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Thread: Best Glide Speed

  1. #1
    Senior Member jiott's Avatar
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    Default Best Glide Speed

    Kitfox publishes a best glide speed for the SS7 of 65 mph. I decided to verify this using the new EAA flight test manual's test cards for this. They have you do a separate test at 3 speeds: Vy, Vy+10, Vy-10. Since my Vy is 65 mph I did tests at 55, 65, and 75 mph. Surprisingly the results showed nearly identical glide performance at all 3 speeds, a glide ratio of 11.5:1 with engine at idle, no flaps, 1150 lbs weight. This seems like good news; you don't have to be very precise in your glide speed to get maximum results.

    I am wondering if anyone else with an SS7 has done these tests and can verify or question my results?
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  2. #2
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    Default Re: Best Glide Speed

    My glide is similar, and pretty much the same from 60-80 mph and if you land at
    anything close to glide speed you use the entire runway in ground effect floating

    Jeff (Series 5, IO-240B)

  3. #3
    Senior Member Dave S's Avatar
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    Default Re: Best Glide Speed

    Recently got the new EAA flight test manual and also decided to revisit Vx, Vy and especially best glide.

    There is going to be some variability due to loading so my results might be a bit different than a more lightly or heavily loaded aircraft.

    I tested at 55, 60, 65,and 70 mph with a weight of 1425#. Did not test beyond 70 so don't have that info.

    What I found at the weight above was not a lot of difference between 70 and 60, although 65 was slightly better than 70 while 60 not too different from 65. Going to 55 reduced the best glide a measurable amount. Looking at my original data and this more recent retest indicates that best glide at 1425# is probably at 62 mph indicated although the difference within the range from 60 to 65 is very slight.

    I suspect 65 published by Kitfox was at the max weight of 1550#.

    Also did a few trials with flaps which made it real clear that if a person wants max glide, don't ever use flaps.

    I came to the conclusion long ago that if a person didn't know any of the V speeds for a kitfox7 - if they simply use 65mph they will be close enough to most critical airspeeds to stay out of trouble.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

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    Senior Member Delta Whisky's Avatar
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    Default Re: Best Glide Speed

    I'm still working on getting enough data to be able to purge the outliers that were caused by poor piloting and air mass movement but right now, it appears my best L/d (cabane gear and round cowl at 1093#) is going to come in at 9.2 at 60.4mph. I'd include a picture of the polar but for some reason I've had trouble this last week adding pictures. Moderator??

  5. #5
    Senior Member PapuaPilot's Avatar
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    Default Re: Best Glide Speed

    I haven't looked back at my data, but it seems I got similar results too. I have been using 65 for Vy and V glide.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  6. #6
    Senior Member Delta Whisky's Avatar
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    Default Re: Best Glide Speed

    I'm still trying to find out why I suddenly can't upload photos to the site. So - if there is a picture of the last set of data points from my glide tests - then I have discovered the source of my problem. (Not the solution, just the source.) Note: the airspeed units are in knots and the VS is in feet per minute.
    Kitfox glide ratio.jpg

  7. #7
    Senior Member jiott's Avatar
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    Default Re: Best Glide Speed

    Wow! Looks like about 47 knots (54 mph) is your best glide speed. That's quite slow compared to the rest of us.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  8. #8
    Senior Member efwd's Avatar
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    Default Re: Best Glide Speed

    That is the speed that mine becomes an elevator. If I am high on final and the winds are not a concern I can slow to 50-55 mph and I fall like a rock. It feels as effective as doing a slip.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  9. #9
    Senior Member PapuaPilot's Avatar
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    Default Re: Best Glide Speed

    Quote Originally Posted by jiott View Post
    Wow! Looks like about 47 knots (54 mph) is your best glide speed. That's quite slow compared to the rest of us.
    I think you have it backwards Jim, DW said his airspeed is in knots. The line intersects the curve at 54 kts, which is 62 mph. That is pretty close to 65.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  10. #10
    Senior Member jiott's Avatar
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    Default Re: Best Glide Speed

    Phil, maybe I need education, but the way I interpret that curve is the horizontal axis is airspeed in knots and the vertical axis is decent rate in fpm. So best glide would be the lowest point on the curve of test points, which looks to me to be about 47 knots. I am not sure what the straight line and its tangent point to the curve is for. Best glide is simply minimum descent rate.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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