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-   -   Helicopter Glide Ratio (https://www.pprune.org/rotorheads/489253-helicopter-glide-ratio.html)

ChippyChop 29th Jun 2012 16:04

Helicopter Glide Ratio
 
Hi all just updated my Sky demon to find they have added a glide safe option. Not really that much help for us as we should know it out the window. But did get me to thinking just what is thaverage glide ratio for a helicopter.

Cheers Chippy

[email protected] 29th Jun 2012 16:23

Always less than you think!

JimBall 29th Jun 2012 16:28

Should be in the POM.

R44 is 4.7:1 on an average day. (4700ft horizontal for every 1000ft vertical.) But depends on airspeed at start of glide....

[email protected] 29th Jun 2012 16:30

It's not gliding - it's falling with style! Apologies to Toy Story;)

DennisK 29th Jun 2012 16:52

Auto descent
 
Hi Jim ... are you sure we are right! Heli descent in auto circa 1800 to 2000 fpm so from 1000 feet we hit the ground in circa 35 secs, which at 60 mph auto speed gives a range of around 2500 feet! Sounds mor elike 2.5 to 1. OR where have I gone wrong! DRK

JimBall 29th Jun 2012 18:29

Well, great DK, I want to bow to you. But the POM for R44 Raven II states 4.7:1.

And, from my experience over the past 9 years flying them, I'd say it's spot on - in average circs. As I intimated, going into a glide from a hover is a different thing - and the ratio shortens.

A 90/90 R44 auto (as taught by the factory on the safety course) gives astonishing results - 90kts at 90% rrpm.

OffshoreHeli-Mgr 29th Jun 2012 19:29

Quote: A 90/90 R44 auto (as taught by the factory on the safety course) gives astonishing results - 90kts at 90% rrpm.


I would imagine the low-rpm horn would be going off.

DennisK 29th Jun 2012 20:12

Auto range
 
Hi again Jim ... I spent yesterday at Dunsfold doing nothing more than practice autos (hi speed-low speed and hi & lo rrpm) and must confess that even the humble Hughes 300C at 60 knots into wind seems to make better than my 2.5 to 1 theory. But figs are figs. Anyone out there who can offer something definitive? Dennis K.

Corax 30th Jun 2012 02:04

Rate of descent varies with size/weight too. I remember seeing 3800 fpm on a Vertol!

lelebebbel 30th Jun 2012 04:41

R44 ROD in Auto at 60kts is less than 1800fpm. The published figure of 4.7:1 can be achieved when using the max. range configuration (90%RRPM/90kts)
At 90kts (= 9,100ft/min) forward speed, you would get 4.7:1 at a descent rate of just under 2,000fpm.



I would imagine the low-rpm horn would be going off.
Yes. Minimum allowable RRPM in autorotation is 90% though, so you are within limits.

Helinut 30th Jun 2012 14:40

I wonder whether the R44 may be a rather atypical helicopter, with a low RoD in autorotation.

I haven't flown one for many years, but recall that although the limitations section include a max DA of 14,000ft, there is another "altitude" limitation of 9,000ft agl "as the aircraft would otherwise take more than 5 minutes to land in the event of a fire". I suspect this implies a low RoD (rather than a rapid escalation of a fire in this type). I cannot recall a similar limitation in other types.

zlocko2002 30th Jun 2012 15:19

in mi-17 speed is 65kt and descent around 2000ft/min, but when you start flare(110-120ft agl), it takes some time and distance to get speed down for touchdown.

good egg 9th Jul 2013 20:30

Rule 5 & alight clear
 
Just curious, if you were cruising in a single (let's say 80kt?) in slack wind, and suffered engine failure (total), what height agl would you need to be to travel 1nm before impact/hopefully-surviving!?

SilsoeSid 9th Jul 2013 22:47

I think we need a hymn sheet to sing off, how about;
R22 autorotation engine off landing to a power recovery | Helicopter Lessons


Once were into wind at 70 knots, we lower the collective lever (with right pedal for yaw), roll the throttle off, and check up on the collective to prevent the rotors overspeeding, and add a touch of aft cyclic to achieve airspeed of 60kts.

So were now in a descent at about 1650feet per minute, with the rotor speed needle at 100%, the engine speed needle at idle, the wings level relative to the horizon, in trim, and looking out for drift. Its quite a stable and manoeuvrable flight condition, as long as we are careful to keep the rotorspeed accurate. So we can put turns in (as in the video here) to shorten our travel over the ground, we can even do a 360 orbit if we have enough height.
60 kts @ 1650'/min sound ok?

1 mile at 60 kts would take 1 minute, then it starts to get complicated :rolleyes:

Aesir 9th Jul 2013 23:49

Talk about complicated :hmm:



Glide ratio for BH206 is 4:1 @ 69kts.

Discussed here: http://www.pprune.org/rotorheads/489...ide-ratio.html

.

Ascend Charlie 10th Jul 2013 00:32

From 1000' you will be on the ground in about 30 seconds, give or take a few. Doesn't leave you much time to enter autorotation, make a mayday call, turn towards a safe spot, hopefully into wind, adjust your glidepath to make that spot, flare and land. About as much time as it took to read that.

You should have a good idea of where your "cone" of landing spots is, under and forward of your machine, and where in the window is the cutoff between making it and being impossible to get there.

pilot and apprentice 10th Jul 2013 02:05

A simple answer for the OP: a good rule of thumb is a 4:1 glide ratio in a helicopter (not perfect but close enough). So to glide 1 nm you need 1/4 nm, or roughly 1500', of altitude.

Given the use of terms, like 'impact' and 'hopefully surviving' I assumed you do not fly helicopters. Some glide better, some worse, but the biggest deciding factor will be pilot actions.

SASless 10th Jul 2013 04:00

Glide Ratio.....about equal to a set of streamlined car keys!

gulliBell 10th Jul 2013 04:08

If I had a 20nm over-water crossing and I was flying a 206L without floats I'd always climb to 10,000' thinking if the donk quit half-way I'd be able to make it to one shore or the other (using the best wind option of course). You "should" be able to go 10nm from that height if you bleed the RRRM back and keep the speed up, assisted by any tail wind in your favor.

rotorfan 10th Jul 2013 05:38

Helicopter glide ratio

Would that qualify as an oxymoron? :hmm:


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