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Thread: Modifying cabin width on plans built

  1. #1

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    Modifying cabin width on plans built

    I'm in the process of building a mock-up of the GP-4 cockpit from the plans so I can sit in in and dream of flying it. I'm also using it to plan the panel and therefore the underlying systems. For instance, I know it's going to be an electric panel with no vacuum systems.

    But the main reason is to design in the creature comforts for flying decent distances. I've got about 1000 hours, mostly in Mooneys on IFR cross-countries of 300 to 550 miles. I've flown Pipers and Cessnas as well. But the best cockpit I've been in so far was the SportCruiser (formerly the PiperSport) with a cockpit width of 47 inches.

    Yes, that itty bitty Rotax powered featherweight was 47 inches wide!

    It was plenty of room for me (5'7" 170lbs) and a large instructor (6'1" 195lbs) and we weren't constantly rubbing elbows. Even my left elbow from the pilots seat had plenty of room. There were storage areas between the seats with the center console being about 4" wide on the outside with an armrest on the top. I think I remember side pockets on the outside stations that also had armrests, but I might not be remembering that correctly.

    I have no real desire to do anything crazy with the GP-4 in terms of major redesigns. But some of the minor things like this seems "safe". I also don't think it will affect cruise speed much since the increased frontal area will also be very minimal.

    Has anyone else thought through this exercise?

    Here are some things I've thought about that might need to be changed to accomodate the width change.
    - T-18 canopy would need to have 7 inches added to the width. Is that even possible?
    - Everything from roughly station 0 to roughly station 70 would need to be proportionally increased in width.
    - If I don't change the length of the spar, I wouldn't need to change the length of pushrods or gear. However, if I did, I would need to add 7 inches to all of the systems that cross the centerline.
    - I don't see any reason to change any dimension forward of the firewall or aft of the baggage compartment.

    I'm not an aeronautical engineer, and I will seek out a proper authority before cutting any wood or metal. I'm still in the "thinking/planning" phase at this point.

    Mark

  2. #2

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    Quote Originally Posted by marktrav View Post
    Here are some things I've thought about that might need to be changed to accomodate the width change.
    - T-18 canopy would need to have 7 inches added to the width. Is that even possible?
    - Everything from roughly station 0 to roughly station 70 would need to be proportionally increased in width.
    - If I don't change the length of the spar, I wouldn't need to change the length of pushrods or gear. However, if I did, I would need to add 7 inches to all of the systems that cross the centerline.
    - I don't see any reason to change any dimension forward of the firewall or aft of the baggage compartment.

    I'm not an aeronautical engineer, and I will seek out a proper authority before cutting any wood or metal. I'm still in the "thinking/planning" phase at this point.

    Mark
    Here are some thoughts on your ideas;

    - Regarding the canopy width, yes, you could probably buy a bigger bubble. You might call someone like Todd's Canopies (you should be able to find him by searching one of the RV boards.) and get an estimate of the cost. Alterntely, you could split the normal canopy and run an aluminum (or whatever) strip down the center to join the halves and provide additional width.

    - Regarding other aspects of the width issue, I suspect you'd need to re-loft most of the stations to provide an attractively faired fuselage.

    - Also, If you widen the fuse, I assume you'll widen the distance between the sticks, the width of the flap extension system, and the mechanism between the sticks. Not hard, but more to think about.

    Speculating here, but if you widen the fuselage, it may reduce the effectiveness of the rudder and vertical stabilizer in high angle of attack situations. Or maybe not. If it does, to maintain the same flying qualities as a standard GP4, you'd need a bigger vertical surface.

    Is there a builder's group for the GP4? Is George Pereira available for questions? Someone may have already worked through these issues for you.

  3. #3

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    Thanks Kyle!

    I've spoken to George in the past, and while we didn't talk specifically about this particular mod, his pat response seems to be "Don't mess with it! Just build it!". The GP-4 forum on Yahoo generates the same response. At least so far...

    Those who have modified them significantly (wing chord change, IO-540 mod) have not been successful and are now in the NTSB database. I think there are only a little over 20 flying. The owners I've talked to love them, but the ones I've talked to all stuck strictly to the plans.

    Seems like a perfect little airplane if you stick to the plans, but there doesn't seem to be many margins (if any) to extend the design, which I guess is the price you pay to get 200 knots out of 200hp.

    I've still got awhile before I need to make a decision, but it is something of high interest to me.

    Interesting point about the rudder... I wonder if the increased width of the cabin would necessitate increasing the width of the horizontal stabilizer as well.

    If adding 7 inches to the width necessitates aerodynamic changes to the flight surfaces, then I'll probably back off the idea and stick with the plans.

    Mark

  4. #4
    Richard Warner's Avatar
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    Changing widths may also require changing the structure of the frames so that strength is not lost. Most people I know who make radical design changes, which is what you are doing, end up with a worthless pile of "aircraft material" so to speak, that never gets in the air. My advice is to find another design that meets all of your requirements.

  5. #5

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    Quote Originally Posted by Richard Warner View Post
    Changing widths may also require changing the structure of the frames so that strength is not lost. Most people I know who make radical design changes, which is what you are doing, end up with a worthless pile of "aircraft material" so to speak, that never gets in the air. My advice is to find another design that meets all of your requirements.
    Well, this design meets all of my "requirements", so I'm not looking for another. I've been looking for a long time. This one's a winner. Just wish it had a little more elbow room is all.

    Mark

  6. #6

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    Jul 2011
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    Just remember that if you're making any significant change (and widening the fuselage is a significant change), you're essentially designing a new airplane... and need to be qualified to do that.

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