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Old 19th Apr 2003, 22:07   #1 (permalink)
 
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Cool Rotor Blade cross section

Are the airfoils used for modern helicopter rotor blades symmetrical or un-symmetrical. what difference does this make?

Why does a manufacturer chose one as opposed to the other ?

Thanks

GR
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Old 19th Apr 2003, 22:30   #2 (permalink)
 
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Try this:

The control arm of a blade maintains the desired pitch angle.

Movement of the centre of pressure on a symmetrical blade has a small pitching moment and therefore small stresses on the control arm.

A large movement of the centre of pressure, as with an un-summetrical blade, will create excessive stress on the control arm.

There are other reasons but this is a starter for 10.
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Old 19th Apr 2003, 23:53   #3 (permalink)
 
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The selection of airfoil depends on many factors, and often ease of manufacture dominates, especially for small, light inexpensive helos or homebuilts. It an also be the fact that the airfoil was approved 50 years ago when it was state of the art , and re qualifying a new, better airfoil is expensive, time consuming and only a few percent improvement.

Old helos used the symetrical NACA 0012 airfoil, which was kind, easy to make and efficient. More modern ones use drooped nose airfoils like the Sikorsky SC 1095 or the Boeing BV 7 which are more efficient, especially at high lift near blade stall.

The blade pitching moment is an issue, where the symetrical airfoils don't take any force on the pitch links to select a new angle of attack, so unbosted controls are OK. New airfoils generally need to be muscled into position, so a boosted control system is often needed to get the increased efficiency (thus the gross weight gained with the new airfoil could be cancelled by the weght of the hydraulics!)
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Old 19th Apr 2003, 23:57   #4 (permalink)
 
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I'm not too sure if this is totally right, but I'm sure an airframe man will correct me if I'm wrong.
I know most modern large helicopters have symmetrical section main blades. One of the properties of this type of blade, is that they do not produce any lift when they are in flat pitch, so when the helicopter is running on the ground with the collective fully down, there is no lift from the blades.
If a blade with an aerofoil section was used, lift would always be produced whenever the blades are rotating, so the only way to prevent lift whilst on the ground would be to put the blades into negative pitch. ( which causes vibration and stress on the airframe).

Mr G
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Old 20th Apr 2003, 00:56   #5 (permalink)
 
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Thanks for the prompt answers

Mr Coyles book states most modern helicopters use non-symmetrical blades.

Mr Schafer states the preferred airfoil is symmetrical.

Just trying to get to the bottom of this as MR CAA wants to know !
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Old 20th Apr 2003, 01:22   #6 (permalink)
 
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So what you are looking for is the CAA answer which may or may nor bare any relationship to the correct answer. All those who have taken CAA exams will know what I mean.
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Old 20th Apr 2003, 02:28   #7 (permalink)
 
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Virtually all modern helicopters use assymetrical airfoils on the blades, with variable airfoil sections a so that the root is different that the tips.


Symmetrical airfoils do not have the efficiencies needed for modern high speed and high load factor flight.

Last edited by NickLappos; 20th Apr 2003 at 03:36.
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Old 20th Apr 2003, 09:36   #8 (permalink)

Iconoclast
 
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Quote:
Virtually all modern helicopters use assymetrical airfoils on the blades, with variable airfoil sections a so that the root is different that the tips.
I was unaware that modern helicopters used asymmetrical airfoils. I believe this was first tried on the Cheyenne helicopter and I also believe it was the cause of that helicopters demise. The rotor head was rigid and the blades were relatively stiff by comparison to most helicopter blades and the constant change of the airfoil shape led to instability of the blades because of the pitching moment caused by the constantly changing unsymmetrical air foil sections. Assuming we can all agree on gyroscopic/aerodynamic precession the blade phase angle which was supposedly 90-degrees could vary all over the place so that if the pilot pushed forward cyclic the helicopter would most likely roll to the right and not to the same degree depending on weight, air density and airspeed. In some cases it would result in divergence resulting in fuselage contact. It took several years and several major design changes to the flight control system to solve the problem. The solution to the problem was so complex that there were almost 100 single point failures that would result in loss of control. As a result of this the Army cancelled the program making way for the Apache.

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Old 20th Apr 2003, 13:39   #9 (permalink)
 
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Lu,

This page lists the blades that are used on approximately 150 helicopters. Helicopter Airfoils
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Old 21st Apr 2003, 02:41   #10 (permalink)
rwm
 
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The Cheyenne had a lot of new and untested ideas on it. The shape of its blades was the least of the problem.
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Old 21st Apr 2003, 03:56   #11 (permalink)

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Not necessarily so.

To:rwm

Quote:
The Cheyenne had a lot of new and untested ideas on it. The shape of its blades was the least of the problem.
If you are addressing the use of gyroscopic forces to control blade pitch (cyclic) the concept was well proven on three levels of prototypes. The concept adopted by Lockheed was to scale up a smaller design using a similar airfoil section that was not asymmetrical as described by Nick Lappos. The original design had the same cross section at each of the blade stations. The Army kept adding weight to the design and soon the helicopter could not perform to the specs. Lockheed requested that they be allowed to increase the blade length to accommodate the increased weight. The Army refused so Lockheed (Ray Prouty) was forced to come up with a blade design that would accommodate the increase in weight while still remaining within the design planform approved by the Army. That is when the problems started to manifest themselves.

The propeller at the rear of the helicopter,which was used for forward propulsion or to retard the speed in a dive was basically proven by attaching a small turbojet engine to one of the smaller prototypes. This engine could not retard forward speed but it was capable of providing forward thrust. It was one of the first compound helicopters.

If I haven’t addressed the shortcomings you alluded to please post again and we can continue this discussion.



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Old 21st Apr 2003, 20:02   #12 (permalink)
 
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Thanks for the link Dave - very interesting.

From this link it would appear that that greater portion use the symmetrical NACA 0012 airfoil.

Found this site to complement Daves link -

http://www.nasg.com/afdb/do-search-airfoil-e.phtml

Does'nt really seem a right or wrong answer to this question.
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Old 21st Apr 2003, 20:34   #13 (permalink)
rwm
 
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To: Lu Z

Could you think of the amount of man hours the cheyenne would need just to keep it in the air? Complex gearboxes that shift the amount of power available to each of its rotating blades. The gyro head. The spring loaded control levers. All this covered in armour and sheet metal. The large avionics and weapons systems. All these componets being made by the cheapest bidder. And most of this technology being designed at the same time as the helicopter.
Like most hi tech military objects, bean counters get involved and prune designs before they become mature. I'm sure you have feelings about the Arrow and Iroquois.

I must admit that it would have been one mean machine.
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Old 21st Apr 2003, 22:52   #14 (permalink)
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A little something that interested me years ago along the lines of this topic. I think Nick can elaborate.
For many years the Cobra used the same old rotor system a six inch cross section of the blade near the hub (24 inch chord?) weighed in at about 15 Lbs, maybe more. Then along came the Kaman composite blades with a (28 inch chord?) That same six inch cross section weighed in at about 3or 4 lbs. The difference of the total weight of the new blades had an amazing result in the aerodynamics. So much so that Emergency procedures differed for the aircraft depending on which blades were aboard. Those lighter blades made a real difference during a needle split. You had better be on it quick or you were into low rotor RPM right now.
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Old 21st Apr 2003, 23:01   #15 (permalink)
 
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B Sousa,
The rotor blade weight is an important contributer to the stored energy that is used to keep rpm up when the power quits. If the blades are light, the inertia is less, and the stored energy is less.

This means that you have to look more carefully, and lower the collective more rapidly in a low inertia system.
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Old 22nd Apr 2003, 02:03   #16 (permalink)

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Thumbs up Yeah but....

To: rwm

Quote:
Could you think of the amount of man hours the cheyenne would need just to keep it in the air? Complex gearboxes that shift the amount of power available to each of its rotating blades. The gyro head. The spring loaded control levers. All this covered in armour and sheet metal. The large avionics and weapons systems. All these componets being made by the cheapest bidder. And most of this technology being designed at the same time as the helicopter.
Regarding complex gearboxes the only thing complex was that it had one input and two outputs. There was no shifting involved. The propeller was controllable relative to pitch and when flying in the helicopter mode and then the propeller was in flat pitch. When in the compound mode the pitch was increased and the propeller provided thrust. When in a dive the pilot could retard forward motion by going into reverse pitch. The propeller was no more complex than that used on an aircraft. When in the compound mode the pilot would use only that amount of tail rotor pitch to counter the torque.

The gyro head was a picture of simplicity. Four weighted arms mounted on a constant velocity joint. The spring “loaded” control levers were not spring loaded and they were not levers. They were shaped similar to a horseshoe with the servo attached to one end and the pitch rod to the other. When the servo was actuated it would compress the “Horseshoe” spring which would transfer the compressive load to the pitch rod. This would apply a perturbing force to the gyro causing it to precess. In precessing, the control gyro would apply a load to the pitch horns causing the blade pitch to change cyclically.

Regarding the high maintenance man loading this is true. The Cheyenne was compared to a Century Series Fighter regarding complexity and was far too complex for Army personnel to maintain it. The same was true for the successor to the Cheyenne, which was the Apache. Because of its’ complexity the Apache is still maintained by contract personnel from Boeing.

Regarding the cheapest bidder I can’t speak to that because I came onto the Cheyenne program after the contract was let and I don’t know if the contract was sole source or if it was competitive. On the Apache the competition was between Boeing, Sikorsky, Bell and Hughes. Hughes eventually won the contract but they were totally unable to design and build the whole helicopter and almost all of it was subcontracted to other firms.


Last edited by Lu Zuckerman; 22nd Apr 2003 at 06:12.
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Old 23rd Apr 2003, 01:18   #17 (permalink)
 
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On the subject of airfoil section development , can anyone shed any light on the story about the VR-8 ( well one of the Chinook Airfoil sections) being discovered by accident during icing trials?

There is an apocryphal story that it was noticed that after a certain time in the hover under the icing rig the Tq required to hover went DOWN noticeably.

This was found to be repeatable so the tests were conducted enough to map the new blade profile that the ice had formed before it melted off.

Anyone out there who "was that soldier"? its a super story and I like to think it has genuine roots

Wunper
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Old 23rd Apr 2003, 04:44   #18 (permalink)
 
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To return for a moment to Golden Rivet's original question - looking for an answer for the Belgrano's stokers....

There's a complete mixture of blade types flying today - why?
Because that's what the clever designer chappie decided, as modified by the beancounter's restrictions. As Rob L suggested, yer man wants to know the ins and outs to see how much you know....and to give you a chance to dig a bullsh!t hole big enough to disappear from sight.

Given the appallingly low level of helo knowledge currently on display from our Authority, it's a better than even chance that you already know much more about your aircraft than he does anyway.

Which isn't unreasonable given that the surveyor probably looks at 25 different aircraft types a week and can't possibly know much about any except those he may have worked on when he was employed by BA.

Considered opinion, when asked this question, suggests that after a few words of wisdom (readily gleaned from real experts like Nick above, read the posts) you could tactfully remind your man that it's a MAINTENANCE licence you're being examined for, not a design licence. Admittedly, it risks a snotty reaction, but a review of the syllabus doesn't seem to require a detailed knowledge of the entire design process, but a general understanding of same combined with a detailed knowledge of the looking after of the beast.

Best of luck with the resit (!)
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Old 23rd Apr 2003, 16:16   #19 (permalink)
 
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Coriolis hits the nail!

Yes it's a MAINTENANCE licence. Who the hell cares what type or shape the blade is? Anyway just what is a "modern" helicopter?

I think the original poster should be learning how to pass examinations and not to be confused by facts!

Tell them what they WANT to hear!

After holding licences and various approvals in about 6 different countries they are all the same.

Years ago we used to make a point of not letting pilots of Allison 250 powered helicopters know anything about the engine prior to sitting the Gas Turbine exam. The examination bore no relevance to reality! If they did have some prior knowledge they would inevitably fail the paper!
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Old 23rd Apr 2003, 19:54   #20 (permalink)
 
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Cool

Thanks John - Did'nt think it was an unreasonable question -

Are maintenance personnel not welcome here ?

If you have'nt got anything constructive to add to the the thread, why bother posting at all ?

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