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Ex-FAAWAFU

The best Destroyer/Frigate helicopter in history

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The experts will astound us with the truth soon however looking at it it replaces the fastest moving piece with a swept wing instead of a plain end

Since sweeping the wings back in prehistory allowed for high speed flight I'm assuming that doing the same on rotor tips has a similar effect

I will now sit back and like you, learn from "The Man"

 

🚁

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The British Experimental Rotor Profile (BERP) blade was/is a very successful programme to improve the aerodynamic performance of rotor blades, particularly in aircraft designed for high speed.  All you plank-wing types will no doubt scoff at the idea of high speed in a helicopter, but you have to keep in mind that aerodynamic issues depend on the speed of the wing (in this case the blade; a blade IS a wing, so I’ll use the terms interchangeably here), not the speed of the fuselage - in rotary wing the difference can be very large, especially at the tip of the blades.

 

Rotary wing aerodynamics are complicated, but the simplified version is that there are two phenomena you have to deal with (though they are inter-related): compressibility towards the tip of the advancing blade (the blade whose tip is going in the same direction as the fuselage); and retreating blade stall on the opposite side.

 

The advancing blade tip in a Lynx is close enough to the speed of sound for compressibility to be an issue, just as it was for early jets.  Again simplified, but basically the air can’t get out of the way fast enough so you get shock waves that put big stress on the blade.  The swept and anhedral / drooped tip help to delay these, allowing the whole aircraft to go faster.  

 

Retreating blade stall is kind of the opposite side of the same coin.  Anyone familiar with how airfoils generate lift knows that speed is one of the key variables, and angle of attack (‘alpha’) the other.  If you think about it, the advancing blade is generating loads of lift because it’s fast - so to generate something similar, the retreating blade has to increase its alpha [because otherwise you’d end up with a large roll moment as one side of the rotor disc out-lifted the other].  As with any airfoil, if you keep on adding more alpha, eventually you stall the wing (or part of it - which would again add huge stresses to the blade).  The “paddle” tip of a BERP blade helps with retreating blade stall.

 

So the short answer is that BERP blades are much more efficient, especially at high speed.  [Bill is basically right!]

 

The aerodynamics have have been understood for quite a while, but early helicopters weren’t fast enough for it to become a major problem - and it was only possible to make complex-shaped rotor blades of sufficient strength once composites came along; metal blades (let alone the wooden blades of really early helicopters!) wouldn’t have withstood the stresses.

Edited by Ex-FAAWAFU

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P.S. The speed thing is why you see BERP blades on the Lynx (normal cruise 120kts, routinely flown to 150 and beyond) & Merlin (no slouch, though I don’t know exact figures), but not Seaking (normal cruise 90kts, maximum c.110 downhill on a good day).  Seaking 4s & 7s in Afghanistan had issues with lift in “hot & high” conditions, and adopted “Carson” blades, which have swept tips but not the rest of the BERP’s features.

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See Giorgio, told you "The Man" would tell us didn't I?

 

I'm glad I was rightish but I hadn't taken the retreating blade bit into account, good job some of us know

 

Thanks Crisp, very useful stuff (again) to know

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Hello Crisp !

Very good job with the jig, I'll take it in mind...

Since the blades of my Kamans are different...

Now are the blades really différents in the real thing or is this a moulding gag from Matchbox or KH :shrug:

There are no curves at the tip but a kind of a flap near the tip… Supposedly for the same reasons !

Now, I'm looking for pics of the ASW compartment and sonobuoys launcher but that's not easy to find out !

Have a great modelling time !!

CC

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CC the flappy thing near the tip of a Seasprite (and I think of all Kaman designs) is for a different reason; it’s the way those designs control pitch, and it’s called a “servo flap”.  

 

Most helicopters rotate (“feather”) the entire blade to alter angle of attack, using a pitch change rod at the root which rotates the blade around a spindle (“the feathering hinge”).  Kaman went down a different route (building on Flettner’s pioneering work during WW2), instead making the blade change pitch via the flappy thing, which acts like an aileron.  The advantage of that is simplicity and less need for heavy hydraulics (which was a big deal in early helicopters when power was very limited).

 

There’s a good explanation of this here: https://www.helis.com/howflies/servo.php

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2 hours ago, Ex-FAAWAFU said:

The advancing blade tip in a Lynx is close enough to the speed of sound for compressibility to be an issue, just as it was for early jets.  Again simplified, but basically the air can’t get out of the way fast enough so you get shock waves that put big stress on the blade.

£10 gets you that somewhere on the Internet is a conspiracy group ('Bladers'? :facepalm:) invested in the idea that helicopters therefore fly better in the vacuum of space....

 

Another erudite exposition Crisp. :thanks:

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Drag is less of an issue in a vacuum, certainly.  Alas, so is lift...

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Thanks for the effort of making all of this clearly understandable, Crisp! :thanks: It was quite an education!

 

Ciao

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1 hour ago, giemme said:

Thanks for the effort of making all of this clearly understandable, Crisp! :thanks: It was quite an education!

 

Ciao

What he said! Crisp just showing what's so great about this forum with a couple of great posts explaining a  complex subject to us non-flying civvies (me) and the wider BM massive. Thank you. 

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Thanks, everyone.  I’ve been trying to remember exact numbers all day, but can’t...

 

The 3 aircraft I’m modelling all had old fashioned straight blades - metal in the case of XZ722 & XZ729, composite for ZD260 (I am going to have to perform the same tip surgery on the Revell 1/32 blades in due course).  But later on in my career I flew Mk3 Lynxes with BERP blades.  The increase in efficiency was such that the torque (power) required to hover on a given day was noticeably lower - it’s been this I’ve been trying to recall.  I think there was about 3% difference from memory, which is a lot to gain from pure aerodynamic improvements.

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9 hours ago, Ex-FAAWAFU said:

Rotary wing aerodynamics are complicated

Not if one has an understanding of witchcraft and other forms of black magic.

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A bit more modelling; I’ve now almost finished the blades.  The resin tips are a little bit wider than the Airfix originals, which works well - it’s clearly deliberate since the instructions mention it, and it allows you a little leeway in sanding back to get the alignment looking right.

 

Airfix provide 4 folded blades (which I am using) & 4 spread, so here you can see the comparison:

resized_b94ea80c-8808-4063-af2e-48ad8b79

 

You can probably see from that pic that this was before sanding away the extra bit  - the 3rd one up is clearly fat at the tip, for example.

 

But here they are after a bit more fettling.

resized_94d5fd64-265f-403c-9563-56cde49b

 

Getting there.  

 

I’ve also been detaching the resin tail from its pouring stub (not the whole tail, just the bullet fairing that covers the tail rotor gearbox), which is quite an undertaking - it’s a complex shape so takes a lot of work.  With the reverse direction tail rotor and a different (longer) horizontal stabiliser, the back end is quite different - and there are quite a few strengthening plates on the Mk8 tail which need removing.  Airfix did a nice job on this kit, though.

 

Happy enough with progress thus far.

 

More soon

 

Crisp

Edited by Ex-FAAWAFU

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Forgive me for backtracking slightly, but the pics of the gearbox don't appear to show a static swash plate or control rods. Is that something to do with its 'rigid' nature?

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Ooh, thanks @DaveWilko - I knew they did a tail fold and nose electronics bay for the Mk.8, and similar for the Mk.7, but hadn’t seen the engine.  This might change my plans; the original idea was to have the 1/32 cab as the engine monster.

 

@rich2010, no swash plate on a Lynx; the head is controlled by a thing called a spider arm, which basically runs through the centre of the head to control runs beneath the gearbox - in the picture of us removing the head from ZD260 at Ascension, you can actually see the bottom of the spider arm dangling below the head - we’re trying to line it up so it fits into the yellow tripod stand thingy.

 

A little more resin work, this time (just because I felt like it) on XZ729; I have built the Mk.44 torpedo which will eventually be strapped to the starboard weapon carrier.  Seen here in ‘before & after’ pose, it’s an Aires “Aero Bonus” thing with 2 weapons per pack & 5 parts per weapon - very nice.  The prop blades are a particularly lovely bit of casting; should look great once painted up.

 

resized_72106af0-1a7b-4865-a9f2-8cea825e

 

More soon.

 

[Hmmm.  That Black Dog site appears to have engines for 1/48 Merlins AND Seakings...

 

Edit: AND some lovely wee 1/350 RN flight deck tractors.

 

Dammit, this could cost money!]

 

Crisp

Edited by Ex-FAAWAFU

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More fascinating aerodynamic explanations Crisp, much appreciated by this reader. Takes me back to some early days of learning for me aspiring to pass various levels of Cadet airmanship exams! Theory of flight always fascinated me the most.

 

Terry

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Watching with interest!

 

A bit more on the BERP rotors, the BERP "spades" are actually transonic, they have both supersonic and subsonic airflow on them. Part the reason they protrude forward is due to the change in lift distribution because of the supersonic flow regime, this creates a shockwave at about 1/4 chord on the upper surface. That notch in the front the of the blade is also to create a vortex which helps keep the airflow attached (otherwise the flow separates due to shock stalling).

 

The BERP IV programme has improved the BERP III and gives another few percent performance increase. There's an interesting paper summarising some of the effects from BERP tips...

https://dspace-erf.nlr.nl/xmlui/bitstream/handle/20.500.11881/1050/2b1.pdf?sequence=1

 

Ben

 

 

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Thanks, Ben.  

 

The HMA8 has a shorter horizontal stabiliser than the earlier marks, and I think that had something to do with vortices generated by the main rotor disk.  [But I’m on pretty thin ice here; this is definitely in the “I think I read this somewhere” category rather than my own experience; I only ever flew various flavours of Mk.3]

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A few years ago (ok, 12) we were up north around the outer Scottish islands taking part in an exercise. At the close of flying one day when all the aircraft were returning to mother, our SPLOT at time (himself an ex-pinky) wound up the speed on his Merlin. It was a super clear day and we could see the helicopter a few miles away low over the sea but clearly moving fast - very fast. It really was a fantastic sight to see the big 14 tonne thing at low level, very fast. As he cleared the ship he hauled it into a very tight right hand turn to approach from the left and land on. Lovely stuff!!

Memories - low level, very fast and the sound of those blades cracking as it turned tighly!!!

 

Jeff

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10 minutes ago, Ex-FAAWAFU said:

Thanks, Ben.  

 

The HMA8 has a shorter horizontal stabiliser than the earlier marks, and I think that had something to do with vortices generated by the main rotor disk.  [But I’m on pretty thin ice here; this is definitely in the “I think I read this somewhere” category rather than my own experience; I only ever flew various flavours of Mk.3]

Yep, the long one got bumped about by the vortices too much, hence the switch to the short one (technical terms, of course!). That did also throw up the issue of the short stabilizer not producing enough downforce, hence the addition of the gurney flap.

 

Extremely envious of your flying experience, I applied to be a WAFU when they were recruiting for whirly pilots, unfortunately, my medical history prevented any chance of that 😔

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