Donut effect on the EH 101
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BIT, they aren't a bit alike, the old BERP blade has a thick, forward bulge at the tip that is almost ax-like when seen from above. In other words, the blade chord increases markedly at the tip.
The S-92 (and UH-60M) blade is a swept, anhedral tip that thins out in thickness and droops down the last 5% of blade span.
The S-92 blade has a figure of merit that is about 4% better than the BERP blade (which means that for the same horsepower, the S-92 blade lifts about 5% more weight than the BERP.)
The S-92 (and UH-60M) blade is a swept, anhedral tip that thins out in thickness and droops down the last 5% of blade span.
The S-92 blade has a figure of merit that is about 4% better than the BERP blade (which means that for the same horsepower, the S-92 blade lifts about 5% more weight than the BERP.)
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Nick, thanks for the FOM data and the detailed design differences between 92and 101 tips, I appreciate your input.
I fly at highish density altitude as fast as possible a lot so what about the blade stall line data comparison?
I fly at highish density altitude as fast as possible a lot so what about the blade stall line data comparison?
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To try and get back to the point -
the BERP blade operates at a relatively low NR and creates a greater proportion of its lift at the tip (as too I suppose does the 92?). The 'Donut Effect' has been routinelly commented on by operators in Iraq - not an absence of brownout, but rather a bubble of relatively better vis within the disc.
In light of the lack of other reasonable suggestions I propose this as a solution - but would be delighted to be corrected (I am certainly not an aerodynamicist...and certainly not a Westlands agent!!)
Anybody got a better answer?
the BERP blade operates at a relatively low NR and creates a greater proportion of its lift at the tip (as too I suppose does the 92?). The 'Donut Effect' has been routinelly commented on by operators in Iraq - not an absence of brownout, but rather a bubble of relatively better vis within the disc.
In light of the lack of other reasonable suggestions I propose this as a solution - but would be delighted to be corrected (I am certainly not an aerodynamicist...and certainly not a Westlands agent!!)
Anybody got a better answer?
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It's so easy to get into aerodynamics theory and arguments about who's got the better tip that its easy to forget what matters.
The line pilots that reported this effect are happy with it. They are probably experienced pilots who have flown similiar conditions in different types (all of which blow dust into a donut, yet don't seem to offer the benefit they've reported). Lets respect them by agreeing that perhaps they have found something unique.
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As far as the blade tip argument, here's my 2 cents. The S92 has about 20 years of development and technology that BERP didn't have. If S92 isn't better then something is very wrong. Similiarly, BERP IV has the advantage of potentially being better than the current S92. That's just how technology develops.
As far as timing and design similiarities, that's easy to explain. Physics doesn't stop at political boundaries. The physics that gives benefits to the BERP blade also work for the S92. As far as timing goes, the demands of the helicopter are very similiar because they are competing in the same global market. This means that the desired performance enhancements are quite close. The timing of producing similiar designs has much to do with the technology that's available. As more is known about composites, and more confidence is generated with their use, designs will start using them. I expect that making a tip like these out of wooden blades would have required so much weight that the gains in performance were lost to AUW.
As far as I know, BERP was the first (at least commercially, not sure about development). That makes it exciting. Bragging about its performance now could sound a lot like bragging about the speed of my 75 MHz Pentium. At its time Pentium was impressive, now I'm left behind with an 866MHz Pentium III. AW appears to understand this, hence the development of a new series of BERP blades.
Matthew.
The line pilots that reported this effect are happy with it. They are probably experienced pilots who have flown similiar conditions in different types (all of which blow dust into a donut, yet don't seem to offer the benefit they've reported). Lets respect them by agreeing that perhaps they have found something unique.
-----------
As far as the blade tip argument, here's my 2 cents. The S92 has about 20 years of development and technology that BERP didn't have. If S92 isn't better then something is very wrong. Similiarly, BERP IV has the advantage of potentially being better than the current S92. That's just how technology develops.
As far as timing and design similiarities, that's easy to explain. Physics doesn't stop at political boundaries. The physics that gives benefits to the BERP blade also work for the S92. As far as timing goes, the demands of the helicopter are very similiar because they are competing in the same global market. This means that the desired performance enhancements are quite close. The timing of producing similiar designs has much to do with the technology that's available. As more is known about composites, and more confidence is generated with their use, designs will start using them. I expect that making a tip like these out of wooden blades would have required so much weight that the gains in performance were lost to AUW.
As far as I know, BERP was the first (at least commercially, not sure about development). That makes it exciting. Bragging about its performance now could sound a lot like bragging about the speed of my 75 MHz Pentium. At its time Pentium was impressive, now I'm left behind with an 866MHz Pentium III. AW appears to understand this, hence the development of a new series of BERP blades.
Matthew.
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For what it's worth i believe the mechanism for the '92 anhedral tips is that tip vortex associated with producing lift is spead out (to make it weaker) and pushed away from the tip path. Helicopters produce lift by ingesting static air and accelerating it downwards through the rotor, so that downwash speed is twice speed through rotor. By pushing the tip vortex away the rotor effectively ingests air from 4% larger area than it otherwise would. So effectively disk loading goes down, or FM in the standard calc goes up.
I won't bore y'awl with the Biot-Savart law applied to vortex fields, and anyhow CFD has made this academic.
BERP I (or II?) was really designed for high speed, by having shaped tip profile stay within transonic Mach cone on the advancing side (sweepback), and incidentally pushing retreating stall up by letting the shed vortex developed during initial stall remain attached to profiled leading edge. Not sure it would make any difference in hover, but have seen no evidence to support or refute that claim.
I would be interested to see Nick's stall data, but my guess would be that once the already shed vortex is lost lift dropoff is severe.
BERP IV will likely be as much a step from BERP I as I was from 0.
Mart
I won't bore y'awl with the Biot-Savart law applied to vortex fields, and anyhow CFD has made this academic.
BERP I (or II?) was really designed for high speed, by having shaped tip profile stay within transonic Mach cone on the advancing side (sweepback), and incidentally pushing retreating stall up by letting the shed vortex developed during initial stall remain attached to profiled leading edge. Not sure it would make any difference in hover, but have seen no evidence to support or refute that claim.
I would be interested to see Nick's stall data, but my guess would be that once the already shed vortex is lost lift dropoff is severe.
BERP IV will likely be as much a step from BERP I as I was from 0.
Mart
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I'm still trying to understand Nick's remark
.... I never realized it required small stones to hover in a black hawk
....Would have tought it required large ones to extend hover in such an efect.
have used small stones and features in Black Hawks
....Would have tought it required large ones to extend hover in such an efect.
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I think the Green Helis blade would fair better in FF Flight. not sure yet. I would like a better picture. or info on which blade that is.
the tip kinda looks like it would be less likly to stall at higher angles. (Is there a Chart that shows info on this? any data?
I would like to look at a chart which shows a line to indicate stall, with AoA up one side, and Air speed along the bottom, or something to that effect)
the tip kinda looks like it would be less likly to stall at higher angles. (Is there a Chart that shows info on this? any data?
I would like to look at a chart which shows a line to indicate stall, with AoA up one side, and Air speed along the bottom, or something to that effect)
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danger,
I am looking for it - haven't forgotten! If you know any EH drivers, the chart I refer to is in the flight manual.
Most flight manuals are not copyrighted, but each document tells you.
I am looking for it - haven't forgotten! If you know any EH drivers, the chart I refer to is in the flight manual.
Most flight manuals are not copyrighted, but each document tells you.
Last edited by NickLappos; 8th Apr 2007 at 13:32.
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confused...
Nick, do you mean that the Flight manual is in the public domain ? (and by EG do you mean EH and it's a typo?)
If so is there a S92 equivalent for comparison?
DM
If so is there a S92 equivalent for comparison?
DM
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Nick, the cruise charts show that vne (indicated) decreases as pressure altitude increases when ambient temperatures are very low. As it warms up, vne doesn't affect max speed in level flight at heavy weights because of approaching transmission limits.
I think the information you're talking about may be embedded in the charts, but it would require quite a bit of effort to draw it out, and I would want to check the legality of posting proprietary information.
If anyone wants to spend $25, AIAA-1990-3008 might have some relevant data. Go to http://www.aiaa.org.
I think the information you're talking about may be embedded in the charts, but it would require quite a bit of effort to draw it out, and I would want to check the legality of posting proprietary information.
If anyone wants to spend $25, AIAA-1990-3008 might have some relevant data. Go to http://www.aiaa.org.
donut is old
Agree with Lappos. This dust donut phenomenon is so old, it appears in photos in an early Army Report published in 1967, written by Sheridan. Maybe that does not ring a bell, but the report was used as the reference to a mil spec on global climactic data, which was then used in another, etc.. until the actual sand/dust density in air around helicopters used for most comercial and millitary test applications came from the same. Not surprisingly once you read it, but dust concentration is highest with higher downwash velocity
Heck, Prouty has this in one of the first chapters of the big red book.So the real question in a hover, is at what blade radius is the peak velocity? More to do with disc loading, so then, gross weight than blade design, as long as you are actually flying that is.
BERP blades sure do look funny, maybe better at something, but I bet the axehead costs more, and may even increase blade stress from CF.
Heck, Prouty has this in one of the first chapters of the big red book.So the real question in a hover, is at what blade radius is the peak velocity? More to do with disc loading, so then, gross weight than blade design, as long as you are actually flying that is.BERP blades sure do look funny, maybe better at something, but I bet the axehead costs more, and may even increase blade stress from CF.
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BERP
Sounds like this thread on BERP has lost the reason the blade was designed. Remember your basic theory of flight? What factors limited forward speed in a conventional helicopter;
1 Limit of fwd cyclic due to flapback
2 Compressibility on advancing blade
3 Airflow reversal - retreating blade
4 Retreating blade stall
BERP blades were designed to compensate for 2-4, focused on forward speed not hover efficiency due to the fact that the 101 role as envisaged was to be a passive sonics helicopter spending most of its time in forward flight. Composite material allowed for exotic shape to the whole length of the blade and the tip has gone through several iterations.
Move forward and the wall comes down, the role changes and you end up with a helicopter that hovers with quite an element of nose up and less efficient in the hover than others. No surprise.
The AW press release cited talks about improved hover, well from a poor base! As for other benefits, it has to be the most quiet helicopter in its class in the hover....
1 Limit of fwd cyclic due to flapback
2 Compressibility on advancing blade
3 Airflow reversal - retreating blade
4 Retreating blade stall
BERP blades were designed to compensate for 2-4, focused on forward speed not hover efficiency due to the fact that the 101 role as envisaged was to be a passive sonics helicopter spending most of its time in forward flight. Composite material allowed for exotic shape to the whole length of the blade and the tip has gone through several iterations.
Move forward and the wall comes down, the role changes and you end up with a helicopter that hovers with quite an element of nose up and less efficient in the hover than others. No surprise.
The AW press release cited talks about improved hover, well from a poor base! As for other benefits, it has to be the most quiet helicopter in its class in the hover....
Point is that the blade design should have little to do with this phenomenon, the thread title anyway, and the report shows this pictured on an H-21 way before BERP. My copy is old, and actually made of paper. No promise, but if I can scan the photo, I will. Correction to some details: Sheridan is the author's first name, and it was 1968, not 1967. Maybe someone can beat me to it. Love the gram/ cubic ft units.... but an Excellent report. Interesting that the video only simulates this condition. Too much to actually fly in it?
Accession Number : AD0669676
Title : EVALUATION OF THE DUST CLOUD GENERATED BY HELICOPTER ROTOR DOWNWASH
Personal Author(s) : Rodgers, Sheridan J.
PDF Url : AD0669676
Helicopters
Jet and Gas Turbine Engines
Accession Number : AD0669676
Title : EVALUATION OF THE DUST CLOUD GENERATED BY HELICOPTER ROTOR DOWNWASH
Descriptive Note : Final rept.
Corporate Author : MSA RESEARCH CORP EVANS CITY PAPersonal Author(s) : Rodgers, Sheridan J.
PDF Url : AD0669676
Report Date : MAR 1968
Pagination or Media Count : 136
Abstract : The dust cloud generated by a tandem-rotor H-21 helicopter was studied as a function of type of soil, hover height and disc loading. A total of 98 tests were made, and three different test sites were used. Samples were collected at 25 locations on the helicopter. Analyses were made for dust content and particle size distribution. Average dust concentrations at the area of highest dust density, i.e., at rotor blade overlap, are given. The highest concentrations were measured at the site of rotor blade overlap, and the lowest concentrations were found beneath the rotor hubs. The maximum particle size decreased with increasing elevation. No particles over 500 microns were found at any elevation. Dust concentrations of 40 mg/cu ft were measured during takeoff and approach maneuvers. With another helicopter hovering in the immediate area, concentrations of 64 mg/cu ft were measured.
Descriptors : *HELICOPTER ROTORS, *DUST, *GROUND EFFECT, PARTICLE SIZE, LIFE EXPECTANCY(SERVICE LIFE), SOILS, SAMPLING, DOWNWASH, ENGINE AIR SYSTEMS COMPONENTS, HELICOPTER ENGINES, HOVERING, INGESTION(ENGINES), VISIBILITY, EROSION, ROTOR BLADES(ROTARY WINGS), PILOTS
Subject Categories : AerodynamicsPagination or Media Count : 136
Abstract : The dust cloud generated by a tandem-rotor H-21 helicopter was studied as a function of type of soil, hover height and disc loading. A total of 98 tests were made, and three different test sites were used. Samples were collected at 25 locations on the helicopter. Analyses were made for dust content and particle size distribution. Average dust concentrations at the area of highest dust density, i.e., at rotor blade overlap, are given. The highest concentrations were measured at the site of rotor blade overlap, and the lowest concentrations were found beneath the rotor hubs. The maximum particle size decreased with increasing elevation. No particles over 500 microns were found at any elevation. Dust concentrations of 40 mg/cu ft were measured during takeoff and approach maneuvers. With another helicopter hovering in the immediate area, concentrations of 64 mg/cu ft were measured.
Descriptors : *HELICOPTER ROTORS, *DUST, *GROUND EFFECT, PARTICLE SIZE, LIFE EXPECTANCY(SERVICE LIFE), SOILS, SAMPLING, DOWNWASH, ENGINE AIR SYSTEMS COMPONENTS, HELICOPTER ENGINES, HOVERING, INGESTION(ENGINES), VISIBILITY, EROSION, ROTOR BLADES(ROTARY WINGS), PILOTS
Helicopters
Jet and Gas Turbine Engines
Distribution Statement : APPROVED FOR PUBLIC RELEASE