New Design

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mstull

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As usual, I went through a lot of napkin drawings before stumbling upon something interesting for my next U/L design. It didn't come out like I expected at all. The parameters are: pusher; tricycle; easy to fly and land; quick and easy to build; light and simple; truly legal; leaning more towards a motor-glider.

The longer, under-cambered wings will use an ordinary, fabric covered, aluminum, ladder frame, with 34' span, 43" chord, supported by flying wires, and ordinary, notched in ailerons. I plan to use a 24 Hp CorsAir engine. It should climb and glide excellent with the long, efficient wings.

I'm planning to put the engine and prop under the wing, figuring on about a 50" prop. Moving the engine forward and the cockpit back, makes the fuselage structure come out compact and simple. I'll hang the fuel tank under the back of the cockpit.

The empennage structure is an open tripod, with one tube on top, and tubes coming from both sides of the lower fuselage. This will be my first design to have a true fuselage and just one rudder. The small wing chord will allow a short fuselage... short enough to fit in my trailer.

I wanted to mount the main gear directly on the cross tube at the bottom of the main fuselage triangle, but then the plane would be a tail dragger. So I had to add a second axle on trailing arms. It adds some weight and drag, but might solve another problem... getting the fuselage to fit through the door of my trailer. If I disconnect the trailing arm bungee loops, the plane will drop down 9".

With these long wings and closely spaced spars (because of the under-camber and small wing chord), the wing could easily twist adversely when the ailerons are deflected. So I added an extra set of flying wires for the outer part of the wings near the middle of the ailerons.

The ribs will be similar to the ones on my Lexan sheeted wings, using aluminum tubing for the bottom of the ribs, with styrofoam glued on top, and 1/32" plywood rib caps. I'll use a Gottingen 387 airfoil for the upper surface.

The bottom fabric will be unsupported, except for PK rivets into the rear spar, to give it the under-camber, like my biplane had in the center of its upper wing.

I'll use false ribs to minimize leading edge sheeting. The full ribs will be about 2' apart, with false ribs spaced about every 6".

I haven't worked out all the connections nor control systems. I like to solve some of that as I build. I wanted y'all to be the first to see and discus my design on these hallowed pages. Let me know if you see something amiss.
 

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Dana

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What's the matter with making it a taildragger?

-Dana

"If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy." James Madison, while a United States Congressman
 

mstull

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Dana,

The weather. About 1/2 of the days that are good enough to fly with a tricycle, it would be too breezy to taxi with the wings at the high AOA of a tail dragger. If I had a fully enclosed cockpit, I could fly it in the early mornings or evenings when it's not breezy. But I prefer an open cockpit. So I fly in the hot, breezy, bumpy afternoons.

You're welcome to explain why it would be just as safe to taxi in breezy conditions, if you think it is. That's my only reason not to make it a tail dragger. I would love to save the weight and complexity, and would enjoy flying a tail dragger. I'm trying to think of another way to partially disassemble it to get it through the trailer door.

A tail dragger would save about 8 pounds. I even had a cool revelation: Tail draggers have more difficult ground handling because the main gear is well ahead of the CG. On my design, the main gear would be barely ahead of the CG if I put the wheels on that cross bar. But I could leave the training wheel on to keep it from nosing over. So it would be a real easy ground handling taildragger.

Do you think it would be possible and safe to hold the nose wheel down by holding down elevator? Having both a tail wheel and nose wheel would counter some of my weight savings. But if I can keep it on the nose wheel most of the time, I could get by with just a tail skid. Convince me... please.

One of my best friends nosed over his Luscombe the day before Thanksgiving, when he slammed on his brakes. He accidentally landed down wind at a fairly short strip. He didn't have insurance. It will/would cost about as much as the plane is worth to fix it. He's thinking of selling it for salvage. It is/was an 8A that fit the LSA rules.
 

Dana

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Well, I'll admit to a bit of a bias in favor of taildraggers. :)

The biggest wind related issue for taildraggers is, of course, crosswind landings. That's mainly an issue of practice, and knowing when to do a 3 point landing and when to do a wheel landing (it took me a ground loop in the T-Craft to learn that one... fortunately nothing was damaged but it took quite awhile to dig it out of a snow bank!)

I never thought of taxiing as being an issue with a taildragger vs. a nosedragger. Certainly you should use good crosswind taxi technique. I suppose it works both ways... you may be more susceptible to a wind gust quartering off the nose, but you're less susceptible to one from the rear. Weathervaning is probably more of an issue, which is one reason I'm glad I have independent wheel brakes on my plane.

In my own case, I figure I can safely fly and taxi in any winds that I can fold or unflod the wings in (
). One one occasion I went up when I shouldn't have... I managed to land and taxi back OK, but as soon as I started the unfolding process I knew I was in trouble... I used my cell phone to call friends who were hanging out at the other end of the airport to come and help me.

One advantage of a taildragger is that the wind tends to hold it down to the ground if you park it tal into the wind. Of course, a pusher ultralight that sits on the tail when the pilot isn't in the seat (something I've never liked, though I recognize the necessity) is no different.

Putting on a nosewheer and tailwheel seems redundant, and if you were going to do that and taxi on the nosewheel (which would need to be steerable), then there would be no need for a tailwheel. If you're worried about nosing over like your friend (a non issue if you take it easy on the brakes at low speed), you could do as I've seen on some Kolbs: a metal hoop under the nose. The later Kolbs (not mine) have the gear farther back than typical taildraggers, and also a lower AOA on the ground, both of which contribute to the reputation for very benign ground handling. You might consider that... though you lose the ability to make a full stall landing (a log of Kolb pilots always do wheel landings).

Actually, I'm toying with (and fighting!) the concept of a nosewheel for the biplane, since the configuration I'm playing with is similar to the nosewheel-equipped Curtiss Pushers and similar early aircraft. But I really prefer a tailwheel... so I'll have to see whether the landing gear drives the airplane's overall configuration, or vice versa.

-Dana

The most valuable function performed by the federal government is entertainment.
 
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Topaz

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Hi Mark,

So I take it this is really more of an ultralight with a better climb rate, than actually an ultralight motorglider? Much as I know you like to be 'out in the breeze', I think you'll find that the design as-drawn really won't have any significant soaring potential. I don't think you'll need to go completely enclosed, but some sort of pod around your body, at least (leaving the head exposed), will help both the minimum sink and L/D. Getting rid of flying wires and going with faired struts will help these numbers, too.

The minimum sink and L/D need to be in the <=250fpm, >=16:1 range before you'll really be able to shut off the motor, find a thermal and actually climb in common conditions. Even 200fpm and 23:1 would put you at the level of performance of just a basic glider trainer, the SGS 2-33, so I'm not talking high-performance in terms of soaring (where the high-end ships are below 150fpm and well over 60:1 L/D), but rather the bare minimum to make this a viable glider with the motor turned off.

Just something to think about.
 

addaon

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Minimum sink for me is 250 fpm, L/D is 7:1 on a good day... I admit flying more than five or so hours is difficult, but that ain't bad for soaring.
 

pwood66889

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Ground handling will always be better in a tricycle landing gear; one sees better, and the configuration is inherently stable.
Conventional landing gear became "conventional" when planes landed at air fields; i.e., fields where one could land in any direction the wind was coming from. Fields that had grass on `em for the most part.
When planes became heavier one could not pave the whole field so the plane would not sink in. Thus the run way. Around that time, the weight combined with the increased speed to dictate the change to trycycle.
The ultimate cross wind landing machine has:
* trycycle gear
* low wings
* sits on the ground at minimum lift angle
Percy (with one of those jobbies for sale) in NM, USA
 

Dana

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Minimum sink for me is 250 fpm, L/D is 7:1 on a good day... I admit flying more than five or so hours is difficult, but that ain't bad for soaring.
That's paraglider kind of numbers... and a lot of soaring is done in paragliders!
Sink rate (250fpm is OK in an aircraft slow enough to stay in the core of a small thermal) is enough to soar; glide ratio gets you from one thermal to the next before you sink out.

I don't remember what the sink rate or glide ratio of my Taylorcraft were, but I successfully soared it with the engine off a couple of times. Got into worrisome cloud suck in the paramotor a few times, too...

-Dana

If there was any logic in this world, it would be men who ride side-saddle, not women.
 

mstull

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Thanks for your thoughts guys,

I wasn't hoping to be able to soar... sorry Topaz. I want an open cockpit, and don't have the patience to build up a even longer wings that are still light enough to pass the U/L weight limit. I am hoping to be able to climb well and cruise reasonably fast with a small engine. I do plan to eventually add fairings to the exposed tubing.

I'm hoping to benefit from my favorite axiom, "You can gain more performance with lighter weight than with any other modification." With the very light engine and super simple structure, I'm hoping to come out around 50# under the limit. I've never made a plane that I could restart in the air, though I don't rule it out with this one. The CorsAir engine is very easy to pull through.

Yes, I think the plane would ground handle okay with the main wheels on the frame bar. It would probably stay on its nose wheel on its own, once there. And I think it would steer okay if I force the nose down with down elevator and some throttle. I'm used to adjusting my piloting technique to suit different planes.

So the only strong reason to put the main gear back on trailing arms would be ease of disassembly for trailering. I don't trailer my plane very often. So I'm leaning towards putting the main gear on the frame.

I completely disassembled Lucky Stars. I was surprised none of the eye bolts broke. I always thought they would be a weak point, so I've been trying to use other types of tubing connections. They sure are convenient, since you can join tubes at any odd angle with them. I guess I'll go back to feeling good about them.

One idea I stumbled upon was to make the top of the main fuselage triangle lower, with the gear leg tubes meeting a foot or so above the engine, and use a single tube up to the keel from there. It wouldn't be good for side loads, like hitting a big bump with one wheel. But it would support the engine and cockpit better/lighter, and could provide a way to shorten the fuselage to fit through my trailer door. I'll pencil it out to check all the forces.

Keep sending suggestions.
 

Topaz

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That's paraglider kind of numbers... and a lot of soaring is done in paragliders!
Sink rate (250fpm is OK in an aircraft slow enough to stay in the core of a small thermal) is enough to soar; glide ratio gets you from one thermal to the next before you sink out.
Exactly. Which is mighty handy when the lift dies on one part of the hill and you want to try out a couple of more promising spots before having to head for the IP.

Call me biased, Dana - A 3,000' AGL tow is $62 for me, so if I can stay up all day by cruising from one thermal to the next (even if widely separated), rather than coming down and towing up again, I'll take it. ;)

No worries Mark - I heard "motorglider" and my ears perked up. :gig:
 

Norman

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I believe that the only legal requirement to call it a motor glider is that the span squared load be less than 0.62. For a plane that grosses 500# a span of 28' is all that's needed to get that span^2/load. With a span of 34' and wing loading of ~4lb/ft^2 (3psf would be even better) his sink rate should be fine as long as he seals the aileron hinges. I do wonder about the shallow angle of the outboard flying wires though.
 

mstull

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Thanks Norm,

I'm planning to fly under Part 103, not as a motor-glider.

I agree that the outer flying wires had a poor angle. When I moved the main gear up onto the frame's cross bar this morning (rather than on trailing arms) I also moved that cross bar (which is now the axle) several inches lower. That helps the angle of the flying wires a little. I also moved their attachment point on the wing in half a foot. Both are pretty incremental improvements. But it does look noticeably better (about 5 degrees better).

Everything in U/L design is a compromise. I wanted longer wings. I made the fuselage relatively tall to help improve that angle (which allowed room for the prop under the wing). At first I wanted 36' span. But I had to compromise that down to 34' because of the angle of the outer flying wires.

Here's the latest drawing.
 

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BBerson

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Mark,
If you want to run direct drive.... a prop shroud is something to experiment with. My research has found papers that claim double the thrust with a shroud for static thrust.

Designing for static thrust is not usual in aviation, but the hovercraft and airboat guys are using shrouds. An ultralight is similar.

I read one NACA report that determined a shroud could cut noise by half or double the noise if improperly designed. (separated flow at lip makes noise).

Just something to think about if your prop space is limited, otherwise I suppose a big geared prop is best.
BB
 

mstull

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Thanks BB,

I'm planning to use the CorsAir engine with its stock, belt, reduction drive, and standard size prop. I don't know if I'll ever try direct drive again... since that probably contributed to my latest engine failure. I enjoyed experimenting with it.

I've read that shrouds can make a big difference too. But the shroud has to be extremely close to the prop. Supporting the shroud stiffly enough and making the shroud stiff enough to accomplish that would be too heavy on an U/L. And the engine couldn't be allowed to vibrate on its rubber mounts, because its motion would allow the prop to hit the shroud. So shrouded props aren't practical on most prop driven planes.

Some designers draw them into their proposed planes or drones. It would help if someone started manufacturing an efficient prop with shroud setup... most likely with a belt to the an engine outside the shroud. But every aircraft design would need a slightly different size unit. So we may never see them... especially at a reasonable price. Isn't there a helicopter that has one on its tail rotor?
 
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BBerson

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Isn't there a helicopter that has one on its tail rotor?
Yes, the French have the "Fenestron". The early models had an annoying siren sound. The new models are much quieter... funny, the Coast Guard helo with the new fenestron just flew past my house as I am typing this. It was quieter.

Another thought to consider is small four strokes. Here is a recent comment from the small four stroke yahoo group.
"For what it's worth my Honda GX 670 modified with 10/1 Weisco pistons ,
stock bore Empi carb 3 degree advance key and some port work is making 255
lb of static thrust. It's swinging a 72/46 Ten. Wood prop. I'm estimating I
m getting around 32 hp. Max RPM is 4400."

These industrial engines usually run 2000 or more hours. Maybe a bit less at 4400 rpm. Still experimental , of course.
BB
 

BBerson

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Another idea is hybrid electric.
With a small ICE for cruise and one or more small electric motors for takeoff and climb.

The cost is a bit high for the whole system, (motor, battery, controller) but maybe using several small units would work.
I was thinking, if the "climb" motors had low pitch props, this would limit the top speed and yet provide a strong climb. No active power limit device would be needed.
 

mstull

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BB,

I like the Honda commercial engine better than others. But I worry about its reliability after boosting its power. When we convert automotive engines for aircraft, we generally de-rate them for reliability. No doubt commercial/generator engines are already de-rated so they can run an extremely long time. So we probably don't need to further de-rate them. But I'd want to see proof of longevity before trying a boosted one. I like that you don't need a big exhaust system with them.

The GX670's 95 pounds, plus the reduction drive, is a big weight penalty to pay for 4-stroke reliability.
 

BBerson

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I have a Honda GX670. With the flywheel removed and alternator coils and starter removed, it weighs 65 lbs. Of course, without the flywheel, some other ignition is needed. The flywheel weighs about 15 lbs. I thought about removing some of the flywheel casting and alternator magnets. The basic engine with flywheel is 65+15= 80 lbs.

I am planning on direct drive.
 

mstull

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That's real interesting, BB,

Most horizontal shaft engines aren't designed to take thrust loads on the crankshaft. I'd also be worried about crankshaft cracking from gyroscopic pitch and yaw loads, unless the crankshaft is at least 1.25" diameter where it exits the engine. And I'd definitely only use a wood prop. It's much easier on the crankshaft.

I would try machining some of the flywheel mass off, rather than removing it completely, like they do on the Big Twin, and try to retain the stock ignition. You'll probably need to machine a taper on the end of the crankshaft for the prop flange. Even the most microscopic wobble of a non-tapered, keyed shaft won't do.

What I've learned from more than a year of experimenting with direct drive, that applies to your proposal: If the RPM is over 4,000, it will sound loud and bad, so keep it below that.

There is a balance between using higher RPM to gain horsepower, and using a larger diameter prop to gain prop efficiency. For example, I used a 42" prop to get the RPMs up where there was 40 Hp, but got just as much thrust with a 48" prop (with the same 22" pitch), at 800 RPM less, where the engine only has 29 Hp. I was amazed how the larger prop is so much more efficient that it can make the same thrust with 27% less horsepower... and sounds so much better.

Direct drive favors a larger diameter with lower pitch props. If you go the other way (smaller diameter with higher pitch) the prop won't hook up well, so it won't make as much thrust.

Direct drive also favors a very efficient prop. I've tried 4 different brands and learned that you get what you pay for. I recommend a Tennessee. Ask for a thin airfoil, with the airfoil surfaces going straight back to a sharp trailing edge. Most U/L prop manufacturers round the trailing edges, decreasing thrust and creating drag and noise.

Prop experts say it's torque that determines how big a prop you can turn. Using a reduction drive multiplies the torque so you can use a much larger prop and get much more thrust out of a low Hp engine. Even though the Big Twin runs at fairly low RPM, like your engine, they still use a reduction drive on it with a huge prop. I asked them about running it direct drive... "not recommended". Yes, the reduction drive and larger prop add weight. But you'll probably need a reduction drive to get decent thrust.

On my direct drive engine, I was purposely de-rating the engine significantly, trying to stay within the U/L speed limit. But your engine is already pretty low Hp. You're going to need to get everything out of it as you can to get a decent climb. My large displacement, MZ201 engine had 2 power strokes per revolution. Your 4 stroke only has 1. That will severely limit your torque, and direct drive prop diameter and thrust.

I'd try converting a Kawasaki 250 Ninja motorcycle engine if I had a machine shop. Good luck with your project.
 
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