Best Glide Speed

[jiott]

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?

[n85ae]

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)

[Dave S]

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.

[Delta Whisky]

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??

[PapuaPilot]

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.

[Delta Whisky]

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.
25578

[jiott]

Wow! Looks like about 47 knots (54 mph) is your best glide speed. That's quite slow compared to the rest of us.

[efwd]

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.

[PapuaPilot]

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.

[jiott]

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.

[Delta Whisky]

Jim - the point at which the line is tangent to the curve is the max L/D speed. The minimum sink speed is at the bottom of the curve. The issue is: which one if the best glide speed? Some say it is the speed that gives the max time to stay aloft (which is min sink; max time to evaluate a landing spot and prepare for the event), others say the speed at which you glide the farthest (max L/D; max distance to hunt more landing spots?). Tomato, tamato maybe?

[PapuaPilot]

Best glide speed is defined as the speed that gets you the most distance. This is where the line intersects the Speed/Rate Of Descent curve. The lowest point of the curve is called minimum sink rate, which will keep you in the air a little longer but you won't glide as far. On the graph that DW made best glide is 54 and min sink is about 44.

It's not one or the other. If you want the greatest distance you should use best glide, and if you want max time aloft you should use the min. sink rate. Glider pilots have a good handle on this, often they want max time aloft, unless they are trying to do a x-country flight.

https://generalaviationnews.com/2016/01/10/do-you-know-your-best-glide-speed/#:~:text=Best%20glide%20speed%20is%20the,the%20lea st%20loss%20of%20altitude.

[jiott]

OK, I needed some education and you guys straightened me out, I agree with you 100%. I was thinking only sink rate and forgetting about distance which is the most important.

[Delta Whisky]

I owe jiott an apology and PapuaPilot a note of thanks for the clarification. PapuaPilot clarified the relationship between the regulated world in which we fly and my response to jiott which was directed to the real world question which wasn't his question. (Real world meaning: best glide speed for what purpose? Are you trying to stay up or go far?)

Past that - I would add a cautionary note to all those measuring their plane's glide performance: do your very best to run them at the same weight. That way, when you are finished, you can adjust your critical speeds to fly (minimum sink and max L/D speeds) by the aircraft's "new" weight. The formula is new speed = measured speed times the square root of the new weight divided by the old weight. In my case my best L/D occurs at 61 mph at a weight of 1081. If I head off on a trip at max gross (1320 in my case) and the good ol engine becomes silent, my max L/D speed becomes 68 mph and my minimum sink speed becomes 54 mph. Another way of saying this - if you run your test glides at various weights, you are really collecting data for a variety of graphs. Note though - the glide ratio stays the same (at the various weights). That could also be one of the may possible reasons we have differences between our planes' measured performances.

I'm not satisfied with the quality of my test points and plan to continue testing into the winter months when the air becomes more stable.

[rtc183]

Did some engine off testing in my S7 with the Airmaster prop. Wanted to know exactly how it would perform with the motor actually off. With the prop not feathered at 60mph and 1225lbs the lowest sink rate was 500fpm. Feathering the prop gave me 400fpm and took 12 seconds. Unfeathering to start took 12 seconds plus about 8 seconds for the 912is computer to test and be ready. The Airmaster prop gives me 25 miles at 10000ft. to find someplace at sea level. It is a nice thing to have.
Bob

[RV6flyer]

Best glide speed is best L/D speed. One should not care how long it takes them to land but how far you can go to get to a good landing spot. The tangent line is the best L/D for a no wind condition. For a head wind you need to be a little faster. If you study the curve you will see that being slightly on the fast side of Best L/D does not effect you much. You are better off being on the fast side rather than then "the back side of the curve".

Flew sailplanes for 25 years.

Allan