Best glide speed not published?

jrh

jrh

Well-Known Member
I just started flying with a student in a 1959 Piper Comanche 250 (PA-24-250).

While reviewing V speeds with him, we ran across something interesting...we couldn't find a published best glide speed anywhere in his documentation.

He was taught by a former instructor to use 85 mph as best glide, which "feels" about right when I flew with him, but there's no written support for this number.

So I'm wondering, what should we do? Use a certain percentage of another V speed, such as 1.2 times Vs1 instead? Flight test his best glide speed somehow? Use the published best glide speed from later model Comanches?

It's kind of a bizarre problem to have, but I want to send this guy for a commercial checkride next week and we need to have solid support for the examiner.

Thanks!
 
It's 87 KTS at max gross weight (I fly a 1958 Comanche 250).

If this is your student's airplane, he =wants= the GAMA-style POH manual that is available through Webco Aircraft (http://www.webcoaircraft.com/). You won't find it mentioned online yet. These are manuals that were prepared through the years by Doug Killough and have been very-well regarded by Comache owners. Doug died last year and the Comanche Flyers Foundatio bought the rights to the manuals, which are being dfistributed by Webco.

They ain't cheap. The International Comanche Society member price is $75, but I think it's well worth it. If he calls Webco for the non-member price, he may figure out that joining is the better deal (I don't know). If he calls, gave him have his aircraft serial number available.

Of course, if you =really= want to impress the examiner, go out to the practice area and find out what the best glide speed is.
 
Thanks for the quick replies! I'll talk to him about that alternative POH you mentioned, Midlife.

MidlifeFlyer said:
Of course, if you =really= want to impress the examiner, go out to the practice area and find out what the best glide speed is.
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Could you explain the proper way to do this?

I think I read a Barry Schiff book a long time ago that explained how to flight test some of the common V speeds, like best glide, Vx, Vy, etc. but now I can't remember how it's done.
 
My two cents:


Airplane flying handbook says 1.4 (x) Vso. I was told 1.2 (x) Vso was minimum sink rate.

Also, I read an article in AOPA (which I used for my CFI checkride) that said deduct 1% airspeed for every 2% under gross weight.
 
jrh said:
Could you explain the proper way to do this?
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Think about what best glide is. It's the speed that gets you furthest for each foot of altitude loss.

Divide your horizontal velocity (airspeed will do) by your sink rate for a number of airspeeds and pick the one with the highest ratio.
 
Arnie7781 said:
My two cents:


Airplane flying handbook says 1.4 (x) Vso. I was told 1.2 (x) Vso was minimum sink rate.

Also, I read an article in AOPA (which I used for my CFI checkride) that said deduct 1% airspeed for every 2% under gross weight.
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Where are you reading that from? I have a 2004 edition of AFH, and I can't find it. Thanks.
 
Arnie7781 said:
Under 180 degree power-off approach...

Page 8-23, 4th paragraph
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I don't think that's what it says. "Recommended" glide speed, "normal" glide speed and the "appropriate" glide speed (PTS) for the maneuver might not be "best glide" (L/Dmax).

1.2 Vso and 1.4 Vso will definitely get you in the ballpark for Vx and Vmin sink (1.3) and Vy and Vg (1.4), but they are just rules of thumb and vary among aircraft. In Cessna singles, 1.4 Vso is just about on target. In a Cirrus SR22 it's about 5 knots off.

jrh, if all else fails, there's that other "rule of thumb." At loss of power, trim for the same pitch as level normal cruise flight. I haven't seen a single yet where that didn't just about nail best glide and it has the advantage of avoiding chasing the ASI instead of finding and heading for a landing site and troubleshooting the failure. Makes for a =very= smooth emergency procedure.
 
MidlifeFlyer said:
I don't think that's what it says. "Recommended" glide speed, "normal" glide speed and the "appropriate" glide speed (PTS) for the maneuver might not be "best glide" (L/Dmax).
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I don't see how you can interpret "manufacturer’s recommended glide speed" as anything other than best glide, since I've never seen any other glide speed published.

Still, best glide speed in the configuration you must use for the 180 power off accuracy landing *must* be slower than the published best glide speed.
 
So, in a 172RG, if after pulling the power, I slowed to a normal abeam speed of 80 (well above 1.4 Vso or best glide), stepped down my speeds as I incrementally put in gear and flaps, kept the pattern tighter to account for the engine being at idle rather than at 1500± RPM, was full stabilized at at 60-65 with full flaps on final, and smothly touched down directly on the designated spot, I would get pink-slipped?
 
MidlifeFlyer said:
smoothly touched down directly on the designated spot, I would get pink-slipped?
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If you're asking me, then, no, not likely, but I've seen some hair-raising maneuvers pass checkrides, so that itself is not a criterion for correctness.;)

When discussing maneuvers, the normal assumption is that we wish to apply textbook procedures. Texbook is to already have the gear down when abeam the numbers and then preserve altitude until the recommended glide is achieved. There is no guidance for the airspeed up to that point, so I think the faster, the better, because it increases your range. Unless your aircraft possesses a gear down best glide, then there exists no "recommended glide speed" for that configuration. I reduce the best glide for the Arrow II I use by about 10 knots; published best glide produces an alarmingly steep angle of descent with the gear down. 1.4VS0 would be crazy.

I haven't done this maneuver in a C172RG, but I've done them a lot in a regular C172. I fly pretty much a normal pattern; the airplane just wants to fly forever. Only in the Arrow II are changes needed in my pattern sizes in order to make the runway. In that airplane, for an 800 ft pattern, you need to be no more than .5 nm away and you've got to turn the moment the throttle is brought to idle.
 
MidlifeFlyer said:
I would get pink-slipped?
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Well, no, you probably wouldn't, but you would not be complying strictly to the PTS standrds in the CAX PTS IV. AoA: T.O's & LDGS TASK K: POWER OFF 180: item 4: "...establishes appropriate glide speed."

item #1 says: "Exibits knowledge of the elements related to a power-off 180 accuracy approach and landing."

This would require the applicant to know the consequences of not using best glide speed as the appropriate speed.

In actual practice, the way you describe it can be done, but it is with the knowledge that you are taking a chance by not using best glide, even when you know you can make it at different speeds, and also that you should be able to vary speeds to arrive at the designated point.
 
nosehair said:
"...establishes appropriate glide speed."
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That's sort of my issue. The PTS and AFH are both pretty good at using the phrase "best glide speed" and it's absence in this task in favor of "appropriate glide speed" (and the lack of a ± acceptable deviation) makes me wonder.

BTW, that's mostly an academic "wonder," although, while significantly faster may not be the greatest idea, slower than best glide, whether minimum sink or short field approach speed, may well be part of what's needed to get the job done.
 
nosehair said:
Well, no, you probably wouldn't, but you would not be complying strictly to the PTS standrds in the CAX PTS IV. AoA: T.O's & LDGS TASK K: POWER OFF 180: item 4: "...establishes appropriate glide speed."
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I'm not sure that "appropriate glide speed" for the power-off 180 maneuver is necessarily best glide speed.

I say this because of a discussion I had with my examiner after my CSEL checkride. In a nutshell, he suggested flying the maneuver slightly above or below best-glide speed. (I can't remember which it was.) He indicated that this would assist in correcting an undershoot condition. If you see that you're going to be a little short, you can then adjust to best-glide to get a little more distance out of your glide.


nosehair said:
This would require the applicant to know the consequences of not using best glide speed as the appropriate speed.
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I suppose this is what the examiner mentioned above was trying to say: the maneuver isn't about holding a certain speed until touchdown. Rather, it's about knowing your airplane's performance in various configurations, and using a commercial pilot's level of decision-making ability to manage the aircraft energy to get 'er down on the right spot.
 
WAFlyBoy said:
I'm not sure that "appropriate glide speed" for the power-off 180 maneuver is necessarily best glide speed.

I say this because of a discussion I had with my examiner after my CSEL checkride.
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I appreciate your point, and because it isn't so clearly stated in the PTS, it does leave it to the examiner. Another point to consider on all demonstrations of proficiency. However, lacking specific examiner preferences, and with the black-and-white text in the AFH describing glides, and their purpose in training, the "best glide", or "normal glide", as described in the AFH is what is referenced in the PTS and is what the applicant should use until directed to a different number by the examiner.
The point of the gliding maneuver is to become proficient at holding best glide with no attention to it, and being able to vary from that speed as appropriate when necessary.
 
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