Best rate of glide?

[1938BF50]

Wx providing, we are heading out on a grand adventure this weekend. Amos and I are going to head north up the eastern shore of Door County, across to Washington Island, to Manistique, MI, over to Beaver Island, south along Traverse Bay, and then on to Big Rapids, MI., to the annual airshow. I will be crossing about 20 miles of water on the Beaver Island leg.

Not knowing if I will be able to do any testing before the trip, I would be intersested in hearing some of your BEST RATE OF GLIDE experience. When testing rate of glide what RPM are you throttling back to?

[flyguy]

Re: Best rate of glide?

This is what I found. If I try to find the speed at which I can maintain altitude with the lowest power setting I get 70 Statute, indicated and 1500 RPMs at say 1500 feet AGL. I take this to be more or less best L/D for the aircraft. This is a BC12D-85 at about 1100 lbs. So 70 is what I use when push comes to shove and everything gets quiet.

Bet that starts an interesting discussion, chuckle.
Darryl

[paulnuss]

Re: Best rate of glide?

There's more to it than any one "best speed". If you must cover some ground (or water) to glide to a landing area, you must consider the wind and your rates of descent at various airspeeds. An example, to illustrate the point: you are without engine, and 5000 ft above the water, with 4 miles to the shoreline ahead. You need a glide ratio a little better than 4 to 1 to make dry land. No problem? Maybe! If headwinds are 30 mph, and you glide at 60 mph, your forward progress is 30 mph. You are sinking at 700 ft per minute. It will take 8 minutes to get to the shoreline. You will have lost 5600 ft, and you only had 5000 to play with. Increase speed to 80 mph, now sinking at 900 feet per minute, your forward progress is 50 mph. You will need just under 5 minutes, and will have lost 4500 ft of altitude. You arrive over the shoreline with 500 ft to spare. So you need to know your sink rate at various indicated airspeeeds when gliding, and know what the effects of any winds might be.

[Pearson]

Re: Best rate of glide?

Well this is going to be an interesting thread. I agree with Paul, there is no one "best speed". But if you are trying to figure out what your sink rate is with zero thrust, it will be almost impossible to achieve without actually stopping the prop. Everyone tries to find the rpm for zero thrust, but what do you do about the drag of the stopped prop. Also the different prop pitches will cause differing rpms to produce zero thrust. I know from experience that in general most aircraft lift over drag speeds are near or just below the top of the white arc, which is flap extension speed. But since we don't have flaps, that doesn't do us any good. What we need is the lift over drag chart. My guess would be that best glide speed in still air is somewhere in the mid 60s, say 63 to 67. But, as Paul mentions, you need to think about what the wind is doing. Weight is also a factor, but I would think at our light weights it is almost negligible.

[M Jones]

Re: Best rate of glide?

Mike,

I'll leave the best speed recommendation to the experts, it looks like you've already gotten some good info.

Washington Island is having their annual fish boil on Saturday the 16th. We're planning to go again, weather permitting. You might consider a day trip up there next weekend if you're free. Let me now if you do plan to go.

PS. I flew a short over water leg from Menominee to Sturgeon Bay a while back and found out that I did think I head funny engine noises. I guess you have to get use to it...

Mike

[1938BF50]

Re: Best rate of glide?

Mike,

The fish boil sounds like a ball! I have not been there in over 20 years. I can say that is the best fish boil I have ever been to. I will check with the social coordinator and see if we are free.

[Larry Lyons]

Re: Best rate of glide?

An old WWII term for flying over water was "Automatic Ruff". It apears just as you get past the glide point! When I was a pip squeak I heard that term a lot!
L

[M Jones]

Re: Best rate of glide?

Hi Mike,

A couple of things about the fish boil on Washington Island you may already know, but thought I should mention... No fuel on the field, so plan for that.

Door County / Cherry Land airport is about 40 miles south and had fuel when there. Ephraim / Fish Creek airport is about 20 miles south and listed as having fuel. (I've landed there but didn't ask about fuel)

If you really want to get a little over water experience, Escanaba in the UP is the next airport/mainland to the north.

By the way, the last time I attended the boil they started serving at 11:30. If you wanted to go early on Friday night, they close the north/south grass strip so people can tent camp. I thought I would do that but the wife is planning to go and wouldn't enjoy that, so we'll leave by 8:00 on Saturday morning if the weather cooperates.

Mike

[fearofpavement]

Re: Best rate of glide?

One thing that has not been stated yet is that there is a difference between "best glide" which gives you the greatest distance through the air and minimum sink which gives the greatest time in the air. I have found that in most aircraft the best glide is similar to best rate of climb but that may be a vague speed too in a Tcraft. I always figure about 70 mph in our BC12D is about the best glide give or take a little.

[magman]

Re: Best rate of glide?

An old WWII term for flying over water was "Automatic Ruff". It apears just as you get past the glide point! When I was a pip squeak I heard that term a lot!
L

Vietnam Era = MPP Maximum Point of Pucker