twisting rotorblades

[downwash]

i plan to build rotorblades by machineing a main spar and have a ribbed tail section.how to twist? my original plan was to twist the billets befor machineing and let them return to there natral state. but i beleive this would be difficult to yeild reliable results. can they be made strait and then bent? would that comprimise the strength? many production blades almost all are twisted. how is this done? any thoughts sugesstions or knowlage would be awsome thanks

 

[MikeHalloran]

I can't recall noticing twist in any rotor blade I've ever seen. Maybe I didn't look close enough.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pressed]

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[URL unfurl="true"]http://www.hartzellprop.com/pdfs/KitplaneBroch.pdf[/URL]

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Propellers are made normally of wood, aluminum or composite with optional angle pitch adjustment in the hub.

Helicopter blades are mostly made of composites due to thier length. Are shaped like a plane wing and are pitched by a swashplate at the mast.

[URL unfurl="true"]http://en.wikipedia.org/wiki/Helicopter_rotor[/URL]

I think the short answer you want is: "they are forged"

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[Boathouse]

The main rotor blades for the CH54 and S64 (CH53 are the same I believe) are manufactured using an extruded aluminum spar with fabricated sheet metal pockets bonded onto the rear face. The spar is a "squashed D" section and is most definately twisted down its length (as most blades are). The process is to purchase the extrusion, machine said extrusion to its blueprint thicknesses and prior to heat treatment, twist the spar at discreet lengths along the span to achieve the overall twist required. Obviously the material is yielded slightly so as to keep the required set and a humping great machine is required.

The process is extremely difficult and costly and is probably why composite blades are all the rage - much easier to lay up than twist

 

[Compositepro]

Actually, the main advantage of composite blades is much greater fatigue life. If I recall correctly the life of an aluminum blade is about 2000 hours and composite is like four or five times greater.

 

[Intermesher]

Here is some of the information on a current project to make rotorblades out of carbon composite. It is strictly R & D to see if active twist by independent root and tip control can be accomplished.

Rotor - Blade - NACA 00xx - IRAT - Torque Tube Method

http://www.unicopter.com/UniCopter_Blade_NACA00xx.html

Dave J.

 

[FredGarvin]

When CH-47s went from aluminum to composite, there was a great advantage to life and weight. There is a very noticeable twist them as well. Sorry I can't shed any light into the manufacture though. I never got to see that aspect.

This page is the best CH-47 resource on the net and has a good decription of the make up of the rotors. The decriptions and pictures are about half way down.

http://www.chinook-helicopter.com/standards/areas/blade.html

 

[Sarge14]

Before Composites were widely use blades were twisted by creating the spar and physically clamping the hub end of the blade and manually twisting along the span. Most likely you will want to do this in a linear fashion (max twist near hub and minimum twist near tip) in order to improve rotor Figure of Merit.

Nowadays with composites there are a bunch of different techniques used. I am not sure of the scale of your blades but I have fabricated smaller blades using Rohacell Foam and Pre-preg carbon fibre. Though the process is fairly lengthy it is still the easiest. First, and most expensively you design a mold that incorporates the blade twist. You then use the foam to shape the blade and cover this with the pre-preg C-F. You then bake the mold in the oven (not normal home oven obviously) and out pops a blade. There is a lot of very specifid other things you must do but that is the gist of it.

If you are planning on machining an aluminum (or whatever) spar I would recommend doing that without the twist and then twisting it after. You will have to be very careful to apply the same twist to each blade.

You have your work cut out for you.

 

[dave048]

Mike is correct there is no twist in helicopter rotor blades, main or tail. Perhaps you are thinking of propellers?
Also ours only have a hollow metal core made from a flattened tube with composite for the rest.
Dave

 

[Sarge14]

Dave,

How can you say that helicopter main rotor blades are not twisted? This is one of the easiest ways to improve the Figure of Merit of a rotor and some degree of linear twist is used on pretty much every main rotor blade in use today. Almost all of the have a linear twist of somewhere between 8 and 12 degrees from root to tip always, to my knowledge, in the form of a nose down twist (negative).

I know for a fact that the Blackhawk has a substantial amount of blade twist (somethign like 11 to 15 degrees) though those blades use a non-linear twist distribution.

For Tail rotors it is more likely that the blades are not twisted but even then a good number incorporate some degree of twist.

Where are you getting your facts?

 

[GregLocock]

dave doesn't work for nasa

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930082219_1993082219.pdf

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dave048]

OK, OK there is 14 degrees of twist. My "facts" came from the fact that I see the blades every day and they looked straight. To me 14 degrees in 26 feet looked straight. We have racks of those blades around here so I went out and looked and then I checked the B/P. You can see the twist if you look down the length and the B/P confirms it. Compared to propeller blades, though, this is a very small twist. Anyway, from now on I'll stick with what I know!

 

[VMSETHAN]

My company is now in the process of manufacturing tail blades for a customer to go on their fleet of CH-54B skycranes. We have pretty much solved all the problems in resurrecting the parts except for the twisting of the aluminum spar. Does anyone have info on a facility that has done similar projects in the past? Perhaps someone has knowledge of how Sikorsky originally did it?
We have CMM'd an original spar and it does not match what the drawing specified, but it is a timed out blade so perhaps I should not take that too harshly.

 

[dave048]

You might try Hexcel in Washington State. Thats all the info I have.
Is this for aerial firefighting use?

 

[dave048]

You might try Hexcel in Washington state.

 

[VMSETHAN]

I have contacted Hexcell, but am not real hopefull. We have talked to them before about composite issues and they are great, but I don't know if they have very much extrusion processing capability. Thanks for the tip.

These blades are going on a restricted category a/c. Fires are one opf their tasks but they do construction and lift jobs also.

 

[Boathouse]

VMSETHAN, I am assuming you are going the STC route for these blades and not trying to PMA the things? That would be extremely tough. Even though restricted category, your blades will be subject to the same Part 29 regulatory "guidance" as an OEM blade.

I think you will find that "We have pretty much solved all the problems in resurrecting the parts except for the twisting of the aluminum spar" means you have not solved a vast majority of the problems. How can you attempt this without the type data to dictate tolerances, spar wall thicknesses etc? You will surely have to certify the bonding process via significant coupon fatigue testing too.

Couldnt you more easily buy them from Sikorsky?

 

[VMSETHAN]

Yes, we are going STC/PMA. The STC approves the product, but the PMA approves the the manufacturing/QA process. They go hand in hand. This has been in progress for almost a year. We have completed several STC/PMA programs in the past for other parts and are fully aware of all the regulatory and certification issues that need addressing.
As to the second comment, we have all the necessary data to do the project. Our customer has 95% of the original Sikorsky data/draings, including the spar and titanium cuff. The remainder we have had to purchase or reverse engineer. There is significant testing that will still be required for the PMA, but we have built the metal bonded items of the blade(pocket and fairing) for some time. The relevant Sikorsky bonding processes we have accomplished in the past and include peel/shear/fatigue tesing.
Sikorsky has not supported the aircraft these are going on (CH-54B/S-64F)since they sold the type certificate to Erickson Air Crane in the 90's. A VERY similar blade is currently used for the CH-53D and MH-53J. I assume Sikorsky still supports these. Besides, our customer is looking at long term supportability of their aircraft. Their fleet is growing.

 

[Boathouse]

They definately will not be going on an F model. Erickson built the only F models and only they and their Italian customer operate F model cranes. Sikorsky never actually built an F model. So it will be the restricted category guys only.

Interesting how your customer has 95% of the drawings since Sikorsky sold the whole package to Erickson. They certainly shouldn't have the reports necessary either since they too were part of the deal.

So which route are you taking? Similarity??

 

[VMSETHAN]

We are one of Erickson's primary sheetmetal/composite parts suppliers and have a long history with them. You are corrct in that they have the package for the S-64(E and F). It is my understanding that they did not, however, get the tail rotor drawings. Rumor has it that Sikorsky held those drawings since they directly relate to the CH-53D fleet still in operation. Sounds reasonable, but who knows?
I should have differentiated the F and B models. Our blade project is for a different customer who operates seven B models and three A's. They are all restricted category. The drawings they have are all Sikorsky and pre-date the Erickson purchase of the type ceritficate. The blade drawing effectivities "generally" show that blades and parts for the CH-54B also apply to the S-64F. Since the CH-54 was military, a lot of info is available through FOIA.

We are doing the STC by similarity. A few minor changes to detail parts that are being blessed by our blade DER and testing.