Making Sense of Best Glide and Glide Ratio

[Jetpilot86]

Had a crankshaft snap on an A36 I flew Part 135 in over 30 years ago.  As a result, the prop flat pitched at high RPM. IIRC, I was 8 miles from an airport at 4000 feet. I barely covered 4 miles and managed to land on an emergency grass strip for crop dusters instead. 

[Hank]

3 hours ago, Jetpilot86 said:

Had a crankshaft snap on an A36 I flew Part 135 in over 30 years ago.  As a result, the prop flat pitched at high RPM. IIRC, I was 8 miles from an airport at 4000 feet. I barely covered 4 miles and managed to land on an emergency grass strip for crop dusters instead. 

Finally! A real example, with numbers, of "glides like a sewer lid"!

[A64Pilot]

11 hours ago, Jetpilot86 said:

Had a crankshaft snap on an A36 I flew Part 135 in over 30 years ago.  As a result, the prop flat pitched at high RPM. IIRC, I was 8 miles from an airport at 4000 feet. I barely covered 4 miles and managed to land on an emergency grass strip for crop dusters instead. 

I had a similar experience with the Prototype S2R-H80 coming back from Oshkosh at 9500 ft when the Gas Generator let go, as in blew apart. The Avia 106” three blade is a dual acting prop, which means it takes oil pressure to both increase and decrease pitch, unlike a Hartzell that a spring drives it to feather, so if oil pressure is lost it goes to feather.

It had a Walter E-11 engine as the GE was in Certification and the Walter was identical in form, fit and function.

So as the oil pump is driven off of the compressor section my prop stuck at cruise pitch. I called Grand Strand approach I think it was and declared an Emergency and asked where the nearest airport was, he said you just over flew it.

I was able to do a 180 from 9,500 ft, fly one mile, enter downwind and land, even made the turn off at mid field.

So rate of descent of I guess over 3,000 FPM? Anyone who has flown turbines knows if you pull one to idle the prop goes so flat it’s a hard decel, and a Crop Duster with a 106” prop is worse.

Disassembled it, put it on a truck and shipped it home.

 

Edited Thursday at 08:25 PM by A64Pilot

[jlunseth]

@A64 In response to your climb rate note, there was a really good article in AOPA magazine a few years ago. The writer actually went out with a safety pilot and did some tests to determine when and if the impossible turn might work. According to their results - and if I recall the article correctly they had some other results on climb rate also- neither Vx nor Vy are very good climb rates immediately after takeoff. Vx is too nose high and in their testing, stall came too fast. Normal pilots, not expecting the engine to quit, would generally react too slowly to avoid a stall. Vy resulted in the plane getting too far away from the runway, so the impossible turn would not work out. What they landed on was a speed about half way between Vx and Vy, which works out to, guess what? Best Glide. So I always do my initial climb at 85 kts., which if you look at the Best Glide chart in my POH is in that range. I am a little inconsistent, if I am trying to stretch out a glide I generally use 82 kts. but when I am climbing out post takeoff I use 85, which gives me a little better cushion if the engine were to fail. In their testing for the article they required the pilot flying to wait for, as I recall, 3 seconds before "realizing" the engine had quit and pushing the nose over. They found that the pilot had the time to respond if the climb rate was at Best Glide, and even better, the plane was not so far from the airport that it was no longer possible to return. 

Sometimes I use a faster climb speed when I am out in a rural area where there are endless areas to land in the event of an engine failure, but my home base, KFCM, is pretty well hemmed in so whatever I can do to get to an altitude where a return to the airport is possible, that is the best choice.

[Hank]

27 minutes ago, jlunseth said:

AOPA magazine a few years ago . . . neither Vx nor Vy are very good climb rates immediately after takeoff. Vx is too nose high and in their testing, stall came too fast. Normal pilots, not expecting the engine to quit, would generally react too slowly to avoid a stall. Vy resulted in the plane getting too far away from the runway, so the impossible turn would not work out. What they landed on was a speed about half way between Vx and Vy, which works out to, guess what? Best Glide.

As I learned early in my engineering career, this is a "case by case" decision, Best thought through in advance, on the ground. 

But the summation "halfway between Vx and Vy works out to Best Glide" is not always accurate.

For my C:

• Vx = 85 mph 
• Vy = 100 mph - Altitude 
• Vbg = 105 mph 
• Halfway between Vx and Vy ~ 92 mph, a much steeper climb than Best Glide would produce.

So I routinely climb at Vx to clear obstacles then lower the nose to Vy to altitude. But Vbg is always there in the back of my head if something happens. 

Many people here recommend a Cruise Climb around 120 mph for engine cooling.

[jlunseth]

1 hour ago, Hank said:

Many people here recommend a Cruise Climb around 120 mph for engine cooling.

Just to be clear, the climb speed I was referring to is intended to get you to 1,000 feet AGL, or whatever altitude in that vicinity allows the pilot to make the 270 degree turn to return to the runway. One of the things they did in creating that AOPA article was to practice the impossible turn, at a safe altitude, in order to determine how much AGL altitude the particular aircraft requires to make the turn, and it is a 270 turn to get back.

I use 1,000 AGL.

[PT20J]

I did a lot of practicing the "impossible" turn in a sim and then verified my findings at the airport. With no wind, I can just barely make it back at 800' AGL using my normal takeoff sequence where I lift off at about 65 KIAS, pitch to 7 deg, raise the gear at positive rate, raise the flaps passing 80 KIAS. Wind is the biggest variable. 1000' is more comfortable. 

[A64Pilot]

If you practice the maneuver be sure to “freeze” the controls for say 2 sec after pulling the power as even Superman has a reaction time.

Secondly if you just barely make it, it’s unlikely you really would due to a real windmilling prop sucks a lot of energy where an idling engine won’t or sucks a lot less anyway, it may even provide a bit of thrust, reason so many have the idle set so low to help prevent float. 

Problem with climbing at best glide is that if the engine quits, you will be below best glide by the time you react you have lost speed.

Personally I have a field in mind just to the left of a house on one end and just a field on the other end. Years  ago the house had a V tail crash in their front yard and the two inside burned, so I’m sure they don’t want another crashed airplane, so I don’t overfly their house.

Very often if an engine quits soon after take off the pilot stalls and of course lose control often being severely injured or worse. I just accept that the impossible turn is just that and hang my hat on that most forced landings in fields etc the people walk away.

Edited yesterday at 12:33 PM by A64Pilot

[Pinecone]

AOPA Impossible Turn Video