View Full Version : BK 117 Down draft
chopmoo
13th Jul 2017, 00:27
Hello
Does anyone have the wind downdraft velocity approximate figures while working under a BK117 up to about 10 metres.
I understand it depends on the weight and conditions, let's say near max all up weight and calm conditions.
jimf671
13th Jul 2017, 15:00
Not seen.
ICAR Air Commission was seeking out information about this for a range of aircraft types a few years ago in relation to the dangers of winchmen and mountain rescuers working under aircraft in challenging locations. Very little was available.
As a rough rule of thumb, if you know what the rate of descent is at best speed in autorotation, it is a pretty good indicator of the downwash speed in the OGE hover. Not exact or mathematical but pretty good as a ballpark figure.
For example 2000'/min equates to just under 20 kts.
Flying Bull
13th Jul 2017, 17:47
http://www.polizeifliegerstaffel.de/userfiles/images/WR30Sep2015/WR30Sep15_05.jpg
http://www.polizeifliegerstaffel.de/userfiles/images/WR30Sep2015/WR30Sep15_06.jpg
/F7ZsxxjPp-A
fadecdegraded
13th Jul 2017, 18:37
Haven't done it with a BK but holding a handheld anemometer under a hovering AS350 at MAUW gave 50kts, the BK would be more.
Definatly more than 20kts.
Note that it is OGE hover so some distance below the disc - not the same as standing directly under it in a low hover.
It is useful in assessing proximity to VRS since, if you know your downwash speed you know where the likely critical RoD is.
If you know a better way then post it.
SuperF
14th Jul 2017, 08:03
The BK has probably be the worst downwash among light and medium helicopters that i have seen. Having followed behind most types on fire operations, the worst machine with a short line on is by far the BK. You need at least a 100' line on a BK, whereas a 206, 500, 350, 204 or 205 can happily work on a 50' without stirring the fire up too much.
rottenjohn
14th Jul 2017, 08:24
Note that it is OGE hover so some distance below the disc - not the same as standing directly under it in a low hover.
It is useful in assessing proximity to VRS since, if you know your downwash speed you know where the likely critical RoD is.
If you know a better way then post it.
I don't know a better way, but I do know from looking down from 10 meters it's a hell of a lot more than that
thechopper
14th Jul 2017, 09:40
Try this link
Calculating Rotor Downwash Velocity - Rotor & Wing International (http://www.rotorandwing.com/2011/11/29/calculating-rotor-downwash-velocity/)
:8
NickLappos
14th Jul 2017, 14:17
Here is a very detailed FAA document with lots of data. It is "Rotorwash Analysis Handbook" FAA AD A283 719:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA283719&Location=U2&doc=GetTRDoc.pdf
I don't know a better way, but I do know from looking down from 10 meters it's a hell of a lot more than that perhaps you could use the condition of the water beneath you and relate it to the Beaufort scale.
Most helos produce something that would equate to a Moderate sea state - that is equivalent to a Force 5 or 19 - 24 Kts:ok:
To get to 50 Kts you are talking Storm Force 10 which would give Sea state 8 - if you have ever been out in those conditions at sea, you will know that is not what exists under a hovering helicopter.
fadecdegraded
15th Jul 2017, 02:02
perhaps you could use the condition of the water beneath you and relate it to the Beaufort scale.
Most helos produce something that would equate to a Moderate sea state - that is equivalent to a Force 5 or 19 - 24 Kts:ok:
To get to 50 Kts you are talking Storm Force 10 which would give Sea state 8 - if you have ever been out in those conditions at sea, you will know that is not what exists under a hovering helicopter.
Well it was 50kts the day I did it with the handheld wind meter and yes I have spent a lot of time at sea and if you enlarge that wind to make it comparable to the size of the sea it would be force 10.
Lots of ifs and scenarios in your posts crab but if you don't know something it's ok, no single person knows everything and it's ok to say I don't know especially if you actually don't.
Or just say nothing or in this case post nothing rather than say what someone else has actually done is wrong or lies or whatever you meant.
A very emotional post fadecdegraded - you seem to be suffering from the modern syndrome that one either has to be right or wrong about everything and just making a suggestion or discussing options or opinions isn't acceptable.
I have suggested answers to the OPs question - whether they are right, wrong or somewhere inbetween isn't for me to say - some of the graphs in Nick's linked document seem to suggest variations between 20 and 50 kts for a CH 53 but there is a lot of data in there and I don't have the time or inclination to digest it.
Phoinix
15th Jul 2017, 10:34
I measured these numbers... but it's on hoist position, in 5 meter increments:
https://www.dropbox.com/s/zj5bdxv9muecbp3/Downwash.jpg?dl=0
So the peak value is 14.8 m/s or 28 Kts:ok:
NL
the bit about the harley-davidson vs the S76 (at about p249) looked like valuable research into this "complex issue"
Not strictly the same issue but, nevertheless, good visuals based upon actual data:
09bvuYRKwwc
Self loading bear
18th Jul 2017, 22:02
Not strictly the same issue but, nevertheless, good visuals based upon actual data:
09bvuYRKwwc
Thanks very good video
SLB