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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 |
Always less than you think!
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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.... |
It's not gliding - it's falling with style! Apologies to Toy Story;)
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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
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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. |
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. |
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.
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Rate of descent varies with size/weight too. I remember seeing 3800 fpm on a Vertol!
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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. |
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. |
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.
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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!?
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I think we need a hymn sheet to sing off, how about;
R22 autorotation engine off landing to a power recovery | Helicopter Lessons Once we’re 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 we’re 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. It’s 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. 1 mile at 60 kts would take 1 minute, then it starts to get complicated :rolleyes: |
Talk about complicated :hmm:
Glide ratio for BH206 is 4:1 @ 69kts. Discussed here: http://www.pprune.org/rotorheads/489...ide-ratio.html . |
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. |
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. |
Glide Ratio.....about equal to a set of streamlined car keys!
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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.
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Helicopter glide ratio
Would that qualify as an oxymoron? :hmm: |
Given the use of terms, like 'impact' and 'hopefully surviving' I assumed you do not fly helicopters. Thanks for replies, always good to further one's knowledge :ok: |
Too complicated for me. I was taught to look out the chin bubble if the engine failed cos that was probably where I would land! Just dumping the collective quick has to be the first priority,not calculating glide ratios.
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Though this was standard issue lesson 1.
Better to be in the spot you can reach rather in one you think you can reach http://i98.photobucket.com/albums/l2...6/IMG_3650.jpg |
"Glide ratio" is like the EPA mileage on the window sticker
Numbers, based on theory, pulled out of some tests that I won't ever be repeating.
I don't know exact distances or absolute altitude, en route. "That" field is my next forced landing site, based on the angle of reference observed in performing autorotations, and varies from airframe to airframe. Never thought about that point being 5000 feet across the ground versus 5500 feet, just that it's within the cone (as someone has termed it). Seems to work at all altitudes, except very low and very fast. Gaining altitude makes the area included larger and a wind shifts it downwind. My experience is that GPS positions lag actual, so distances derived therefrom to known points in a dynamic, non-surveyed situation can be dangerous. |
Glide ratio varies a lot with airspeed and Rotor rpm.
Min allowed NR and max allowed IAS in auto gives the best glide ratio. Can differ quite much from 'standard' 70kts and 100% NR... |
Gulibell,
Seriously? |
Having come from the Chippie 10, to Culdrose for the 'wings course', the main thing I remember from the famil flight in a Hiller12e was the auto to EOL. I thought 'farkinell' that was quick'!
Eventually got used to that, and on moving to the Whirly 7, (showing a bit of age there!), was amazed at how gentle the auto was, almost enough time to have a ciggie on the way down! |
Try it in a Wokka and ad in a 180 or 360 turn for giggles.....you can catch your car keys as they catch up with you!
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The glide ratio for an S-76 is about 4.2, at min rate of descent and maybe 4.7 at best glide.
The lowest rate of descent in the glide is driven by the power required to fly at best rate of climb speed, since the engine is no longer the power source, the power is taken from the potential energy of the helicopter - its altitude. Lowest rate of descent is at or close to Vy (best rate of climb) but might read differently on the airspeed indicator due to calibration differences between the steep descent of auto and the steep climb of a max climb rate. In other words, the airspeed might READ differently in auto, but most likely the actual airspeed is the same as a climb. Similarly, the best angle of descent is close to the best range speed, for the same reason. Best range is where the fuel burned takes you the farthest. If you substitute fuel burned for altitude lost (same thing, really - energy consumed is energy consumed) then best glide is farthest flown for a unit of descent. The steep glide angle relative to an airplane is due to the basic inefficiency of the rotor as compared to a wing. Most airplanes have a glide ratios of 8 to 12 to 1, and sleek airplanes can be 18:1. Gliders can be impossibly efficient, with 40:1 in the world class ones. Rotors are less efficient than wings onlu until the winged machine tries to hover, that is......... |
Gliders can be impossibly efficient, with 40:1 in the world class ones. |
R66 - efficient
... over 5:1 in an R66
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A good glider these days can make about 55:1. That's roughly 10 miles per 1000'. You can't see the surface at 10 miles from 1000' due to the curvature of the earth. |
You can't see the surface at 10 miles from 1000' due to the curvature of the earth. |
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