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Thread: What is factory Vne on Model-4 and Speedster ???

  1. #11

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    Thumbs down Re: What is factory Vne on Model-4 and Speedster ???

    What would the Vne be limited by if the windshield were not an issue?

    I originally thought the wing design might cause excessive stresses using normal pilot induced control forces. But after reading this thread, I was surprised the windshield (a simple structure) is the main factor behind the present Vne.

    My Vari-eze cruises all day at 200 mph. So if the Kitfox firewall were horizontally extended to include part of the top of the engine cowling area to maintain the firewall integrity. And, a canopy were devised similar to faster aircraft. The windshield shouldn't be a significant modification.

    What is the next limiting factor for Kitfox Vne?

    With engines up to 200 hp being mounted on a Kitfox, 140 hp engines commonly available, and windshields that certainly can handle higher air speeds, why isn't the Kitfox flying at higher cruising speeds?

    Possible thoughts are:

    a) the wing sail area at 125 mph in relation to the shear forces on the folding wing critical structures?

    b) wing profile related pilot induced control forces?

    c) aileron induced instability?

    d) .... ?

    But these are not supported in the provided literature.

    Given the aerodynamic design of the Kitfox, it just seems like it should be cruising at 160 mph unless wing design is a limiting factor. And wings on the Kitfox can easily be replaced.

    One set for STOL, another for high cruise.

    Though I'm not at all disrespecting the Kitfox relatively high range in airspeeds between stall and Vne, as compared with other aircraft.
    James Dunn

  2. #12
    Super Moderator Av8r3400's Avatar
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    Default Re: What is factory Vne on Model-4 and Speedster ???

    The newer Kitfoxes with the stronger, molded windshield and two balance weights per flapperon have a VNE of 145 mph.

    With enough horsepower, I'm sure you could go faster than the plane is designed to go. More horsepower equals more weight. More horsepower equals more fuel. More fuel equals even more weight. Weight is the detriment to speed. This becomes a never ending circle of more power to overcome more weight.

    Making a plane like a Kitfox go fast becomes like a dog chasing his own tail, power, weight, fuel, structure, etc, etc…

    If you want to cruise at 160 mph, build an RV.
    Av8r3400
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  3. #13

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    Default Re: What is factory Vne on Model-4 and Speedster ???

    I'm fickle and moody. Sometimes I want the STOL for playing in the local mountains and rivers. Then less often, I want to go back East 2000 miles to visit family.

    The Kitfox obviously has the roots to grow capacity. Simply look at other discussions here:

    http://www.teamkitfox.com/Forums/sho...?t=5964&page=3

    The common Kitfox is no where near its performance maximums with existing 70 hp engines. But there are other turbocharged engines that actually save weight at 140 hp; though durability is questionable (not yet known, not that it isn't).
    James Dunn

  4. #14
    Super Moderator Av8r3400's Avatar
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    Default Re: What is factory Vne on Model-4 and Speedster ???

    Sounds to me like you just need two airplanes.
    Av8r3400
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  5. #15
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    Default Re: What is factory Vne on Model-4 and Speedster ???

    You know James, I used to think the same way - I owned a Mooney and it was great for long distances. But then I realised that the commercials do high and fast much better and cheaper than I could! But they couldn't low and slow and off field... no brainer solution to me!

    r
    Ross
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    OZ
    Sold to Richard and Scott Taubman in OZ, 2019. Kitfox SS7,Rotax 912is Sport, Airmaster CSP 75" blades.
    Landcruiser and Cub off road camper (doesn't get any kudos on this forum!)

  6. #16

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    Default Re: What is factory Vne on Model-4 and Speedster ???

    The acrylic windshield upgrade is too easy a fix for it alone to limit Vne. The FAA testing for the windshield might be far less expensive than another component, so I get that. It is cheaper to stop at the windshield which is easily observable and measured without involving a wind tunnel.

    As is shown elsewhere, there are Kitfox flying faster than Vne. Kitfox itself modified the 4 to create the Speedster.

    So my question:

    After the windshield, what is the next Kitfox feature that limits Vne?
    James Dunn

  7. #17
    Senior Member t j's Avatar
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    Default Re: What is factory Vne on Model-4 and Speedster ???

    I sense a little confusion about V speeds for amateur built experimental airplanes. The FAA doesn't do the testing on them to set Vne, the manufacturer (builder) does that however they want.

    To answer your question though, my opinion is the fabric covering is the factor limiting speed to about 150 mph for any tube and fabric airplane. I suspect no one has felt the need for such speed enough to go there.
    Tom Jones
    Classic 4 builder

  8. #18
    Senior Member Esser's Avatar
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    Default Re: What is factory Vne on Model-4 and Speedster ???

    The fabric isn't the limiting factor. Pitts have a Vne of over 210MPH.

    I imagine the limiting factor is the flexible wing of the kitfox. Susceptible to flutter.

  9. #19
    Super Moderator Av8r3400's Avatar
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    Default Re: What is factory Vne on Model-4 and Speedster ???

    A fabric covered Beech Staggerwing would cruise almost 200 mph.

    Esser, I believe you are into the main limiting factor being flutter. Either from the flexing of the wing or the flapperons or a combination of both. No matter how you cut it, these aircraft were not designed to or intended to go fast.

    The Denny claims of 150+ mph cruise may be possible with a 914 turbo engine at 20,000 feet, as a computed TRUE airspeed. But down low this kind of speed is just looking for trouble using an airframe designed for 100 mph.

    In the 30+ years of Avids, Kitfoxes and the rest, there has never been an inflight structural failure of the airframe. This is one of the main reasons I chose to own this aircraft type. Putting enough horsepower to muscle the plane to these higher speeds is just looking to ruin this record of safety.



    Just my opinion. It is experimental, so you can do as you please. I just think it to be very ill advised.
    Av8r3400
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  10. #20
    Senior Member Dave S's Avatar
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    Default Re: What is factory Vne on Model-4 and Speedster ???

    For me, chasing Vne is not that important. Certainly, I did test our S7 to 140 mph to verify the plane was controllable throughout the design envelope.....Knowing the wing loading of the aircraft....I hate to think what kind of whack on the fanny a person would get if they hit a good pothole in the sky going really, really fast - I've never been able to figure when the sky would be guaranteed 100% free of potholes - routinely cruising around in the yellow arc, irrespective of where Vne is planted - just not me I guess.

    I never bought a kitfox to see how fast I could get out of the sky and back on the ground

    Sincerely,
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

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