Glide Ratios

[Dale Young]

I wish I had a little more glide! (Grumman Yankee)

 

[p1l0t]

13:1 with the prop stalled, MAYBE. 10-11:1 otherwise.
That's recollection from 1,000 hours of ownership of an M20J.

Ed will be along shortly!

Well yeah, that's clean with the prop at highest pitch/low rpm. And its not a maybe its 13:1

<---<^>--->

 

[bbchien]

OK, but curious what 1,000 hrs has to do with it? It's a simple datapoint that must be measured (one time will do). Extent of experience would have no correlation with the accuracy of your "recollection"...not that I don't believe you of course.

Doesnn't ahve a lot to do with it except I had time to do things like the attachment....

PS there is an arithmatic error in it (corrected in ink)....

 

[wabower]

Well, at least it's an arithmatic error rather than a virbal one.:wink2:

Doesnn't ahve a lot to do with it except I had time to do things like the attachment....

PS there is an arithmatic error in it (corrected in ink)....

 

[ClimbnSink]

Point is none of the GA fleet qualifies as 'efficient airframes' some are better than others but they are all pretty bad, flying on motor.

They may fly on excess power, but when they glide they have a fixed power, usually zero. The best glide is based on the best L/D and is a fixed angle. Usually the more efficient airframes have a lower D and therefore a higher glide ratio.

 

[dell30rb]

Allow me to qualify your comment ...

AC engines aren't 'low HP'. They are rated at a lower horsepower because they turn at a lower RPM. The 300HP cars aren't generating that power at 2,700 RPM they're doing it at 6K-7K RPM or more depending on displacement.

Example: I had a 600cc (36 cu in) motorcycle that was rated for 110HP ... @ 10,000RPM! But at 2,700 RPM I doubt it would get 15HP.

Cars only need 20-40 HP to maintain highway speeds. Your Cessna needs 120HP (75%) to maintain it's rated cruise speed. Try to run your car at 75% power and see how long it lasts.

They are low HP... 160 HP is less than 300 something.

You could put a gearbox on that 600CC motorcycle of yours and turn 10,000 RPM while delivering the full 110HP to a propeller turning 2700 RPM. In fact your motorcycle already has a gearbox. I'm pretty sure your rear tire does not spin at 10,000RPM

The reason most airplane engines are built like they are is for simplicity / reliability. No gearbox means less moving parts, less weight. Air cooling = simplicity. You either build a big, simple motor that is detuned (compared to, a car) and air cool it, or you build a small, hot motor with a gearbox and liquid cooling. Either way you're delivering the same horsepower to the propeller.

As an example - compare a rotax 912S to a lycoming O-235C. About the same HP (100). The 912 is 83ci with a 10.5:1 compression ratio and the 235 is 233ci With a 6.5:1 compression ratio. Both deliver the same HP.

 

[Jaybird180]

They are low HP... 160 HP is less than 300 something.

You could put a gearbox on that 600CC motorcycle of yours and turn 10,000 RPM while delivering the full 110HP to a propeller turning 2700 RPM. In fact your motorcycle already has a gearbox. I'm pretty sure your rear tire does not spin at 10,000RPM

The reason most airplane engines are built like they are is for simplicity / reliability. No gearbox means less moving parts, less weight. Air cooling = simplicity. You either build a big, simple motor that is detuned (compared to, a car) and air cool it, or you build a small, hot motor with a gearbox and liquid cooling. Either way you're delivering the same horsepower to the propeller.

As an example - compare a rotax 912S to a lycoming O-235C. About the same HP (100). The 912 is 83ci with a 10.5:1 compression ratio and the 235 is 233ci With a 6.5:1 compression ratio. Both deliver the same HP.

What do you think the bike is doing at redline and 6th gear? I don't feel like doing the math, but it's probably up there.

 

[Jaybird180]

Allow me to qualify your comment ...

AC engines aren't 'low HP'. They are rated at a lower horsepower because they turn at a lower RPM. The 300HP cars aren't generating that power at 2,700 RPM they're doing it at 6K-7K RPM or more depending on displacement.

Example: I had a 600cc (36 cu in) motorcycle that was rated for 110HP ... @ 10,000RPM! But at 2,700 RPM I doubt it would get 15HP.

Cars only need 20-40 HP to maintain highway speeds. Your Cessna needs 120HP (75%) to maintain it's rated cruise speed. Try to run your car at 75% power and see how long it lasts.

HP is a factor of RPM and torque. Engine HP data is published at MAX HP. Some engines are setup for high torque at low RPM (eg. V-Twin cruisers) versus High RPM screamers (inline 4's) which usually has peak torque in the 8-10k range and a gear change crossover point around 12,500 rpm with overrev in excess of 14,000.

 

[dell30rb]

What do you think the bike is doing at redline and 6th gear? I don't feel like doing the math, but it's probably up there.

I checked and with a 25" diameter tire you're doin about 1650rpm at 120mph

Work is boring today

 

[wabower]

When you guys are done with Horsepower 101 could you do about 20 posts on octane just to keep it interesting?

HP is a factor of RPM and torque. Engine HP data is published at MAX HP. Some engines are setup for high torque at low RPM (eg. V-Twin cruisers) versus High RPM screamers (inline 4's) which usually has peak torque in the 8-10k range and a gear change crossover point around 12,500 rpm with overrev in excess of 14,000.

 

[Jaybird180]

When you guys are done with Horsepower 101 could you do about 20 posts on octane just to keep it interesting?

That's a different forum, Wayne. But nice catch

 

[Sac Arrow]

What do you think the bike is doing at redline and 6th gear? I don't feel like doing the math, but it's probably up there.

The above quoted 600 cc motorbike normally won't redline in 6th gear. It will do about 160 mph or so, depending on how well you are tucked.

 

[Cap'n Jack]

Probably closer than you think. Airplanes fly on excess power, not grace, not engineering.

Could you define "excess power"? I heard it said that planes climb on excess power. A 150 HP plane climbs faster than a 120 HP plane because it has more "excess horsepower". Without "excess power", the plane maintains altitude (or sinks if there isn't enough power).

I'm just trying to understand the term.

 

[dell30rb]

When you guys are done with Horsepower 101 could you do about 20 posts on octane just to keep it interesting?

Be careful what you wish for. I'm the only one at the office today