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Thread: A much faster Kitfox?

  1. #41
    Senior Member PapuaPilot's Avatar
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    Default Re: A much faster Kitfox?

    [QUOTE=PropellerHead;43847]Could it be that, with the smaller elevator, your friend was just running out of elevator authority and not really stalling?

    That's what I think was happening.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  2. #42
    Senior Member av8rps's Avatar
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    Default Re: A much faster Kitfox?

    Hi Tom,

    Your information here is a really good example of what one can do to improve efficiencies of our planes. By cleaning up your airframe and replacing a prop you in effect have improved the cruise speed of your 503 Model 4 Kitfox by 34%.

    That is a pretty amazing number as there are some unfaired Kitfoxes flying around with almost double the horsepower you have and not going as fast. So your modifications prove exactly what I was getting at by saying I think we have some additional opportunities to improve our speeds. Granted, once a person installs the most obvious fairings, the changes and improvements tend not to be as noticeable. But they do all add up.

    I used to fly an Avid Flyer A-model that was super light with a Rotax 532 that produced 74 hp (I know that because we dyno'd the engine on three different dyno's). On a set of straight fibreglass floats and a super efficient propeller it would go 115 mph wide open straight and level. My buddy with his 150 hp Super Cub on straight floats couldn't even think about keeping up with me if I was on the throttle. But that little Avid was pretty well streamlined, and it was super light (empty weight on wheels was only weighed 396 lbs, and approx 500 lbs on floats). And the prop was a hand carved scimitar shaped, multi-lamination thing of beauty.

    Now most would never believe you could get an old draggy Avid Flyer to go that fast, but they're wrong. It is possible with everything being right. That airplane and yours proves that.

    Paul

    Quote Originally Posted by t j View Post
    Here's my speed story. Kitfox classic 4 with 52 HP 503 Rotax with 3:1 gear box. First built, 68" 3 blade GSC taper tip prop. No fairing on struts or the stock tube landing gear. 6000 RPM was 65 mph.

    Put the wood fairing on the landing gear and changed to a 72" 2 blade Square tip Warp prop. 6000 RPM was 75 mph.

    Added the PVC fairing to the wing struts. 6000 RPM went up to 85 mph. Full throttle level flight, 6600 RPM and gets 87 mph.

    I have the prop set for climb not cruise. I think I have hit the max speed wall for this prop/pitch and the little 503 engine. I have the prop set for climb not cruise. I can get it up to 100 mph in a dive but I have to reduce the throttle to less than 4000 RPM or the EGTs get too high.

  3. #43
    Senior Member HighWing's Avatar
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    Default Re: A much faster Kitfox?

    Quote Originally Posted by PropellerHead View Post
    Could it be that, with the smaller elevator, your friend was just running out of elevator authority and not really stalling?
    I think you nailed it. The math likely defines the minimum theoretical stall speed. What we have in a practical situation are many deviations from the ideal. Another example would be the airfoil where the only areas exactly on the Riblett design are over the rib capstrips and the eighth inch wide upper false ribs. The rest of the wing is a compromise determined by the geometry of the shrunk fabric surface.

    A good friend and Model IV builder tells of the time he heard Harry Riblett discuss the Kitfox use of his design. He mentioned that the most critical part of the design was the first five inches. The LLE from Kitfox adresses that issue. I put an aluminum leading edge on my wings and compared with my first Model IV, the new one stalls three mph lower - 40 vs. The previous 43.

    Then as to the elevator, for years fliers have been doing all sorts of things to increase elevator authority in the early Kitfoxes. The most common are the tape gap seals. The more recent availability of the larger elevators has attacked the deficiency at its source. That is essentially what we did for my friend's Avid.

    I guess it would be safe to say that, although a great airplane, the Kitfox is not perfect out of the box.
    Last edited by HighWing; 10-21-2014 at 08:27 PM.
    Lowell Fitt
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  4. #44
    Senior Member av8rps's Avatar
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    Default Re: A much faster Kitfox?

    I think weight is the most critical component for stall speed, assuming the airframe is the same.

    My early Avid Flyer would stall around 20 mph solo. Yup, I did say 20 mph (it was even advertised at the time at 22 mph. My old flight instructor didn't believe it until I demonstrated it to him).

    By comparison I did all the initial test flying on a friends Avid Flyer Mk 4 and it stalled solo at 36 mph. But the Mark 4 weighed 55% more empty than my A-model did (400 lbs vs 625 lbs). So the empty weight is 55% higher, while the stall speed is up 75%. Oh, and my old Avid has a small elevator, while the Mark IV has a much larger elevator. AND, the center of gravity on the early airplane has a forward CG, while the Mark IV has a rearward CG. Typically nose heavy planes stall sooner than ones with a rearward CG. So if one could move the cg back on my old A-model, it might actually stall even slower? (hard to believe, but maybe?)

    Now this isn't very scientific, but rather is based on some real life comparisons to almost identical airframes that have very different empty weights.

    Quote Originally Posted by HighWing View Post
    I think you nailed it. The math likely defines the minimum theoretical stall speed. What we have in a practical situation are many deviations from the ideal. Another example would be the airfoil where the only areas exactly on the Riblett design are over the rib capstrips and the eighth inch wide upper false ribs. The rest of the wing is a compromise determined by the geometry of the shrunk fabric surface.

    A good friend and Model IV builder tells of the time he heard Harry Riblett discuss the Kitfox use of his design. He mentioned that the most critical part of the design was the first five inches. The LLE from Kitfox adresses that issue. I put an aluminum leading edge on my wings and compared with my first Model IV, the new one stalls three mph lower - 140 vs. The previous 143.

    Then as to the elevator, for years fliers have been doing all sorts of things to increase elevator authority in the early Kitfoxes. The most common are the tape gap seals. The more recent availability of the larger elevators has attacked the deficiency at its source. That is essentially what we did for my friend's Avid.

    I guess it would be safe to say that, although a great airplane, the Kitfox is not perfect out of the box.

  5. #45
    Senior Member AirFox's Avatar
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    Default Re: A much faster Kitfox?

    I'm pretty happy with the Super sport speed with 26" Airstreaks.
    20141021_124403.jpg
    This pic was today with a nice tail wind. I average 26 mph cruise at 5200 rpm. At 5400 I'm in the 130's.

  6. #46
    Senior Member PapuaPilot's Avatar
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    Default Re: A much faster Kitfox?

    Quote Originally Posted by av8rps View Post
    Typically nose heavy planes stall sooner than ones with a rearward CG. So if one could move the cg back on my old A-model, it might actually stall even slower? (hard to believe, but maybe?) Now this isn't very scientific, but rather is based on some real life comparisons to almost identical airframes that have very different empty weights.
    That is true, a plane at aft CG will have a lower stall speed then the same one with a more forward CG. The reason is because the elevator & horizontal stabilizer provide down lift. With a forward CG the tail has to provide more down lift to balance these forces. The total lift of the wing produces is equal to the weight of the plane plus the downward force of the tail. When you go to an aft CG the total lift the wing is reduced lift due to the lower down force of the tail. When the total lift of the wing is reduce so is the stall speed, it's like flying at a lower weight. A lighter plane stalls at a slower speed.

    There are many advantages of an aft CG. The nose will pitch easier so you can get a deeper flair. The elevator is more responsive, but the plane is slightly less stable in pitch. With a slower stall speed you get a reduced ground roll. This can help too when you are doing a soft field takeoff (easier to get the nose up and a lower liftoff speed). You also get a higher cruise speed because the wing has to make less lift (which means less drag too). All things considered an aft CG is better choice.

    I am not promoting flying a plane outside of its CG limits. That can be deadly.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  7. #47

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    Default Re: A much faster Kitfox?

    Excellent information guy's! All the more reason to build as light as possible when you can and pay close attention to weight placement. Which covering/painting system provides the lightest end result?
    Kurt A

    Kitfox II,
    Rotax 912,
    1100 gross
    Fixer Upper Project

  8. #48
    Senior Member av8rps's Avatar
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    Default Re: A much faster Kitfox?

    Kurt,

    The lightest covering system I am aware of is dope and fabric. But you can't get into wanting a deep shiny finish. Just put the paint (butyrate or nitrate dope) on heavy enough so it covers just barely, and stop. It's going to be flat finish and kinda boring looking, but its going to be lightweight.

    The only other very light process is this new stuff from germany that you iron on like we used to monokote model airplanes. But expensive and limited life.

    Paul

  9. #49
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    Default Re: A much faster Kitfox?

    The only other very light process is this new stuff from germany that you iron on like we used to monokote model airplanes. But expensive and limited life.
    What process are you referring to and what information can you share on it's "limited life"? Or is this an opinion?

  10. #50
    Senior Member av8rps's Avatar
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    Default Re: A much faster Kitfox?

    Oratex covering...see www.betteraircraftfabric.com

    I read a recent article about this fabric process as it looked interesting. But I somewhat lost interest when in the article the writer said they expected a 6 year life, and it would cost 5 or 6 grand to cover an airplane.

    But I do see on the Oratex website they now say 10 yrs or more.

    Quote Originally Posted by Danzer1 View Post
    What process are you referring to and what information can you share on it's "limited life"? Or is this an opinion?

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