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Strakes, how to deal with them?

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Mac790

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Ok, it's time for another question:), I was looking at another Russian Long Ez's clone (pic1), and I'm wondering about their mods.

It's fairly easy to calculate wing surface area for conventional planes (pic2), but what about planes with strakes. May I use a conventional approach to calculate wing surface area? Or should I take those strakes into account?

For example:
VariEze pic3, Berkut pic4

Note, they are going to use flaps, and "moving" canard pic5.

Seb
 

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autoreply

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Given the very low aspect ratio of the strakes I would go for the wing area, including the fuselage part where the projection of the leading edge would protrude, just like in conventional designs. While both the strakes and fuselage produce some lift that's accounted for by the wing area "inside" fuselage and strakes. In this case that gives two straight leading edges that cross inside the fuselage.

In reality those strakes might also gives some vortex lift. I don't know of a simple approximation for that and given the high amount of variables I doubt there's one, though you might be able to approximate the higher aoa at which the wing stalls (due to the extra vortex energy on top of the wing).

Why are the canards moving? I can't think of a reason except for mach effects (and looking cool)
 

Mac790

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Why are the canards moving? I can't think of a reason except for mach effects (and looking cool)
I'm guessing but I think it's related with flaps (pitching moment), if they move canard forward they also move CG point forward, Beech Starship had similar solution.

For more info about their concept try those links.
Google Translate

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Seb
 

orion

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In reality those strakes might also gives some vortex lift. I don't know of a simple approximation for that and given the high amount of variables I doubt there's one, though you might be able to approximate the higher aoa at which the wing stalls (due to the extra vortex energy on top of the wing).
For this particular instance I would tend to ignore the strakes and use the theoretical wing area only. While it is true the strakes might provide a bit of lift, it is going to be very minimal, especially considering they sit in the downwash flow of the canard (the latter is actually one of the biggest omissions canard designers tend to make - they forget about the downwash field off the canard, forcing the outer wing to carry more load than it probably should).

As far as vortex lift is concerned, keep in mind that a canard design is an aoa limited airplane. In order for a strake to generate a meaningful vortex flow field it needs two things: 1) a reasonably small leading edge radius and 2) a relatively high angle of attack.

The strakes on the canards are usually configured to carry fuel so a need for volume results in them being fairly thick with rather large leading edge radii. This will keep the flow attached, even to relatively high angles of attack.

However, since the canard configuration is angle limited, it is unlikely that the airplane ever reaches an attitude that might initiate a vortex flow field of any significance.
 

Mac790

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Thank you guys:)

For this particular instance I would tend to ignore the strakes and use the theoretical wing area only. While it is true the strakes might provide a bit of lift, it is going to be very minimal, especially considering they sit in the downwash flow of the canard.
If I understand it correctly it means that theoretically it could be possible to decrease the strakes size without limiting wings lift significantly. Sounds like a relatively safe mod, would you recommend analysis for it?

The strakes on the canards are usually configured to carry fuel so a need for volume results in them being fairly thick with rather large leading edge radii.
The Long Ez/Berkut strakes like you said are configured to carry fuel (outer parts) and also to carry baggage (inner parts). The inner parts could be removed (to reduce wetted area, and improve visibility), if someone doesn't need a baggage area.

Seb
 

orion

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If I understand it correctly it means that theoretically it could be possible to decrease the strakes size without limiting wings lift significantly. Sounds like a relatively safe mod, would you recommend analysis for it?
I guess it depends on how comfortable you are with the design. In general, yes, I'd probably recommend at least a cursory examination of any significant mod. In general, it seems like a lot of the early canard designs were almost eyeballed - even Rutan goofed in that he totally ignored the downwash field on the initial VariEze configuration and it wasn't until after Dave Lednicer pointed it out, did he make any changes.

But yes, given the interesting flow field in that area, I'd say that any significant configurational change should be checked.
 

wsimpso1

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The story on speed strakes on low wing conventional monoplanes is that they really do not add any additional lift, and you should skip their wing area - just use the theoretical wing area.

On the canard ships, the strakes are there to put the fuel load on the cg, and little else. And +1 on everything Orion and Led have said about the topic.

Geez - flapping the main wing and swinging the canard forward to keep the ship in balance - some folks just don't have enough to do in getting an airplane flying and working well...

Billski
 

Mac790

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orion said:
But yes, given the interesting flow field in that area, I'd say that any significant configurational change should be checked.
At the moment it's only a theoretical discussion, definitely I'll ask some experts about their advices, opinions etc. before I decide to make any significant changes to the existing design.

...and it wasn't until after Dave Lednicer pointed it out,
I've heard about it before, I was wondering, is there any report about it?

wsimpso1 said:
Geez - flapping the main wing and swinging the canard forward to keep the ship in balance - some folks just don't have enough to do in getting an airplane flying and working well...
I have same opinion, Long Ez is just too small aircraft for those devices, they not only add complexity but also dead weight, I won't be surprised if stall speed for flapped version will be higher than for unflapped.

Seb
 
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deskpilot

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Seb, I'm so glad you raised this question as I'm having the same problem with my Eagle-Ray design. I've been working out the Aspect Ratio and haven't been able to get it down from 8.1. The question arises as to how much of the plan view area does one use. On my current model, the stakes are pretty small but I am thinking of widening them by giving them a truncated delta leading edge. Why, well I want to decrease the overall width but maintain as much lifting area as possible.
Another question arising is, does one take the plan view area of the winglets and fins into account, after all, they do provide some lift due to their angle.

Sorry to patch into your thread but it's a very interesting one and somewhat 'outside the square' compared to conventional design work and theory.

Orion, care to reply?
 

ragflyer

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The strakes have a low AR and as such, as every one has pointed out, contributes little to the total lift. However, it has an impact on stability-it will change the NP of the aircraft if you remove it.

The strakes are not unlike a fuselage, ( low aspect ratio) that contributes little toA the lift but have an impact on the NP.

As for calculating its effect on NP an equation is given in the book
Introduction to Aeronautics: A Design Perspective

The equation if I recall is from datcom, and they use it to calculate the NP of a number AC including the F-16 and compare it to actual data (very favourably).
 

Mac790

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As for calculating its effect on NP an equation is given in the book
Introduction to Aeronautics: A Design Perspective
Thanks for the book title, fortunately a few chapters are available online.

Chapter 6 Stability

http://www.soton.ac.uk/~jps7/Aircraft%20Design%20Resources/Brandt%20Introduction%20to%20Aeronautics/Ch6Stability.doc

Chapter 5 Performance

http://www.soton.ac.uk/~jps7/Aircraft%20Design%20Resources/Brandt%20Introduction%20to%20Aeronautics/Ch5Performance.doc

Seb
 

orion

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I'd say you really don't want either. First off, your airplane concept is "cute" the way it is. Second, both mods would introduce potential penalties you probably don't want to deal with. Regarding the strakes: Much like a dorsal fin on a vertical stab, the principal purpose of strakes is to generate a large low pressure field at high angles of attack thus increasing maneuvering agility. For instance, in a high G turn the strakes on an F-16 are generating about 70% of the lift. But along with the lift there are also dramatic shifts in pressure distribution over the strake and over the wing. Cm vs. alpha plots of straked configurations tend to look like abstract string art - something you really don't want to deal with in a light plane. The only straked light plane (built by Rans) was abandoned due to unpredictable flying qualities.

Forward sweep is a bit less risky but due to structural requirements, even a moderate forward sweep will significantly affect the structural weight of the wings. If your goal is light weight, forward sweep will penalize you. And as a side note, even the slight aft sweep you have now will cost you a bit in weight over a straight tapered wing.
 

Turner

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Orion, how about a slight strake (like a inner section of the wing with a slight sweepback. Actually I'll draw something in MS Paint (laptop atm) :gig:.

fenix.jpg

I see too much angle on the 'strake' now but generally how about that?

I appreciate the feedback too as it was I who suggested a slight forward sweep and strakes to topspeed. :)
 

orion

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Ok, what you show is not a strake for two reasons. First off, "strakes" extend from the leading edge forward. Furthermore, their geometry is such that they present a "significant" amount of area and are also at a relatively shallow angle to the airflow (large sweepback angle so that they form a vortex flow at higher aoa's). In essence, you can think of them in the same manner as for a dorsal fin in front of a vertical tail. In the case of the wing though, the application has to be one very carefully since trim effects can be significant and destabilizing. For example, look at the strake on an F-16 or an F-18. Their purpose is to move the Cp at higher aoa forward thus providing the plane with high rate turns and increased pitch agility.

The root extensions such as you show are generally used to deal with the wing/fuselage flow to decrease interference drag. A good example can of course be seen on most modern airliners on the high end; and the Hershey bar wing on the Cherokees on the low end. The leading edge extension is referred to as a "glove"; and the trailing edge extension is called a "yehudi".
 

topspeed100

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They got rid of the glove when redesigning P-51D as P-51H with a straight wing and smaller dia wheels. Possibly the field operation were trickier in Korea for the small wheels and no H models ever saw combat.
 
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