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Thread: Ultralight Design

  1. #1
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    Ultralight Design

    Hello all,
    Me and my friend love flying. After a trip to 2009's AirVenture, we discovered the world of Ultralight aircraft! After some research, we were quickly overwhelmed with some of the price tags stuck on these aircraft, and decided that we would build our own. This is coming to about our 6th month of research and designs, and we believe we have decided on one. Now you may be thinking "13 year olds, building and flying ultralights...?" And I've gotten the reaction from many people. The truth is, I am no stranger to aircraft. Me and my friend have done much research and practically know a plane inside and out! So, lets end this introduction and get on to the plane!
    Our Ultralight Design
    For our Ultralight design, we looked at some of the most successful light aircraft, such as the 162, 172, and 182. We decided on an overhead wing design. The wing span will be 23-27 feet, and the chord length will be 4-5 feet. Instead of cutting space for the ailerons inside the wings, we are going to attach them straight to the end, as well as the flaps, to increase the chord length without adding sufficent weight (They will be flat of course). We would also be able to apply droop to the ailerons and flaps. The length of the aircraft will be 15-20 feet. The cockpit will be under the wing, and will be open. Behind the wing will be the Fuel tank, and a storage area big enough to hold a small 1-2 person tent, a sleeping bag, and some food and water (Our goal is to fly this up to Airventure after completion). We would also be able to add fabric doors that we could zip up to protect the cockpit from rain on the ground or keep out harsh cold wind in chilly weather. In the nose of the aircraft would be the engine, preferrably a 2-Cylinder engine, but a 1-Cylinder engine would work just fine. the Vertical and Horizontal Stabalizors would be fairly large, and same with the control surfaces of the whole aircraft. For framework we were planning on using relitivly small and light weight Aluminum tubing, and for the skin uisng Decron fabric. To keep the aircraft as light as possible, we would make things like the seat Cusions, and other unneeded items easily removable, so they dont count agents weight on our airplane. Our control cables will be as simple as possible, using simple cable methods to control the aircraft. It will be a tail dragger, with the rear wheel attached to the rudder (again, for simplicity) and the main landing gear just below and infront of the cockpit. Instruments would also be very simple, using just a GPS for direction and altitude, and a pitch indicator and possibly an airspeed indicator (very simple things sense theres no point in putting too much stuff in there concitering ultralights can only fly by VFR rules). The cockpit would also be conciterably roomy, and comfortable to an extent.

    Thats pretty much our design. Although we are still doing more research and profecting our design, this is what it has come to. We have found an airfield just outside of Madison that is often used by Ultralights, and I am working with a few pilots from Civil Air Patrol to help iron out some of the bumps. Please FEEL FREE TO LEAVE QUESTIONS, COMMENTS, AND SUGGESTIONS! It would be greatly appreciated, but dont criticize us, we're not stupid, and we know what we're doing.

  2. #2
    Registered User Kristoffon's Avatar
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    Re: Ultralight Design

    While I wish you the best of luck, and it looks you'll have a lot of fun, you do appear to have a lot to learn. How is removing seat cushions an advantage? Unless you want to fly without them?

    I recommend you read "Simplified aircraft design for homebuilders" which is very easy to read yet covers all the basics needed, I think, to get you off the ground safely.

  3. #3
    Registered User rtfm's Avatar
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    Re: Ultralight Design

    Hi guys, and welcome to the forum.

    I began designing my airplane about three years ago, and it has been a great deal of fun. Some of my early ideas have been dropped, because they proved to be not so good. But other early ideas are still there, because they were good. You will probably find the same thing happens to you as you learn more about designing a safe airplane.

    Have you drawn it yet? That was the first thing I did. TERRIBLE drawings, but I thought they were cool. I have kept most of them, and when I page through the old notebook, I am quite embarrased sometimes - but that's all part of it. Over the months, the designs got better and better. Kristoffon has recommended you get a copy of Dan Raymer's book - do it. You will be so pleased you did. It is quite thin, it's a little paperback and looks like it might be a bit Micky Mouse - but far from it. You'll find it a superb reference book, which is pretty straightforward and sensible.

    So, let's SEE your plane. Post a few sketches, so the guys can comment more easily.

    Again, welcome to the forum, and all the very best of luck to you.

    Cheers,
    Duncan
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    Builder's Blog:
    http://rtfmaero.wordpress.com/

  4. #4
    Moderator Dana's Avatar
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    Re: Ultralight Design

    Well, good luck... you're not as unrealistic as some new people here. But I do have a few coments:

    Putting fuel and cargo behind the wing means significant changes in center of gravity (aircraft balance point) depending on fuel and cargo load, probably beyond safe limits. A nose heavy is difficult to control, a tail heavy airplane is impossible to control.

    You can't exclude removable parts of the plane from the aircraft's empty weight, unless you can fly without them.

    Attaching the tail whee directly to the rudder means the rudder (and its hinges) would have to be much stronger than otherwise. Putting the tailwheel on its own strut would actually be lighter. and prevents damage to the rudder (a critical flight control) in the event you hit it on something on the ground.

    GPS is not accurate enough for elevation to substitute for an altimeter... though of course for a true ultralight an altimeter is not legally required. As a minimum, an altimeter watch would serve, or a GPS with a pressure altimeter built in. You definitely should have an aispeed indicator.

    You mention a "1-2 person tent" and "we" but you know that ultralights are limited to single seat only, ight?

    If price is your primary concern, buying a used ultralight will nearly always cost less than building.

    -Dana

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    Re: Ultralight Design

    Yes we have drawn our aircraft, and I hope to post some of them in the next week or so. Most Ultralights actually have the tail wheel attached straight to the rudder, and we would check to make sure it is supported properly. But I do understand where your coming from and have noted the safety concerns. Could you please explain to me how you would put the tail wheel on its own strut? I have a good idea attaching the strut to what would normally be the rudder, and then running rods back to the rudder, but I would like it better explained. Also, the fuel tank and cargo area would still be under the wing, just behind the pilot. Like I stated before, the pilot is sitting under, and to the front of the wing. We took some of our ideas from the Snedden M7 project, and they also had removable cusion seats to help with weight. The point was anything that was more for comfort or looks could be easily removed, meaning the aircraft could still easily fly without them, and none of them added to the strutural integerty of the craft. And finally, yes we understand that an ultralight can only carry one person, but we would have the other ride up there and would meet. Thanks for some of the advice and tips, Ill see if I can get some sketches up for you.
    Alex
    Last edited by alexkmmll; July 14th, 2010 at 09:32 AM. Reason: Forgot some stuff...

  6. #6
    Moderator Dana's Avatar
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    Re: Ultralight Design

    The most common tail wheel arrangement, both on light planes as well as ultralights, is to mount the tailwheel on a spring (either a leaf spring or round tube) attached to the fuselage structure. It is then linked to the rudder via springs, so ground shocks are not transmitted to the rudder and control cables.

    In some cases the tailwheel is attached to the lower portion of the vertical stabilizer, where the vertical stab extends below the fuselage, but not to the moving portion of the rudder.

    -Dana

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  7. #7
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    Re: Ultralight Design

    So Dana, would you recommend using a tail wheel, or going with a tricycle design?

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    Re: Ultralight Design

    Ultralight Design-scan0002.jpg Heres my design. Sorry if its a little hard to see. I didnt do frames or interiors or anything but you should be able to get a good idea of what we're wanting to do.
    Some basic stats if you cant see the picture well

    Length: 19'
    Height: 8'3"
    Wing Span: 22'
    Wing Chord: 5'8"
    Wing Area: About 128 sq. ft.

    My concerns are its too tall, not enough surface area on the wing, and too much drag. Does anyone see those as a problem in the design? Drag is easily fixed by angling the wind shield a bit better, but I kinda like the height because it would make it more comfortable inside. Does it need to be this tall? (Just as a note, I left a foot of space at the bottom of the aircraft for room for the control cables)
    Last edited by alexkmmll; July 14th, 2010 at 04:13 PM.

  9. #9
    Registered User Autodidact's Avatar
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    Re: Ultralight Design

    That's kinda cute; the wing area is about right, and the proportions in the top view are about right. I don't think it needs to be 6+ ft deep from the bottom of the fuselage to the top of the wing in the side view, though. The pilot does not have to sit that far off the floor, it is extra structural weight, more drag, etc.

    Also, in the front view, the spanwise lift distribution from the wing root out to the tip is sort of elliptical shaped with the lift tapering off out toward the tip. In other words, the spanwise "balance" point of the lift will be about 40-45% of the way out from the root, and that is a good place to attach the strut to the wing. Once you make the fuselage less tall, the struts wont look funny with the attach point moved in toward the fuselage. (It doesn't "have" to be that way, though, like all engineering, there is more than one way to do it.)

    Look at other ultralights like the Hi-max, Preceptor cub, etc., and take note of their basic dimensions. As for the controls, you won't need a foot under the seat; some aircraft are designed so that the controls go around the seat, and not under. There are different ways to do it. You can find pictures on the web.

    Look up a web site called Chilton-aircraft.co.uk, they have pictures of the control stick assembly and how it attaches to the wing spar and how the cables are run around the seat; should be easier to do in a high wing.

    Make an accurately scaled pilot figure (side view) with brads or rivets in the arm, leg, and hip joints and design your fuselage around that.
    Last edited by Autodidact; July 14th, 2010 at 04:40 PM.
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    Re: Ultralight Design

    Oh, and one other thing, buy Raymer's Simplifed Aircraft Design book, it's perfect for guys your age, even though you will need help from a math teacher or even a friendly engineer (or the guys on this forum) for one or two of the equations.

    It's a good idea to have at least a third of the rudder hanging below the horizontal tail. You could just make yours bigger; it wouldn't hurt anything.
    "Milk cures wing dope poisoning."

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    Re: Ultralight Design

    Ultralight Design-scan0003.jpg Ok so I took your advice and went back and took a whole foot off of the bottom. I shortened the landing gear also. After taking off the foot it made the tail look real bad looking, so I made it a little taller, which also made the transition from the tail to the wing/cockpit area a little smoother. Lastly, I moved the wing struts back so they're about 40% from the root, and made the angle of the windshield about 45%. Your right, not only does it look better, but it really will cut down on weight.

  12. #12
    Moderator Dana's Avatar
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    Re: Ultralight Design

    On first glance, I'll echo Autodidact's comments about the depth of the fuselage and the rudder. I would recommend a tailwheel vs. nosewheel, but that's personal preference... though a tailwheel configuration is lighter, and can usually handle rough fields better. You'll need some tailwheel training though.

    Your proportions look about right, but the aspect ratio looks a bit low... longer skinnier wings will be more efficient, though it's always a tradeoff against structural strength and weight. The fuselage also looks like it might be a bit on the short side for stability, but it's hard to say without numbers.

    I've posted this before, and recently, but this is a good place to start for a conventional design like yours:



    -Dana

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    Re: Ultralight Design

    Thats pretty hard to follow for me, but I understand where its getting at.

  14. #14
    Moderator Dana's Avatar
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    Re: Ultralight Design

    Study it. I don't think airplane design can be made any simpler. Whether you go by these rules of thumb or not, you absolutely need to understand all the terms if you intend to design an airplane. Take as much math and science as you can in high school, and get involved with your local R/C model club too (a great introduction to flying and aircraft design).

    -Dana

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  15. #15
    Registered User rtfm's Avatar
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    Re: Ultralight Design

    Hi,
    I must echo what Dana has said. This is about as simple as airplane design gets. To attempt designing an airplane without reference to this set of guidelines (at very least) or without spending the few dollars to get Raymer's Homebuilder book is not wise.

    And once you read Raymer, the above diagram will all make perfect sense. Trust us on this.

    Regards, and welcome to the ongoing challenge of designing a safe aircraft.

    Duncan
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