thread, one as a participant and the other as a moderator:

1. Sweep doesn't matter when setting incidence. The designer of the plane has specified an incidence that both wings need. If this is your own design or you are tweaking a plane to change the performance, each wing's incidence setting will affect the plane's flight. There is no rule that the top wing needs to be at zero or 1 or any other number.

2. Niceness gets you more help and makes you more friends than rudeness. Some people just really like to post pictures. Let it go.
as I see it sweep does matter. positive incidence produces anhedral geometry and negative incidence produces dihedral geometry, both in relation to the bottom wing and the line of thrust. no plane yet, just a question to discuss incidence. I realized the geometry as I thought about incidence on swept wing biplane and wanted to see if anyone else ever considered what I envisioned. also as the sweep increases in angle the effected geometry is increased in both instances.
what I am wondering is that if swept upper wing has positive incidence is the resultant droop built out of the wing by shimming the wingtip leading and trailing edges and if the wing has negative incidence, is the rise built out by shimming the root leading and trailing edge ?.
I don't see how the relationship between wing sweep angle and incidence can be ignored.

Last edited by r ward; 09-13-2018 at 04:33 PM.
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Old 09-13-2018, 04:32 PM
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Originally Posted by speedracerntrixie View Post
Especially when the question asked is rather vague and the OP is quite rude when he gets answers other then what he wants. This thread is starting to smell like a trolling effort. Best of luck with your project.
why would you think i'm trolling, when I ask a legitimate question about a specific detail in the construction and geometry of a biplane wing. what reads as rants and raves, is simply factual truths plainly stated.
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Old 09-14-2018, 12:33 PM
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It seems to me that the selection of an airfoil section is just the first step of a design process. These sections are generally presented (on airfoil websites) at what they consider a zero angle of attack. The section is seldom (maybe never) actually put into an application at zero AoA. It would first be set to the "design" angle and THEN the wing DESIGN process starts. In other words, using the section at the DESIGN angle, the rest of the wing is designed, with dihedral, washout, etc. THEN established.
The assumption that a wing "section" should be assumed from the start to be at zero is flawed.

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Old 09-14-2018, 02:48 PM
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Allan, I see where you are coming from and to a certain degree will agree with you provided we were discussing full scale design theory. Let's take Dan's airplane for example. It appears that he has the engine, both wings and the stab set to zero. He does not indicate what airfoil was used but for conversation let's assume symmetrical. We know that a symmetrical airfoil is not going to support the airplanes weight at zero degrees. This means that the airplane will need to carry some up trim to provide the required force to hold the wings at enough of a positive AOA to support the airplanes weight. Two things result, the airplane as a whole will be flown at a positive attitude and the airplane will never hold a pitch trim over different air speeds. For a sport model most guys would be perfectly fine with all that.

Now let's take my Divergent as an example simply because it's flight performance is under a much greater scrutiny. As I have previously stated I have everything set to zero except the bottom wing which is currently set to positive 1 degree. The philosophy behind that is I am aiming for the bottom wing to be supporting the bulk of the airplane weight while in level flight while upright. While inverted by keeping the upper wing at zero my theory is that less fuselage AOA will be required to maintain level flight as now the top wing is doing most of the work. No way to avoid some fuselage AOA while inverted however the theory is that in that orientation it's not as noticeable to the judges. Since there is no up trim required for upright level flight and the fuselage is being flown level also no engine up thrust is present in that situation as well along with a properly adjusted CG the model will hold a constant pitch trim from 1/3 power to full power.

Now the reality. We throw these incidence specs around as if they are a hard number when the reality is that they really are not. We use them as a baseline so that we know where everything is however that does not mean they will actually be at those angles while in flight. That's when we have to note how the airplane is flying and make adjustments accordingly. The end result is to get the airplane flying to a point that meets our expectations and/or requirements. For a pattern airplane such as mine that is pretty much a never ending battle.

Concerning the topic of this thread. I did some light research last nite and looked up some plans for models of the Christen Eagle, Pitts Special and Steen Skybolt. As best as I could determine all examples I viewed showed the top wing at zero incidence. Not all that conclusive as the font was too small to read but most had a center line drawn through the airfoil center that ran parallel with another reference line on the plan sheet. Finding information on the full scale aircraft was non existent except a build log on a full scale Christen Eagle that indicated that the top wing is set to 1.5 degrees positive. The Biplane incidence thread here on RCU in the pattern forum has some pretty noteworthy players in the pattern community. Reading through that thread I see no mention of incidence being influenced by wing sweep. IMO it simply is a non issue. I see where Rward is coming from but at the end of the day the wing droop associated with running a small amount of positive incidence on a swept wing will not create any adverse aerodynamic traits. Introducing washout to eliminate the situation depending on the model type could lead to worse flying performance then leaving it alone.
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Old 09-14-2018, 03:11 PM
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Speedracer, you obviously have a lot more practical RC flying experience than me. Your kind of flying calls for a very subtle setup on all things. I am "sport flyer" typical and just try to keep from crashing. I do have design experience and my planes have always performed well, by my meager standards at least.
Your last sentence hits me as especially important, Too many people overthink things and get into more trouble than needed.
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