Glide planning for Bahammas Trip

[Dan O'Brien]

Hi all,

I'm thinking of flying to the Bahamas, first Nassau, since Laura has a gig there. I'm rather neurotic about planning landing sites in the event of an engine out, and it appears to me easy to fly high enough to glide to an airport if the route is KPBI MYGF MYNN (West Palm Beach to Freeport to Nassau). The longest stretch over water between airports on this route is 68 miles, which requires a glide of 34 miles in the worst case to reach an airport. I estimated my glide ratio at 14.4:1 during flight testing, which gives a glide of glide of 42 miles at 17,500 feet. Easy peasy, right? Well, maybe not. The 14:4 glide ratio I estimated was when the prop was pulled. It was conservative for that conditions, but an engine out with no oil pressure means a lower glide ratio if the prop can't be pulled due to a lack of oil pressure. I didn't test my glide when the prop was full forward. Did any of you? Generally, I'm interested in what glides you get in your ES when the prop is forward and when it's pulled.

Happy and safe flying,
Dan

[George Wehrung]

I know our ES w prop pulled is close to 15:1. Non pulled, not sure. But my wife and I have flown to the Marsh harbor a number of times.

I fly over at 11,500. Rent a life raft from Ft Pierce. You need to make a reservation to rent the raft.

If plan to file IFR you will be with Miami center and then handed off to local approach but it’s more like VFR flight following. In that there’s always clouds so you will have to descend early to find the airport.

Reach out if you have more questions.

In summary I don’t worry about the glide…


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[Ryan Riley]

Dan, my glide is the same as George's ES with the prop in course pitch (pulled back) . That is a good question, though. Here's my take on this. First off, if you have an engine failure that is not an engine seizure (still have oil pressure), then you should have the prop in fine pitch, full rich, and try for a restart. If you can't get a restart, then I'd pull the prop back and see what happens. You'll feel a noticeable reduction in drag if the windmilling prop is generating enough oil pressure to allow the blades to move.

If you pull the prop back and nothing happens, then you have a choice. Accept the glide as is, or you can slow below best glide and approach your clean stall speed to try and get the prop to stop. A stopped prop will be better than a windmilling fine pitch prop.

Now back to your mission planning. I planned for the same flight last year and didn't go due to scheduling conflicts. I planned to do the same thing as George. Fly mid-alt and have a raft. If you do the math, you're out of glide for only a short while.

[Dan OBrien]

Interesting point about slowing to stop the windmilling. I wonder how close to stall one gets before it stops or if it stops before stall? My ES stalls gently when coordinated, so if I could get it to stop at or near stall, that might be a good strategy. Kinda makes me wonder about the glide ratio for (1) course pitch; (2) stopped; (3) windmilling. But maybe the raft is the way to go given that it sounds easy to do.

[Ryan Riley]

Dan, when you have some time, 6-3-3 Ditching Procedures is a good read to brush up on prior to your trip. I just re-read it myself since you posted.

https://www.faa.gov/air_traffic/publica ... ion_3.html

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[Dan O'Brien]

Thanks Ryan

[Robert Pastusek]

Dan,
I've been intending to send you some "thoughts" on this subject for some time...
Back in 1976, I had a crankshaft failure in my Cherokee 235 near Winslow, AZ. It was mid-afternoon and we were on our way back to Clovis, NM from the Reno AIr Races. I was at 13,500' (to be as cool as possible), and that provided some airborne time to evaluate and act. I lined up on I-40 (no visible traffic) and raised the nose to try to stop the prop, as the windmilling engine vibration seemed likely to separate it from the airframe. Looking back, that was unlikely, but the noise and vibration were seriously attention-getting, and I was able to stop the prop rotation. This improved the glide range enough that I was able to land at the Winslow Airport (KINW). I had no oil presssure (back part of the engine was not turning) so the prop was in fine (min) pitch from the time of failure, but stopping it more than doubled--actually almost trippled the glide range. That was not expected, and I'd honestly not given the subject much thought. After the Cherokee was back flying, I did some glide testing with the engine idle/off and the prop in min pitch, max pitch, and stopped. Min to max pitch almost doubled the glide range of the Cherokee 235, and stopping the prop added at least 50% more. Attention getting...
So when I built my IV-P 30 years later, I installed an MT counter-weighted prop that goes to max pitch with loss of oil pressure. This was a weight/complexity/cost compromise to not installing a feathering prop, but the engine-out glide results were about the same as for the Cherokee. At 120 KIAS with the engine at idle/off (makes no measurable difference), with the (4-blade) prop at min pitch, N437RP glides at 10:1. At max pitch, it glides at just better than 18:1 (less than 500 FPM down). I believe it would do even better with the prop stopped, but I am not able to stop the prop before the airplane stalls, so I've not been able to evaluate this.
So I have two significantly different glide ratios for a windmilling prop in coarse/max pitch or fine/min pitch for my airplane. IMHO, the difference in glide range is something every pilot flying behind a constant-speed prop should evaluate for him/herself. I've found that it varies a lot among different GA airplanes...with a range from about 8:1 to better than 20:1! The engine-out glide testing on N437RP validated my decision to go with the counterweighted prop, significantly reducing the odds that I'd be caught with an engine failure and the prop stuck in fine pitch. In any case, I'd strongly recommend you evaluate these flight conditions for yourself...in your airplane... Some day you will be a glider pilot, but it can end well if you are prepared--and practice...
Bob Pastusek

[Dan OBrien]

Thanks Bob,

I went back to my flight test logs and realized I did calculate in a glide the ratio for the prop set to fine pitch as well as coarse pitch. A more careful averaging of the data logs than we did from jotting down numbers in flight shows that at 100 kts, the ratio averaged 14.7:1 with prop at coarse pitch and 9.1:1 with the prop at fine pitch. That's a huge difference, which seems broadly consistent with your findings for the IV. While you have higher wing loading, you also have less drag. I did not experiment to try to optimize the speed. Depending on the circumstances, it definitely seems worth trying to get the prop to stop if oil pressure is gone, which presumably would make the ratio even better than 14.7:1. In any event, this makes me want to set my G3X glide ring and iPad glide rings to the upper and lower numbers to cover both cases where oil pressure is gone and can't get it stopped and where there is oil pressure to allow coarse pitch.

Interestingly, the glide ratio in the Columbia 350 manual (airplane is similar size to the ES, a little bigger, and has a different wing) is about 11:1 in their chart, which is between the two numbers I got for my ES. The chart doesn't state whether it assumes coarse or fine pitch.

[Dan OBrien]

BTW Bob, just for level-setting on glide and ratio at 120 kts and descent rate, for an 18:1 ratio I get 675 ft/min descent:

675 ft/min = (120 nm/hr airspeed) / (60 min/hr) x (6076 feet/nm ) / 18 (glide ratio)

As it works out, my flight testing shows 691 ft/min descent at 100 kts. Close to the same descent rate at 100 kts in the ES as in the IV at 120 kts.

Let me know if I did my calcs wrong!

[George Wehrung]

Interesting Dan. In my ES at 95 knots-ish, weight dependent I get 500 fpm.


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