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Discussion in 'General Experimental Aviation Questions' started by Dave Hodges, May 17, 2019.
What percentage of an airplane's weight should the wings typically be?
Thurston's Design for Flying has plotted data on the topic. For a flapped wing he shows 19% at 1050 gross and 12.5% at 2400. He has lower weights for simple wings. He also indicates this is Part 23 aircraft, and given the era of its writing, is probably derived from sheet metal airplanes.
Raymer probably gives some guidance too. I have trouble with these simple curves - use them for basics only and up date when you have done detail design. Airplanes designed to higher g's or exceptional wing spans will have more wing weight, advanced composites and good design work can reduce weights. My 6 g wing with 30 foot span is about 10% of my gross and uses a FOS of 2.0 not 1.5.
My 6.7 g wings with a FOS of 1.5 wings are 15%.
It probably scales with aspect ratio. My glider has an empty weight of 440 lbs, and the wings weigh 220 lbs for the pair.
I know that Autoreply might disagree, but in practical terms it also makes a difference whether the wings are cantilever or externally braced.
But for typical airplanes of typical size and shape, 15% to 20% sounds about right.
The deeper I got into weight estimation (with different results from different authors' methods), the more I started to think it's better to design the plane (to a point), then weigh it (on paper), and iterate.
In other words, take any form of estimation as a mere estimate.
People should not confuse “estimate” with “SWAG”. SWAGs are more prevalent than estimates.
I think parts count, where the structural divide/stub spars are and the complexity of the systems is a way bigger factor than AR.
Pretty sure that if you double chord and halve the span (AR/4), the lower spar weight is far offset by heavier skin (thicker foam cores against buckling, heavier control system and controls). Obviously there's a point where shear bending moment starts contributing disproportionately, but that's typically when the AR exceeds the owner's average age ;-)
My average age is in the 30’s, but I’ve been flying much longer than that.
If the wings are removable, the fittings will make the wings heavier than one with the wing fixed permanently. How much heavier depends on the method used to facilitate wing removal.
I believe a Tailwind wing panel weighs about 70 lbs ready to fly. So times two plus struts. Materials picked are going to sway it. Load it needs to carry is going to sway it too.
So you think that you are an above average pilot?
No, just a slow learner.
Ercoupe wings come in two flavors: Cloth and metal covered. The cloth ones are 35 pounds lighter. C and CD models are 1260 pounds gross weight. Don't have weights off the top of head. You are probably going to have to do the build/iterate route.
Googling around indicates:
"The Wittman W-8 Tailwind is a popular two- seat light aircraft for homebuilding. ... Empty weight: 685 lb (311 kg); Gross weight: 1,235 lb (560 kg); Powerplant: 1 × Lycoming O-320, ..."
So 140/685 is a little over 20%.
Ercoupe fabric wings are 60 pounds each, according to the parts manual online. Just the outer panel, not the center section.
You are right on with the added 35 lbs for the metal covered Ercoupe wings. I have had fabric covered and metal covered Ercoupe wings.
One time I ask Fred Weick what he thought about metal covered Ercoupe wings. He reply was "Its not an Ercoupe ".
I believe the W10 Tailwind grosses at 1450. W8 is the lower, although it is mostly the same wing. I think the heaviest W10 gross I know was set at 1700. Lots of fuel.
'One time I ask[ed] Fred Weick what he thought about metal covered Ercoupe wings. H[is]reply was "Its not an Ercoupe ". '
Fred was correct, of course. At the time of "Metalization," the Grade A Cotton was just starting to be replace with Dacron. Grade A had to be replaced on a rather frequent basis; my last wing cover lasted from 1984 to 2017 when I took it off.
Thanks for pointing that out, Pops.
I will add a partially relevant wing weight question here, and ask for a SWAG answer from any of those with more design/engineering experience than myself.
I have been making sketches of an aluminum version of the Debreyer Pelican, which is a Fauvel style flying wing, but much shorter span than the Fauvel gliders. The Pelican has approx. 24 foot span and 2 meters chord at the root. Straight center section, tapered tips (outboard half), straight spar with no sweep. It's a very thick wing section, 17% thick airfoil on the prototype, which equates to something like a 12 inch spar depth (!).
I am looking at aluminum tube and gusset truss main section ribs, because the're too large to make out of sheet metal without wasting a lot of material. The ribs thus far are using 1/2 inch x .035 6061 tube in a Warren Truss shape. The nose ribs are small enough to be formed sheet metal with a flanged lightening hole.
The main spar will be a vertical sheet metal .025 web with thicker .063 sheet metal upper and lower flanges (bent angles) riveted on. Additional upper and lower spar cap and center section doublers will be 1/4 x 3/4 aluminum strip. So the inboard 6 feet of span will have the heavy sheet spar flanges top and bottom, plus two cap/doublers trips on top and two on the bottom. The mid section (mid-center section to just outboard of the planform break) will have the heavy bent flange plus only one strip top and bottom. The outer tip section ribs will only have the sheet metal flange serving as the spar cap.
These are not set in stone at all, there will be a real live engineer involved to optimize this all, remove metal where it is not needed, or add metal where it is necessary. At this stage it is a ROUGH sketch for very basic visualization. What I am asking for here on HBA is also ROUGH guesses from people who can guess-timate things for the sake of initial discussions only.
Using VERY rough measurements and estimates, and using the aluminum weights listed in the Aircraft Spruce catalog, I came up with about 68 pounds for the basic wing structure (spars, main ribs, nose ribs, rear spar channel, .016" leading edge D-tube sheet). NO control surfaces or control systems were included in this, no fins/rudders, and none of the fuselage or powerplant. No fabric covering was included. I did include an additional 20% added to the shear web weight to account for doublers and triplers where the fuselage/seat will bolt to the spar. Although the leading edge D-tube should probably handle drag/anti-drag loads I added a 7/8 x .035 x 6 foot diagonal tube in each wing panel for a drag brace if it becomes necessary.
So the big question... does this wing weight seem "in the ballpark" to you guys? OR does it seem like this is way to optimistic, or heavier than it ought to be, etc.???
The WILD GUESS is that this aircraft would tip the scales at right about Part 103 weight, or perhaps a little less. It should have a little better than "ultralight" performance on account of reduced drag. But this is NOT a 150MPH aircraft, if I see 80 MPH I would be pelasantly surprised. When the real engineer becomes involved, I'm going to have him adjust the structure for a 6.6G ultimate, which should be very difficult to achieve with a flying wing that will have a tendency to pitch-dampen itself in gusts as the planks tend to do.
Does this 68 pound number seem reasonable, ridiculous, or ?????
A wing panel for the JMR Special weighs 65 lbs ready to cover. Stressed for 10.1 ultimate. Without out the 8.3 gal aluminum wing fuel tanks. 28' 6" wing span, 4' cord.
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