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Thread: Early Foxes Vne Limiting factors?

  1. #1

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    Default Early Foxes Vne Limiting factors?

    Although I wouldn't call my search exhaustive the only reason I've found for the 100 mph vne on the models 1-3 is that the tail surfaces might flutter. I realize with the undercambered wing it will only go so fast, but I've spoken with a gentleman who has a model 1 with a rotax 670 and he informed me that he can exceed 100mph in level flight.

    I'm currently doing a ground up rebuild adding more hp and doing a significant amount of aerodynamic cleanup. So given the fact that I also expect the be able to get to Vne in level flight I'm interested in knowing if there's ways to bump up Vne for the simple reason that I can see where I could easily get myself into trouble. I'm not looking to go racing. I would have chosen another plane if that were the case, this is safety I'm talking about. I don't plan to ever INTENTIONALLY exceed Vne but since I expect the aircraft to be faster its now a concern and if I can add a little more margin so much the better.

    So are there additional factors besides tail flutter? Obviously balancing control surfaces would seem like somewhat of a solution but as far as I know none of the totally stock factory kits have mass balanced tail feathers, so is there some other reason the tail is prone to flutter? I am planning on making the vertical stab a lot stiffer.

  2. #2
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    Default Re: Early Foxes Vne Limiting factors?

    With the .060" thick lexan windshield on my Avid Flyer, it would start to buckle in at 100 MPH. Might have something to do with it as well. JImChuk

  3. #3
    Senior Member Av8r_Sed's Avatar
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    Default Re: Early Foxes Vne Limiting factors?

    My Model III will pitch down at about 90 mph. It’s pronounced and uncomfortable. The windshield also begins to buckle/ oil can. This behavior dissuades me from a quest for speed.
    -- Paul S
    Model III SN910
    582 IVO Med

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    Senior Member t j's Avatar
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    Default Re: Early Foxes Vne Limiting factors?

    There have been instances of butt rib cap strips detaching resulting in turtle deck leading edge popping out from under the skylight.

    I used to tell people that poo-pooed the 85 mph cruise of my 503 powered Classic 4, "When I want to go fast, I ride my 670 powered Ski Doo".
    Tom Jones
    Classic 4 builder

  5. #5

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    Default Re: Early Foxes Vne Limiting factors?

    Quote Originally Posted by avidflyer View Post
    With the .060" thick lexan windshield on my Avid Flyer, it would start to buckle in at 100 MPH. Might have something to do with it as well. JImChuk
    I had seen that somewhere a long time ago but forgotten about it. Thanks.

    Quote Originally Posted by Av8r_Sed View Post
    My Model III will pitch down at about 90 mph. It’s pronounced and uncomfortable. The windshield also begins to buckle/ oil can. This behavior dissuades me from a quest for speed.
    Any idea why it does that? I have heard of a few guys with an early fox talk about flying 90+ mph and they didn't mention that issue. Maybe some quirk in your aircraft?

    Quote Originally Posted by t j View Post
    There have been instances of butt rib cap strips detaching resulting in turtle deck leading edge popping out from under the skylight.

    I used to tell people that poo-pooed the 85 mph cruise of my 503 powered Classic 4, "When I want to go fast, I ride my 670 powered Ski Doo".
    That's interesting. Just thinking about how I would reinforce the butt ribs the first idea that came to mind was completely encapsulating it in fabric. That would hold the cap strip on better I think.

    And I'm not interested in speed. After talking to Lowell Fitt and hearing about how his aircraft with a lot of drag reduction only used 60% as much fuel as another aircraft flying the same leg of a cross country, and since I've only got 12 gallons of fuel capacity I want all the range I can get. Also I live and will be playing in the Utah Rockies. Density altitude is going to be really high and since drag reduction also helps with rate of climb, i want all the performance I can get.

    I do plan to fly my Fox to Oshkosh and being able to cruise a little faster(still under Vne) without the concern of something breaking would be nice.

  6. #6
    Senior Member t j's Avatar
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    Default Re: Early Foxes Vne Limiting factors?

    Service Bulletin #3 VNE Flaperon flutter encountered. They were working on developing counter weights to add to the flaperons.
    Attached Files Attached Files
    Tom Jones
    Classic 4 builder

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    Default Re: Early Foxes Vne Limiting factors?

    I'm not sure that the Kitfox 4 has a stronger butt rib construction, and yet is has a higher VNE. Can't remember hearing of this before, but then again, my father got alzhiemers, and I'm 68 so..... JImChuk

  8. #8

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    Default Re: Early Foxes Vne Limiting factors?

    Quote Originally Posted by t j View Post
    Service Bulletin #3 VNE Flaperon flutter encountered. They were working on developing counter weights to add to the flaperons.
    Flaperon counterbalances were made available and fitting them returned the Vne to 100mph.

    My model 2 has them. We did Vne as a part of test flight a couple of weeks ago. It was pretty much level light and no problems.

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