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Date: Mon, 13 Dec 2004 14:11:53 -0500
From: "canarder@frontiernet.net" <canarder@frontiernet.net>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Glide distances
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Actually, it can get complicated with airplanes that common, nominal
winds amount to 1/3 of glide speed.  Lowest sink rate is NOT best range
glide.  Matter of fact, best range occurs at a measureably higher speed
than best glide.

Best glide speed put out in POH by designer (Vans, Cozy, etc.)is a good
point of departure, but you DO have to depart from there.  If you're
gliding downwind (in say a 20 kt or > tailwind), you want to be at the
speed that gives you lowest sink rate so as to maximize the opportunity
of the wind to blow you farther downwind.  OTOH, if you're stuck gliding
upwind, if you use best glide range speed (or worse yet, lowest sink
rate), you'll be pretty much "hovering" and not going much of anywhere
at all.  This can all be quantified, but I don't think any of the
designers have done it very extensively.

My plan is to use take the recommended best range glide speed put out by
the designer and declare it best no-wind range at that weight - Vo.  I
then carefully measure my sink rate at that IAS from maybe 10,000 msl
to something like 1000-1500 agl (where I can comfortably relight, and
failing that comfortably dead stick the rest of the way).  Next, I
repeat the exercise at 5 and 10 (or 5, 7, 9, etc.)kt slower IAS until I
find the lowest sink rate.  With those two airspeeds, I have the no wind
(Vo) and tail wind (Vo-10) glide speeds and can determine the upwind
best glide airspeed with reasonable accuracy for our purposes by
declaring it to be Vo+10.  Delta-V can be as much as 10 kt IAS.  Then I
check my sink rate at Vo+10).  I now test again and record sink rates at
Vo+10.

Armed with these numbers and always being acutely aware of the
difference between my GPS GS and my cruise TAS, I'll have a pretty good
idea of whether I'm dealing with a headwind or tailwind the minute the
engine quits, and can quickly decide whether to go at or 5 or 10 kts
slower or faster than optimum no wind IAS to wherever I have chosen to
set it down.  With the readily available E6B programs available, and
knowing your sink rate at optimum glide ground speed (GSo), GSo-10 and
GSp+10 you can do some reasonably accurate calculations and tablualte
some ranges on a spread sheet.

I think it's a safe bet most of us will be more or less astounded by the
difference in range.  Like spittin' distance from double for 30 kt
headwind v. 30 kt tailwind.

Just a theory ... Jim S.


Quoting Ed Anderson <eanderson@carolina.rr.com>:

> On my 12 mile engine out glide, I punched up the NRST button on my
> GPS to find the two closest airfields were both 12 miles away
> (gulp!).  Since I knew I was going to be gliding around 85 mph that
> any sort of headwind would considerably shorten the glide.
> Fortunately I had been paying attention to the wind direction as I
> was outrunning a cold front, so turned 120 degs for the downwind
> field.
>
>   From 9500 MSL I arrived over the selected airfield with approx
> 1500-2000 ft AGL.  I had altitude to do a 360 and a couple of hard
> "S" turns to loose altitude and even then came over the fence at 100
> mph.  This with a stubby wing RV-6A.
>
> I did have time in that 8 minute glide to find out that my best glide
> speed (at that weight)  was 87 MPH which produced a 750 fpm rate of
> descent.  Any slower the rate went up and any faster the rate went
> up.  Also were I found what airspeeds the prop would stop at as well
> as what it took to get it going again.  So a rather productive 8
> minutes all things considered {:>).
>
> Ed Anderson
>   ----- Original Message -----
>   From: Perry Mick
>   To: Rotary motors in aircraft
>   Sent: Monday, December 13, 2004 9:29 AM
>   Subject: [FlyRotary] Re: Glide distances
>
>
>   That's true, if you are flying cross country, and are halfway
> between two airports, you would want to pick the one that's downwind,
> not necessarily the closest field.
>   I figure with 1nm per 1000' of altitude, just orbiting the field on
> a test flight, there is plenty of margin built in since I can
> supposedly glide about 1.65 nm / 1000' with no wind.
>
>   Tom wrote:
>
> Perry,
> There always something.  Gotta remember winds.  Being downwind of the
> airport
> at the distance limit would be a problem.
>
> By the way, when flying skydivers I figured out a very easy way to
> determine
> winds aloft with a GPS.  Fly a circle keeping your airspeed constant.
>  Your GPS
> groundspeed will be highest when heading downwind and lowest when
> heading
> upwind.  Do the simple math on speeds to determine how much.  I'm not
> suggesting you do this during engine-out, just when you're curious
> and have a
> desire to fly in circles.
>
> Tom
>
> --- Perry Mick <pjmick@viclink.com> wrote:
>   I've never tested engine out glide, but I also plan with 1000
> ft/min.
> And if you assume a 10:1 glide ratio, you can glide 10,000 fpm, or
> 1.65
> nm or 1.9 sm per 1000 ft. of altitude. When I do test flights, I try
> to
> stay within 1 nm of the airport for each 1000 ft of AGL. For example,
> 5000 ft AGL, stay within a 5 nm radius. Easy with GPS.
>
>
>
>
>
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