I am in the process of building a Pilot Christen Eagle scaled up to 68-inch wing span. The CE uses a fully symmetrical wing. The wing cord will end up at 11 inches and should only be about 1.5 inches thick.

What if I make the wing thickness bigger, perhaps 1.7/8 inches thick, did I improve anything in terms of slow flight speed or did I just add useless drag?

tonyc

Realistically, not enough to make any difference. Slow flight is mainly a function of area, not wing thickness (that was a really broad brush statement with lots of assumptions but close enough - maybe)

You'll just move the top speed closer to the stall speed.
Eagles, Pitts... very antsy airplanes.
Must fly fast.
Add LOTS of lightness!

Is that why Chip Hyde likes to have thin Ultimate wings, to increase his top end speed?

It sounds like perhaps I am going in the wrong direction, I should leave well enough alone or go thinner with the wing?

I know the Pilot Eagle is a great flying airplane, why fool with something that works.

tonyc

There's another broad brush generalization which IMHO is fallacious, TallPaul. For years the full-scale Pitts has deserved its reputation as a squirrel, a treacherous flyer. In the last five or six years, and in my personal experience with the Cermark, and then the GP Pitts, the designers have done their homework and these airplanes fly just as well, if not better than most of the sport monoplanes on the market; that is, of course, unless you choose to load a couple of extra pounds of engine on the nose, increasing the sink rate and stall speed and pucker factor. I wouldn't mess with the thickness of the airfoil. But then again, that's just me...JIM

Pitts and Eagles, along with Gee-Bees, Comets... locally, are all miserable failures!
I know of a Christen Eagle that cost over $3000 to build... has yet to fly, but it sure breaks props when the guy tries!
OTOH, locally Charlie Richardson(now deceased) designed and built an Eagle using fibreglass molds... (lots of lightness) which flies well.
When an EXPERIENCED flier is flying it.
Pretending such planes are easy to fly is disengenuous.
They attract a lot of enthusiastic but somewhat inexperienced fliers, who try and fail to get the d**n thing into the air.
Experienced folks who have no problems with these things and get all upset when less experienced folk do, need to fall back and regroup.
These are NOT for the inexperienced flier, no matter how simple someone WITH the experience demonstrates "ease of flight" can be..
Sucessful Gee-Bees, Pitts, etc will be LIGHT, flown fast, and by experienced pilots.
.
A scale Ultimate, which I fly, has the same brick tendency as the SIG Ultimate, which I also fly, with power off.
Both are a blast to fly with power on although the thin scale wings can't do much to overcome all that drag!
And the SIG has more drag than any two airplanes should!
The Stearmann... fine under power, must be on final when the engine quits...
Lightness AND experience make these things flyable.

The others are right.
Making the wing thicker just adds drag.
The full size makers use thicker wings because it is easier to make them stronger, but any more than 12% thick is just extra drag to reduce the top speed.

If you want a slower stalling speed you can either add camber to the wing (use a Clark Y section or Selig 3021) or, if you want to keep the inverted flight capabilities, temporary camber in the form of flaps. I doubt if the tail is big enough to cope with the pitching effect of flaps.

The only way left is to keep the wing loading low. The lower the wing loading the slower the stalling speed.
Alasdair

On a bipe -- as Bro Lanternman noted -the area is king.
thickness only needs to address structural rigidity.
Most smaller model kits of bipes are grossly overweight and all the futzing with airfoils is wasted effort .
about 10% is a decent thickness for structure.
blaming airfoils for flying character on our models is as futile as baying at the moon.
Build a few bipes-- it all comes rapidly into focus.
Our first home design bipe was a Ardun powered controlline model -in 1951-
it had a "low " glide ability.
We did some TOC bipes which were extremely successful- wing loading was in the low 20 ozs to the ft.
I also did a 50% Challanger this year, for a friend --and we just assembled a 44% Ultimate for another friend.
These newer designs -the good ones - use thinner foils .
I hate rigging bipes -so I don't go to the 7% type foils .

Jim mentioned the Cermark Pitts. Thats the biplane that I fell in love with. Its a Dave Patrick design, which I am sure you know. After I flew this biplane I picked up the Carl Goldberg Ulltimante and scaled it up to 62 inches. The Ultimate came in at 15 lbs or 30 oz wing loading. Its a very good flying biplane, a Dave Patrick design. But it needs to be a little lighter. If I were to build it over again I know I could save 2 lbs.

I have been following the reports on the GP Christen Eagle. Including watching the video on the GP web page. My first impression is, well not much. Watching the video and reading some of the builders comments tells me that the GP Eagle may be a little heavy. I have read 18 to 20 lbs using a gas engine. This comes out to 29-32 oz wing loading.

My target weight for this Eagle is under 14 lbs with a Moki 210. This will be the same size CE as the GPCE, only the wing loading will be in the low 20's, 22-23 oz wing loading.

Dick is right, make it light and all else falls into place. I had one of his pattern planes years ago, it was light and flew great. Remember the Tipo Surpass?

tonyc

tonyc- You would probably agree that the Cermark is a easy flying airplane; take-offs and three-point landings at walking speed are easy, and they don't take a grey-beard on the sticks. TallPaul, I am sure there are lots of bipes(vast majority?) that are treacherous buggers, and my experience is probably less than yours, but all I can speak to is my own limited experience. I heard a lot of "How hard is it to fly?" from a lot of fellow flyers with a couple of years on the sticks; I let several try the sticks, and without exception they all remarked how gently it flew compared to my club's unofficial "mandatory" models, the Sig Somethin' Extra and the GP Dazzler. If anyone cares, I can furnish more info on the Cermark setup-incidence, design changes, etc...JIM

A couple years ago I lifted one of Mike Cross's Goldberg Utimates.... it didn't weigh anything! Probably the lightest plane for its size and type I've ever seen.
I asked him how he did it... "Don't add anything!" he said. We tend to "improve" the structure etc when it is already adequate.
This particular plane... on takeoff, just before rotation, he'd cut the power to idle, then rotate and it would go up about 50', then he'd add power and hover.
A .91 motor, if memory serves.
I have a Uravitch (from MAN plans) Fokker D-VII. Made it more scale, yet it's still very light. Dead sticks are of no concern, unlike my Dr-1 (scaled down Ziroli), which is a beast, taking off and landing! Deadstick, fugedaboudit! Lightness makes up for the additional drag on a bipe.

Since you are trying to reduce level flight stall speed, the wing's lift must remain constant until it exceeds the critical angle of attack and it stalls. [&:]

The "Lift Equation" that gives the amount of lift generated by a wing is:

L= 1/2 * air density * velocity (squared)* wing area * Lift Coefficient


It is worth noting that the lift generated depends directly on each of the terms (air density, velocity, wing area and lift coefficient) - that if any one increases, lift will increase, and that if any one of the terms decreases, lift will decrease. One final observation is that if you want to decrease speed, but hold lift constant, we can increase CL by increasing the wing's angle of attack, which is what we do when we enter level slow flight.

Lift Coefficient (CL) is an adjustment factor for the airfoil section being used and the angle of attack being flown. CL increases with more cambered airfoil sections, extension of flaps, slats, etc, and increasing the angle of attack.

Using the lift equation, if we wish to slow the airplane below its "normal" level flight stall speed, the air density is fixed and we want a lower velocity, so we must either increase wing area or CL.

If we increase the thickness of a non-symmetrical airfoil, we increase the airfoil's average camber. Increasing camber causes a small increase in CL (on the order of less than one tenth of a CL unit or so - depending on the change imposed and the airfoil section chosen). As I recall, normal CLs for "clean" wings are in the range of 1.2 - 1.5 or so, so it is easy to see that getting a large increase in lift from increasing camber alone is difficult. while increasing wing area will increase lift directly (meaning a 10% increase in wing area will increase lift by 10%), so it will do so more efficiently than increasing camber for most airfoil sections.

BTW - those are some COOL Mo-Fokkers!!

HTH

Jim

Bipes simply can't get too light -
Mike Cross is very good at keeping things light - he also did the Great Planes 330 - a terrific design.
I first ran into him when he was flying one of my IMAC designs - the EXCESS (doubled as a 330)
In my little foamies - I strive for zero weight - -so far -no cigar - but as you guys who actually fly have found out - the planes take on a whole new character if the weight is at rockbottom
even the CG becomes very non critical (there I go again )and allow you to run the CG around as desired for 3D or best precision stuff.

If you want a bipe that is excellent in aerobatics yet will have a good glide ratio then the Goldberg Bucker Jungman is the ticket. Unfortunately it is out of production. I have sent emails suggesting they bring it back but to no avail. Anyone who has flown one knows what I am talking about. Just an fyi.

Frank