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9 Answers

Glide Speed

Asked by: 5352 views Flight Instructor

I just got a 1961 Aztec (PA23) and can't find any published best glide speed. Any thoughts?


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9 Answers

  1. Patrick Flannigan on Nov 12, 2011

    I’m not familiar with the Aztec, but I can tell you that best glide occurs at the airspeed for L/D max. This is the airspeed that produces the lowest total drag and will give the longest range and best power-off glide range. 
    Check your performance charts for a L/D (Lift / Drag) chart. You could also look for a minimum drag chart, as Vmd (minimum drag speed) is equal to L/D max and best glide. The three speeds are one and the same.
    If you are unfamiliar with L/Dmax or Vmd, review your aerodynamics and performance material to brush up.

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  2. DeWitt Barker on Nov 12, 2011

    The Aztec is a twin Piper, 250HP each side. This is a 1961 aircraft, the POH is about 20 pages long. No L/D charts. FYI I’m a retired airlline pilot, not new to aviation.

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  3. Nathan Parker on Nov 12, 2011

    The Piper Seneca I didn’t publish a best glide speed either; perhaps Piper thought it didn’t make much sense for a twin.  Part 23 only requires the best glide speed to be published for single-engine airplanes.
    The only way to really calculate it yourself would be to feather both engines and then compute the descent angle at various speeds.  Not recommended.
    Do you have any particular reason to know or is it just curiosity?

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  4. Mathieu Desjarlais on Nov 12, 2011

    Yhea, you probably inherited of an owners manual with very limited information. We operate 5 PA-23-250 and are stuck with the owners manual as reference. Contact me by email if you want and maybe I can help you.

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  5. Brian on Nov 13, 2011

    You could get a reasonable approximation through experiment. Now the following you may already know, but there are many readers here that don’t. So feel free to skim, I’m not assuming your knowledge.
    A few things to know:
    Minimum sink = Minimum power required. 
    L/D max ~ (approximately equals) minimum sink / 0.86
    We can approximate minimum power required speed through experiment in the airplane. Find the lowest power setting where you can maintain level flight in a clean configuration at some airspeed. Note that airspeed and convert it to calibrated airspeed with performance data. If that information is unavailable you can approximate CAS 2 knots below IAS. Most systems I’ve seen (not too many yet) CAS will only be one or two knots below IAS at this minimum sink speed.
    Once you have minimum sink (minimum power required) speed in CAS, take out a calculator, and divide it by 0.86. The result will be at least a reasonable estimate of best glide. It won’t be perfect, but it will be more perfect than nothing if you happen to fly any aircraft, single or twin, where this speed isn’t published.
    I urge those reading to test it out in an aircraft with a known best glide speed. Remember if you’re flying under gross weight your speeds will all be below published. Hope that helps DeWitt. 

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  6. Nathan Parker on Nov 13, 2011

    “We can approximate minimum power required speed through experiment in the airplane.”
    Can’t easily be done power on.  The drag profile is really very different….the prop wash over the fuselage is a source of drag, and the power available will vary with airspeed.  The .86 applies only to the power off condition.

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  7. DeWitt Barker on Nov 13, 2011

    Thanks for the different answers. The reason I am looking for the best glide is because I just had installed a new G-500 panel and you can set the airspeed indicator with all the differant V speeds.

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  8. Brian on Nov 13, 2011

    “Can’t easily be done power on.”
    Oops! Silly me, do the same experiment except power off searching for the speed where sink rate is lowest. Again it won’t be perfect with the engines still running, but it can give you something to work with.

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  9. Bob Watson on Nov 13, 2011

    I’d start with the best rate-of-climb speed and experiment from there. Vy is where you have the most excess horsepower so that should give you an idea where you need the least. In a pinch, that should get you close, if you don’t have the actual number but need a quick approximation.
    As far as experimenting, for ME training we use a “zero-thrust” setting (12″ in the Seneca-I) to simulate a feathered engine for single-engine practice. I suppose you could verify this by comparing the handling and performance of flying with one engine feathered to that with a low power setting and fine tune the zero-thrust setting. Granted it might not be as pure of a test as actually shutting the engines down, but you should be able to get close enough (e.g. throttle back to where the windmilling drag counteracts any propwash effect over the wings) for this test.
    I haven’t tried it, but I’d imagine that you could (at a suitable altitude, of course) set that power on both engines to simulate two feathered engines and (have your co-pilot) start taking notes. For me that would induce much less “pucker factor” than trying to take notes with two feathered props!

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