undercamber VS. flat bottom (airfoils not fannies)

[radioinred]

Hi all, Im the kid who is designing the UL bipe with the little 250 snowmobile engine. I am currently looking for the best airfoil for my wings. Things I know: 1) I want to cruise at or around 55mph 2) MTOW is 600lbs 3) horsepower is about 27 with redrive and powerfin.

I have been going back and forth between an undercambered or flat bottom airfoil like clark y. Will the undercamber create too much drag or is the stability more important at this slower airspeed?

Here are some quick drawings.....

[mstull]

Radio,

I have experimented successfully with adding undercamber on two of my designs. It doesn't affect stability at all. It does improve aerodynamic efficiency. Many, if not most U/Ls have a flat bottom airfoil just to make construction simpler, and you can save weight if you design for it. At U/L speeds, the loss of efficiency of using a flat bottom is pretty insignificant.

But you can gain a lot of low speed efficiency by adding under-camber. The slower you fly, the more you gain. So near stall speed, you'll get much more lift and efficiency. In climb you'll still get significantly more lift and efficiency, that translates into a better climb rate. As you get up near your proposed cruise speed, the gain would be small. Much faster than that, and you can lose efficiency... depending on how deep the under-camber ,and wing loading, and other factors.

The way under-camber works... Picture the air flowing off the back of your wing. The air flowing over the top is angled down a lot because of the shape of the airfoil. Now picture the air flowing under the wing. With a convex under surface, like a Clark Y, the rear part of the airfoil curves up toward the trailing edge, so it doesn't force air down much. With a flat bottom, it's better. But with an under-camber, the air is directed down adding lift.

In addition, there is inefficiency because the air flowing off the bottom collides with the air flowing off the top. The greater the included angle of the trailing edge, the greater the inefficiency. With under-camber, the included angle is much less, so the air doesn't collide. The air flows smoothly down gaining lift while decreasing drag. Ideally, the included angle should be as small as possible, like the airfoil I used on the U/L in this picture. That airfoil is a Gottingen 387 on top, with my own design on the bottom.

A deep under-camber like that improves your stall speed by at least 3 mph. And it helps the plane climb well with low horsepower, which would help your plan. And it makes the plane glide much better.

One disadvantage of adding under-camber is all the extra work in fabric covering. And you'll need pretty closely spaced ribs to keep the fabric from dipping between them. So you have to be careful not to add weight in the process.

Another disadvantage is the under-camber doesn't leave much, if any, room for a rear spar. Some designs compromise the ideal under-camber shape to allow room for a rear spar, by only making the very aft 1/8 or so of chord concave. I've seen that on some gliders.

Yet another disadvantage is it increases adverse yaw from the ailerons. You might need a larger rudder, or move the rudder farther aft to improve its leverage. And it could make it difficult to design the aileron spar with enough torsional stiffness. If you make the aileron chord greater to gain room for a larger aileron spar, it becomes aerodynamically advantageous to continue the concave shape on the ailerons... which adds even more work.

Adding under-camber to an U/L airfoil is not rocket science. You want smooth, gentle curves... as smooth as possible... both in the concave part in the aft 1/3 of chord, and the convex part at the front 1/3 of chord. The deeper you make the under-camber, the more you gain. The more smooth and gradual you make the curves, the less you lose at higher air speeds.

Be aware that adding under-camber moves the center of lift of the airfoil aft significantly. A biplane can help resolve the structural problems, if you use struts between the wings and crossing wires. You can get your torsional stiffness from that, and get by with a rear spar that's farther forward.

Good luck with your design.

[Dana]

Mark already covered most of what I would have said and then some, so I'll just add a couple of comments:

Even the original Clark Y airfoil isn't completely flat bottomed; if you plot it accurately there is a very slight undercamber. Most people modify it to make it flat, though, for ease of construction.

For most ultralights, airfoil selection isn't critical, and most designers use the TLAR section ("That Looks About Right"). One designer, when asked what section he used, gave a numeric designator, which turned out to be the size of the automobile tire he bent the ribs around.

"Undercamber", as a term, really isn't descriptive of an airfoil. An airfoil's shape is defined by two main things: The thickness distribution, which for the NACA 4-series airfoils is defined by the same equation for all the airfoils, multiplied by the thickness, and the shape of the camber line, which is halfway between the upper and lower surfaces. Thus for a given camber, a thicker section might be convex on the bottom while a thinner section with the same camber might be flat bottomed or undercambered. As an aside, the NACA thickness equation was derived by analyzing two of the best airfoils of the day, the Clark Y and the Gottingen 398, which turned out to have the exact same thickness distribution, and working out the equation.

I mention the above because you can see that an undercambered airfoil will be thinner than a flat or convex bottom section with equivalent mean line camber, and besides the structural issues Mark mentioned, the stall behavior may not be as friendly.

-Dana

You do not need a parachute to skydive. You only need a parachute to skydive twice.

[lr27]

You'd probably really have to give us more details before we could recommend anything in particular with any authority.

An "undercambered" wing, will, all else being equal, have more pitching moment and more camber. But all else isn't usually quite equal.

There are some airfoils which are convex on the bottom which still have high lift coefficient and also have a small pitching moment. If you want to keep the tail small, this may be a good idea. 23012 is a traditional one, though it's said to have a sharp stall. 23016 was used on the Sky Pup, I believe. An example I like is one of the FZX airfoils, I think the FZX ng7, which may be found over at that Xfoil Yahoo group. Low pitching moment, high max lift, 15 percent thick gives room for structure. Some Russian outfit, perhaps TSAGI, had an airfoil that was very similar. And it's interesting that the Driggs Dart, from back in 1924 has an airfoil which looks similar, at least at the root (1931 Flying and Glider Manual, EAA carries it) If pitching moment doesn't matter, the old NACA 4415 is pretty good. The MH92 is like this but probably goes too far trying to get rid of pitching moment. And the 43012 belongs here too. None of these are horribly draggy in cruise. What's odd is that, according to Xfoil the FZX has the highest max lift, and the 43012 is next. Neither has much pitching moment. The FZX gets even higher lift than the Gottingen 535, and the 43012 gets almost as much. But the Gottingen has lots of pitching moment.

BTW, negative (the usual) pitching moment is like aft center of lift, except that it doesn't go to infinity the way the center of lift does when you fly fast. The pitching moment doesn't change all that much with speed, but if you're getting a big nose down twist while the wing is hardly lifting, that center has to be way back to give you the twist actually experienced. So pitching moment is easier to deal with.

[bmcj]

I instructed in Eipper Quicksilvers and got to fly both versions, flat bottom and undercamber. I much preferred the undercamber. It flew a little slower at the top end, but also at the low speed end. It would also climb like a homesick angel and carry a good load in the process.

Bruce

[Dana]

The Quicksilver is an extreme case, but it just shows how almost anything will work on an ultralight. The first Quicks were single surface (a single layer of fabric, shaped by battens slipped into sewn in pockets), so the airfoil had no thickness at all, except for the pocket at the leading edge where it wrapped around the front spar. These models were very slow but as you say climbed very well. Later, they added the lower fabric and made it into a flat bottomed airfoil. Current models have either the flat bottom wing or a partial double surface, where the leading edge pocket goes back to maybe to 1/3 or 1/2 of the wing chord,with a single surface behind that.

-Dana

A seminar on Time Travel will be held two weeks ago.

[BBerson]

Radio,
The best airfoil for your I beam spar shown in the drawing would be thicker than average.
As mentioned, an undercambered shape is thin and not the best choice for your case, which may be a cantilever wing.

I would look at some modern airfoils such as the Eppler E580 used on gliders or search for a thick airfoil designed for the ultralight speed range.
The Eppler airfoils have a convex "cusp" near the trailing edge that adds lift without the high drag of a full undercamber.
I agree with Dana, in that I don't think "undercambered" is a term that modern airfoil designers would use. They just use the word "cambered".
Just my thoughts.
BB

[bmcj]

Radio,
I agree with Dana, in that I don't think "undercambered" is a term that modern airfoil designers would use. They just use the word "cambered".
BB

Not to nit-pick, but you can have a highly cambered wing with a flat lower surface, convex lower surface, or concave lower surface. When I hear "undercambered", I know they are referring to an airfoil with a concave lower surface.

Radioinred states that he is aiming at an ULTRALIGHT biplane with only 27 HP. For this, I think that undercambered is still a very viable choice, for both it's lift curve and potential for light weight and slow speed.

Bruce

[BBerson]

My Eppler airfoil book does not have any "undercambered" airfoils for any use other than the very low Reynolds number range below 1 million for models. I can only assume that "undercambered" airfoils have no use in the Reynolds number range of ultralights and gliders. Ultralights and gliders both operate in the Rn range of 1.5 to 3 million and should use the same airfoils, I think. Am I wrong?

I think undercambered airfoils are for slow models.
BB

[Dana]

No problem, I was just making the point that the term "undercambered" is neither sufficient by itself nor necessary to describe an airfoil... while still being a useful term to describe what it looks like.

The mere fact that the lower surface is concave really means nothing; it's just a by-product of the camber and thickness, which do matter.

-Dana

Dullard: someone who can open an encyclopedia or dictionary and only read what they'd planned to.

[radioinred]

Excellent point Dana, Im concerned that an "undercamber" airfoil will produce more drag than my 27 ponies can handle. I love the idea of "TLAR" though. Thats how the pioneers did it! I plan on having both a for and aft spar mostly because I cant imagine engineering a suitable wing structure without an aft spar. A flat bottom wing means less headaches when building and covering but perhaps its worth it for the gain in lift at slower airspeeds. THE WORLD MAY NEVER KNOW.........

[bmcj]

No problem, I was just making the point that the term "undercambered" is neither sufficient by itself nor necessary to describe an airfoil... while still being a useful term to describe what it looks like.

The mere fact that the lower surface is concave really means nothing; it's just a by-product of the camber and thickness, which do matter.

Cambered/undercambered. I'm still trying to figure out if it is SIX or HALF A DOZEN. Don't even get me started on whether the glass is half full or half empty!

Bruce

[bmcj]

Excellent point Dana, Im concerned that an "undercamber" airfoil will produce more drag than my 27 ponies can handle. I love the idea of "TLAR" though. Thats how the pioneers did it! I plan on having both a for and aft spar mostly because I cant imagine engineering a suitable wing structure without an aft spar. A flat bottom wing means less headaches when building and covering but perhaps its worth it for the gain in lift at slower airspeeds. THE WORLD MAY NEVER KNOW.........

As Mark Stull has pointed out, he has used an undercamber with low HP.

Bruce

[Dana]

Well, first you need to decide whether your wing construction will be the traditional ultralight style of sailcloth with leading and trailing edge spars and hand bent battens (ala Quicksilver), in which case the "TLAR" airfoil is about the best you'll do, or the "airplane" style with precisely shaped ribs, in which case you might as well use a proven airfoil.

If you prefer the latter (which is more work), take a look at the Kolb construction. They use built up aluminum ribs (still with a "TLAR" airfoil, though) slipped over a single aluminum tube main spar. The drawings for the older Kolb UltraStar are posted in the files section of the kolbultrastar yahoo group.

I'm leaning towards this type of construction for my own ultralight biplane that I'm working on the concept for, though in my case the goal is aerobatics so I'll be using a semi-symmetrical airfoil, likely a 23012 or 23015, and flaperons to meet the FAR 103 stall speed requirement. If I didn't want to fly upside down and have a sharp stall for snaps and spins, I'd probably choose the Clark Y.

Don't even get me started on whether the glass is half full or half empty!

A pessimist says the glass is half empty.
An optimist says the glass is half full.
An engineer says the glass is larger than it needs to be.

-Dana

[bmcj]

A pessimist says the glass is half empty.
An optimist says the glass is half full.
An engineer says the glass is larger than it needs to be.

-Dana

And a planner says that the extra is there for future growth.