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Main wing position, question.

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Mac790

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Finally I have a little bit more free time (unfortunately only a little bit more), so I'm reading again. Today I was reading about stability in the "Simplified Aircraft Design" by Raymer, and I came across this " after calculating your design's static margin, you may need to move the wing for the second drawing" to the rear for too low static margin, and to the front for high static margin. To that point everything is "clear", (it's about the distance of the center of gravity and the neutral point in the horizontal axis if I understand it correctly), but I was wondering, what will happen if I decide to move wing a little bit higher or lower let's say about 5-6inches (for example for better visibility, etc, I'm thinking about changes in a existing plane). So next I picked his "thicker" book "Conceptual Approach", but I haven't found anything so far, no relation (main wing "vertical" position vs stability, etc). Next I checked our archives, I found a few similar threads (low wing vs high wing, etc) but no answer to my question. I'm aware about structural changes, but what about stability, handling qualities, etc. Could anyone give me the answer or point me to the right book, I'm still digging in Raymer's books.

Seb
 

djschwartz

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The wing vertical position has little effect on stability unless the vertical distance from the wing to the C.G. is very large, as in a weight shift ultralight or powered parachute. The primary effect of wing vertical position is the trim change with power and that is a function of the relationship of the wing position to the thrust line. And even that effect is small unless the airplane has an unusual configuration. There will be some effect on static roll stability due to wing position. This is why low wing airplanes tend to have a bit more dihedral than high wing airplanes.

As for the distances you mention, how extreme that move is depends on the overall size and mass distribution of the aircraft. If you're talking about something the size of a KR1 then this could produce some noticeable changes in behavior. For something the size of a Cessna 172 the effect on stability would be small.
 

orion

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The only measurable physical affect of wing position (vertical axis) versus static stability might be the level of downwash the horizontal tail sees during the different phases of flight. True, it's the longitudinal position that has more bearing on the numbers but vertical position will have some affect also, albeit it'll most likely be relatively minimal. For level flight, the downwash field will be a bit more "effective" for a high wing than for a low one however for most conventional small airplane layouts, the changes will be no more than about 5%. I don't recall a specific calculation for this - the number came from a presentation I attended about fifteen years ago, so I don't even have a reference for you.

Overall though, whether you select a high wing or a low one will most likely be more a function of your own goals and the airplane's other requirements more so than any stability concerns.
 

djschwartz

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FWIW, having flown many examples of both, the Cessna 172 and Piper Cherokee are about as similar as two different aircraft can be. Differences in performance and handling are very small and mostly attributable to design choices like differences in control surface configuration and size of the flaps. The biggest difference a pilot must adjust to between these two is cockpit layout. And that can be an issue even between old and new vintages of the same type.
 

bmcj

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FWIW, having flown many examples of both, the Cessna 172 and Piper Cherokee are about as similar as two different aircraft can be. Differences in performance and handling are very small and mostly attributable to design choices like differences in control surface configuration and size of the flaps. The biggest difference a pilot must adjust to between these two is cockpit layout. And that can be an issue even between old and new vintages of the same type.
Without going out to the airport or searching on the internet, does the Piper Cherokee have more dihedral than the Cessna 172?

Bruce :)
 

Dana

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The Cherokee has a lot more dihedral than a Cessna... but the Cessna gets dihedral effect from being a high wing, whereas the Piper does not. That's one effect of wing vertical location...

-Dana

A flying saucer results when a nudist spills his coffee.
 

orion

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... and the Piper could actually do with a lot less. Most single engine Pipers have 5 degrees - 3 would have been plenty. I think I'd like my Pathfinder's handling, especially in roll, a lot better with less. But, then on the other hand, it is a very forgiving a docile airplane. So I guess the bottom line is simply what qualities you're going after.
 

Mac790

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Orion, Dj,
Thanks for your responses.
Orion said:
The only measurable physical affect of wing position (vertical axis) versus static stability might be the level of downwash the horizontal tail sees during the different phases of flight
Ok so if understand correctly if I lower both wing and the H-stab about let's say 4-5inches, it shouldn't be any major difference (pic1). It seems that it's possible for conventional planes, but what about Canard planes, I know it's a different story even Raymer said, that he wouldn't trust his life to a new canard design without hiring somebody to run aerodynamic analysis on it, etc. Today I was looking closer at Stagger Ez, Cozy III (pic 2,3), and Long Ez, Russian Intruder (pic 4,5). Both Stagger and Intruder have lowered wing, so I'm wondering is it possible to lower both wing and the canard for this type of planes without extensive aerodynamic analysis.

Seb
 

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orion

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If no other changes are made then I'd say yes. But I strongly agree with Raymer - canards can bite you. Even small errors can potentially result in disaster. On a conventional airplane the tail is simply a trim and maneuvering device so minor flow changes in its vicinity will have little effect. On a canard however the fore-surface has a very significant affect on the primary lifting surface (flow quality and angle, lift distribution, upstream turbulence, etc.) of the vehicle so changes should be undertaken with extreme care.

It's interesting to note that on the first VariEze series Rutan did not account for the downwash off the canard and its effect on the main wing. Only after Dave Lednicer pointed out the deficiency did Rutan incorporate changes.
 

Mac790

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Thanks again, no other changes same distance between wing and a canard, both in the vertical and the horizontal axis, same surface area, dihedral, cg, etc. I know it can bite, Peter Garrison said even more "The basic problem of the canard configuration is that if you get it wrong, you're probably dead"
It's interesting to note that on the first VariEze series Rutan did not account for the downwash off the canard and its effect on the main wing.
Interesting, I haven't heard about it before.

Seb
 

bmcj

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Quote:
It's interesting to note that on the first VariEze series Rutan did not account for the downwash off the canard and its effect on the main wing.

Interesting, I haven't heard about it before.

Seb
Yep, doesn't matter how sharp you are... sometimes you miss things.

And if people like Orion, Rutan, and Kelly Johnson can miss things, what hope do the rest of us have?

Bruce :ponder:
 

djschwartz

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Yep, doesn't matter how sharp you are... sometimes you miss things.

And if people like Orion, Rutan, and Kelly Johnson can miss things, what hope do the rest of us have?

Bruce :ponder:
I think the point is: if anyone thinks that aircraft design can be done on a bit of wishful thinking and a few quick calculations they're in for a big disappointment. And the farther you want to wander from well established conventional designs the more difficult, and potentially dangerous, it becomes. Sure, it's not to hard to make a small change in a well established design with little more than a bit of common sense and an understanding that ensuring safety means not getting the most performance possible when the design resources are limited. If one wants to make a significant contribution to the state of the art it's going to take a lot of work, not just in the initial design phase but also in the amount of careful testing and revision that will be required. Designers like Lance Niebauer, Burt Rutan, and Dick VanGrunsven did not achieve the be-all-end-all design on their first attempts. They're still improving their designs today.

Too many people here seem ready to believe that somehow all who have come before them have missed something big. There are reasons that aircraft look the way they do today and it's not because the designers were dumb or there was some conspiracy hiding the magic silver bullet solution. While it is certainly true that we do not yet know everything there is to know about aircraft design, the last hundred years has produced a huge body of knowledge on the subject. It behooves anyone seriously interested in design to take some time to study up before launching into a new design project.
 

Mac790

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Sure, it's not to hard to make a small change in a well established design with little more than a bit of common sense and an understanding that ensuring safety means not getting the most performance possible when the design resources are limited.
Dj what do you mean but small changes, do you think that for example lowering both wing and the canard about 3-4 inches, without changing other things like, airfoils, c.g, surface sizes, etc is a small or rather a significant change. Please keep in mind that I'm not going to build anything (at the moment), I even haven't decided what I'm going to build in the future, I'm only collecting opinions and ideas.

Seb
 

djschwartz

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I hate to be vague, but "small" is relative and depends on the design you're starting with as well as your level of design experience. If you start with a design that is very critical, having been highly optimized for a particular mission or is the result of extensive design work by an experienced team; then, almost any change becomes "large". On the other hand, take a very conventional design like a Cessna 150 or Aeronca Champ that is based on older design knowledge and has been shown to have lots of margin in its design then the kind of change you're describing would probably count as small. As Orion has said, canard designs bring about a whole different set of issues from conventional aircraft and the pool of knowledge and experience for them is no where near as large. I have little experience with canard design so I would hesitate to make any comments there.

Which brings up my final point. If you do not have the knowledge or experience to comfortably make your own determination of whether the proposed change is large or small; then, it is large for you by definition.
 

djschwartz

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Let me add one more thing. Just because a change might be "large" for a person doesn't mean they shouldn't do it. Just that they should be prepared to spend time, energy, and probably money researching the subject, trying out designs, and having those designs reviewed by some one with much more experience in that area.
 

Mac790

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Which brings up my final point. If you do not have the knowledge or experience to comfortably make your own determination of whether the proposed change is large or small; then, it is large for you by definition.
Very smart response, so I'll read more about it, make a few examples (there are also other possibilities) and I'll come back to this later. I think it's also a good idea to get in touch with that other guy (Stagger Ez owner).

Edit
trying out designs, and having those designs reviewed by some one with much more experience in that area.
Yes I was thinking about it too, but it will be hard Raymer is very sceptic about Canards, Orion doesn't "love" them too, Burt gave up after 30 years, and I'm not a millionaire to hire John Roncz. Definitely a lot of reading is necessary. We'll see.


Thanks
Seb
 

Mac790

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Ok a few more updates just in case if anyone is interested. I was digging today a little bit and I came to the one conclusion, there is a one "golden rule" for most canard planes, don't change distance between canard L.E and the wing L.E.

I found also some info about the Stagger Ez, so now I know that the engine in that plane was raised up by 7", the result is a slight pitch change, which was compared to the change in the pitch perfomance similar to what retracts do to a Canard plane. info here

I was very surprised when I found out that the distance between the canard L.E and rear of the spar is exactly same for both the Varieze and the Long Ez, it's 106.3" (pi 1,2), I was expecting some difference because Vari is smaler plane than Long.
I also think that the Stagger Ez is based on Cozy, distance between Canard L.E and wing L.E is same 95.9", for example for Long and Vari it's 94.7". It also seems that only guys from BerkutEngineering were "brave" to enlarge distance between canard L.E and wing L.E for about 8.7" (pic3), with that mod they could use Lycoming 540 without lead ballast in the nose.

But the most interesting was what I found in the VariViggen flight report
"VariViggen was designed in the wind tunnel rather than by calculations. I started out with a canard configuration quite a bit different and kept changing until it flew in the wind tunnel the way I wanted the real thing to fly" .
it was a car-top wind tunnel (pic4) :), not the real one.

Seb
 

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bmcj

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Ok a few more updates just in case if anyone is interested. I was digging today a little bit and I came to the one conclusion, there is a one "golden rule" for most canard planes, don't change distance between canard L.E and the wing L.E.
NOTE (added after the original post): When reading this post, please be aware that Seb (Mac790) was correct, but my reasoning below was completely wrong and apparently the result of a major brain-fart. Read Topaz' explanation in the next post to see why my post here was wrong. I don't want anyone to to get hurt because of my mis-stating the facts. bmcj.

That actually makes sense. The safety feature in a canard is that the canard surface stalls before the wing ever reaches its stall AOA, thereby preventing the main wing from ever stalling. If you shorten the arm between the two wings, then the main wing will be pulled to a higher AOA when the canard stalls (due to the geometry), possibly stalling the main wing. On the other hand, I don't see as much harm in going with a longer arm because the wing will actually be at a lower AOA when the canard stalls... but the downside is that your minimum flying speed will be higher because the main wing will not be able to reach as high on the Cl alpha scale.

Bruce :)
 
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