Da-2 major modification

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Kurt Ayres

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Oct 12, 2019
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I'm a big fan of this airplane, and am wondering what might be involved in modifying the design to somehow "nudge" the stall speed down into Sport aircraft territory (the published S1 is 62 mph!). A larger wing would seem to be an answer, and any general guidance you might provide that would point me in the right direction would be appreciated.
 

ToddK

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You would be looking at a totally different wing minimum with flaps, probably a bit longer, with a bit more chord to get it to fly slower, maybe even a different airfoil. Its only got 83 sq feet of wing area, that is going to need to go up. Plan on hiring an engineer. Fully cantilevered aluminum wings are not something you just want to eyeball.
 

Vigilant1

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Larger wing, then a bigger tail is needed. The snowball grows.

If the DA2 was a very heavy plane, in theory you might get to the LSA stall speed just by reducing max gross weight. But Leeon Davis didn't design any fat airplanes.

An LSA will be a different airplane. You might start with the DA-2, but you might as well start with something else.
 

Dan Thomas

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When the DA-2 came out, its stall speed was too high for Canada's amateur-built rules, so Davis designed flaps for it and renamed it the DA-2A. I can't remember how much the flaps cut the stall speed. That info might be buried on the blueprints I have here, some 300+ square feet of them. The DA-2 website would be a better source.
 

TFF

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Over the years people have added a foot or two to the span. It gets better but it’s still too small for LSA. The fuselage gets to be too short and tail gets too small so there is a limit to scab it on. It needs span and a 200 lb loss in weight. Using it for a template idea, develop a new plane like it, same construction, but strength will need to be checked.
 

lr27

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You might be able to get there with complicated high lift devices, but I wonder if the tail wouldn't be too small for slow flight. My rough calculation is that you'd need 37 percent more lift, if the original stalled at 60 mph. That's a big increase. How much do vortex generators help?
 

lr27

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I read someplace that slats might add 0.5 to the Cl, but that may be for 2d, not a real wing. If I'm not mistaken, slats require an especially nose high pitch attitude. Plus I wonder if a typical engine for a DA-2A has enough thrust for that kind of draggy, slow flight on short wings. Also if the tail would be large enough.
 

erkki67

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Ir the DA-2 please so much, go with a proven design like the Zenair CH601 as a base, you’ll have the tapered wing or squared wing as an option, nose or taildragger gear. You’ll have to compare the fuselage design and the tailsection. You might want to modify only the engine cowl windscreen and turtledeck to make it look a like a DA-2
 

Kurt Ayres

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Thank you for the responses, everyone. I figured it would be impossible, but I wanted to toss the idea out to the experts here in case I missed something. The problem with adding high lift devices is that those wouldn't help with the Sport limits -- it's based on S1, not S0. I'm a scratch builder, and, with the trend going away from plans to kits, there aren't a lot of choices out there. My plan at the moment is to wait for the Thatcher CX-7 plans to come out.
 

bob21921

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The solution is already been designed. It is called the DA-3, the fuselage and wing chord are extended to lower the stall speed or carry more weight (4 seats). There were only two built, one in LA, and another in MI? The LA owner owns the plan copyrights and has the copyrights and plane for sale.

Bob
 

BoKu

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...BTW, Vans missed the LSA stall speed with the [RV-12] prototype, and had to add a little wing area...
...And ended up just about redesigning the airplane end to end. It was still pretty much the same collection of parts, but every part in the wing and mid-fuselage ended up being changed slightly to accommodate the greater area. I was at the Vans factory to help a friend pick up a finish kit at about that time, and got to see the red RV-12 prototype parts next to the re-design. They were same-ish but not same-same.
 

wsimpso1

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I get that S * deltaCl will have to increase 42% over the current bird to go from 62 mph down to 52 mph. That is a long ways without adding wing area.

Slats? They have to be fixed permanent to be counted, so they are a bit of a disadvantage. The 2D data at zero flaps says about 0.5 deltaCl, but make those slats full span on a 3D wing, and it you multiply by about Pi/4, which is 39%. Actually you won't do that well, because the slats make the wing behave like it has a somewhat lower aspect ratio, costing you some lift everywhere. Tails with permanent slats are several: Va based upon wing loading must drop while wing spar stresses at any g loading you pick will go up too, probably forcing you to reduce Va some more or risk having a does of big turb bend the wings...

VG's will buy you a little, but mostly these things are for tailoring your stall and STOL behavior. Mostly you make the air behave like the wing is a little thicker from the point of the VG's on aft, which is not a little more cambered like you might want. Can you fly slower with VG's? Usually a little, but tough to count on that...

The right answer is more wing and/or more airfoil camber. Span increase is the most elegant, and they will all require a structural review perhaps a beefup. Then there is the little issue of stability and control - tail sizes may need to be bumped too. Oh, and once you do a bigger wing and tails, the fuselage should be checked out. If you do not have background in structures, an professional who specializes in sheet metal aero design is in your future.

Trying to make a DA2 into a Light Sport is a reach.

This is the big reason I suggest that in general, you establish your mission, then build or buy an airplane that does that mission, suits your flying preferences. And if you are building, it must be built of materials you love building in.

Billski
 

litemite

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Maybe another way to tackle the stall speed thing is to change the airfoil. Leon designed it with a Clark Y section which has about a Cl of about 1.4 clean where as a Riblett 615 has in the neighborhood of 16 The computed drag is also less by .02. Just tossing that out to see if it might work.
 

tdfsks

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A more modern airfoil would help a little but it is not going to be enough. The DA-2 is going to need more wing area (like the DA-3). However, PA-28 Cherokee style slotted flaps would help enormously, especially if the extra span was all flap and the ailerons were kept the same size. Tail area would also have to be increased as tail volume is marginal.

The gross weight of the DA-2 is 1150 lb. Wing area is 82.5 sq ft. The required stall speed for LSA aircraft is 45 kts CAS (75.9 fps). CLmax = 2 W / (ro x V^2 x S) = (2 x 1150) / (0.002378 x 75.9^2 x 82.5) = 2.03 .... which from memory is about the same CLmax as the above mentioned PA-28. However, a Cherokee has a bigger flap as a percentage of the span than could be fitted to a standard DA-2 (fuselage occupies a bigger percentage of span) and aileron already takes half of span outside fuselage). Based on this, I think a 2 ft span extension (1 ft each tip) with the existing 4 ft aileron and a 5 ft slotted flat (flap on DA-2A is about 4 ft) would likely get the required 45 kts (also need to increase tailplane span by say 6-8" a side).

With regards to a modern airfoil, remember that the Clark Y has a flat bottom aft of about 25% and that shape is built right into the DA-2 fuselage bottom (lower wing skin is fuselage bottom) and so it would not be a trivial matter to change the airfoil.

However, this is not the main issue with the DA-2 ...... there is a much more difficult issue to be solved. The reason why you can no longer build a DA-2 to Leon Davis's original drawings is that there is no source of the 2024-T351 angle (2"x2"x1/4" from memory) in the lengths required. The wing spar is one piece and each cap has a kink at the fuselage side for dihedral. Yes, I know plans are available, but the wing spar extrusions are unobtainium. There is a design for a multi-piece wing but I think only one or two aircraft have used it and it is not as nice looking as the original (dihedral break is outboard of fuselage side) and heavy/complicated. The DA-2 needs a new wing spar designed using available materials. I have done some work on a new spar design but would need some encouragement from a couple of serious builders to finish it.
 

poormansairforce

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A more modern airfoil would help a little but it is not going to be enough. The DA-2 is going to need more wing area (like the DA-3). However, PA-28 Cherokee style slotted flaps would help enormously, especially if the extra span was all flap and the ailerons were kept the same size. Tail area would also have to be increased as tail volume is marginal.

The gross weight of the DA-2 is 1150 lb. Wing area is 82.5 sq ft. The required stall speed for LSA aircraft is 45 kts CAS (75.9 fps). CLmax = 2 W / (ro x V^2 x S) = (2 x 1150) / (0.002378 x 75.9^2 x 82.5) = 2.03 .... which from memory is about the same CLmax as the above mentioned PA-28. However, a Cherokee has a bigger flap as a percentage of the span than could be fitted to a standard DA-2 (fuselage occupies a bigger percentage of span) and aileron already takes half of span outside fuselage). Based on this, I think a 2 ft span extension (1 ft each tip) with the existing 4 ft aileron and a 5 ft slotted flat (flap on DA-2A is about 4 ft) would likely get the required 45 kts (also need to increase tailplane span by say 6-8" a side).

With regards to a modern airfoil, remember that the Clark Y has a flat bottom aft of about 25% and that shape is built right into the DA-2 fuselage bottom (lower wing skin is fuselage bottom) and so it would not be a trivial matter to change the airfoil.

However, this is not the main issue with the DA-2 ...... there is a much more difficult issue to be solved. The reason why you can no longer build a DA-2 to Leon Davis's original drawings is that there is no source of the 2024-T351 angle (2"x2"x1/4" from memory) in the lengths required. The wing spar is one piece and each cap has a kink at the fuselage side for dihedral. Yes, I know plans are available, but the wing spar extrusions are unobtainium. There is a design for a multi-piece wing but I think only one or two aircraft have used it and it is not as nice looking as the original (dihedral break is outboard of fuselage side) and heavy/complicated. The DA-2 needs a new wing spar designed using available materials. I have done some work on a new spar design but would need some encouragement from a couple of serious builders to finish it.
I've had some interest in this plane as well. You numbers pretty much confirm mine in what needs to happen to get this to LSA. I've also got the multi-piece folding wing option PDF. But I've also thought it'd be nice just to design a new wing that's a little bit longer. Consider me an interested builder!😉
 
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