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I'm wondering how the height of the mast is determined. The new R66 has a taller mast than the R44. The 500 and Eurocopters have practically no mast. What are the design factors? Anyone have an easy to understand explanation?

Edited by Little Red 22
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Technically the mast is the shaft that comes out of the transmission. You may find that even helicopters with "almost no mast" have quite a sizeable one, but most of it will be hidden by the airframe. The R22/44/66 seem to have taller masts than most helicopters, but only because the transmissions ride pretty high on the frames. Helicopters like the MD500 have a mast about the same height, but the transmission is in the lower middle of the airframe.

 

I do know what you mean though. You are talking about mast height above the airframe. RagMan nailed that one.

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Technically the mast is the shaft that comes out of the transmission. You may find that even helicopters with "almost no mast" have quite a sizeable one, but most of it will be hidden by the airframe. The R22/44/66 seem to have taller masts than most helicopters, but only because the transmissions ride pretty high on the frames. Helicopters like the MD500 have a mast about the same height, but the transmission is in the lower middle of the airframe.

 

The 500's main rotor transmission is in the ceiling of the rear cabin. I'm guessing the drive shaft of the 22 has gotta be about twice as long as the 500's.

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In many cases, the mast height is to do with the CG spread.

 

In simple terms, the CG allowable is governed by the max forward tilt of the disc and the max aft tilt - draw a line at 90 degrees to the tip path plane in both these positions, and you will get an inverted "V" from the rotor head.

 

The CG must fall inside this V - and not near the limits, as other factors must be considered. The R22 had certain limits to CG, and it was a simple one to look at because there can only be 2 options - either one person in the right seat, giving the most rearward CG, or two people, one in each seat, to give the most forward.

 

With the R44, there are more options. One pilot; both front seats filled; one in front, one in back; one front and 2 back; two front and one back; all filled.

 

To get a bigger spread of CG without a major control design change, Frank simply raised the mast height and thus the inverted V had a larger spread available.

 

And this came direct from Frank, at Heli=Expo in Miami in 1993.

 

With rotor heads like the AS350, or articulated heads, or rigid heads, there is more control power available, so the longer mast requirement isn't so important.

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The 500's main rotor transmission is in the ceiling of the rear cabin. I'm guessing the drive shaft of the 22 has gotta be about twice as long as the 500's.

 

Oops! Your right! I'm thinking of the engine drive shaft TO the transmission. Must have been tired last night!

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For teetering head helicopters, the mast must also be high enough for the blade not to hit the tail boom when at full down deflection. When the blades get longer, like on the R44, the mast has to go higher.

 

Non-teeterers have less problem in controlling the deflection of the blades, so they can be closer to the fuselage.

 

Look at the experimental Sikorsky, with 2 co-axial rigid systems, just a few feet apart. No problems there with excessive flapping.

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In many cases, the mast height is to do with the CG spread.

 

In simple terms, the CG allowable is governed by the max forward tilt of the disc and the max aft tilt - draw a line at 90 degrees to the tip path plane in both these positions, and you will get an inverted "V" from the rotor head.

 

The CG must fall inside this V - and not near the limits, as other factors must be considered. The R22 had certain limits to CG, and it was a simple one to look at because there can only be 2 options - either one person in the right seat, giving the most rearward CG, or two people, one in each seat, to give the most forward.

 

With the R44, there are more options. One pilot; both front seats filled; one in front, one in back; one front and 2 back; two front and one back; all filled.

 

To get a bigger spread of CG without a major control design change, Frank simply raised the mast height and thus the inverted V had a larger spread available.

 

And this came direct from Frank, at Heli=Expo in Miami in 1993.

 

With rotor heads like the AS350, or articulated heads, or rigid heads, there is more control power available, so the longer mast requirement isn't so important.

 

That’s an excellent explanation, very knowledgeable. It deserves an inverted “V” figure.

 

Rotors without a flapping hinge offset (teetering system) obtain all of the effect of flapping from the tilt of the thrust vector. Therefore, the control moments become a by-product of thrust (no thrust – no control moment). So the teetering system obtains advantage when placed higher above the CG, maximizing CG range and increasing overall control moments.

 

Rotors with hinge offset are normally closer to the fuselage and CG since they have the advantage of being able to develop control moments on the fuselage without developing thrust. Hinge offset allows centrifugal force to produce control moments. Any development of thrust further increases the magnitude of the overall control moment.

Scan-1-7.jpg

 

 

Scan-11-2.jpg

Edited by iChris
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  • 1 year later...

Please check, Frank Robinson's comment about tall mast:

http://www.youtube.com/watch?v=a0Iu0kW7gdU

(At 25'25'')

Frank Robinson says the tall mast allows him to use larger diameter rotors, which is more efficient. He didn't mention it is because of CG spread.

 

Comparing Bell 206 and R44, they are very similar in size, but R44 mast is about 1 foot taller (tail is about 1 foot longer).

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For teetering head helicopters, the mast must also be high enough for the blade not to hit the tail boom when at full down deflection. When the blades get longer, like on the R44, the mast has to go higher.

 

Non-teeterers have less problem in controlling the deflection of the blades, so they can be closer to the fuselage.

 

Frank Robinson says the tall mast allows him to use larger diameter rotors, which is more efficient. He didn't mention it is because of CG spread.

 

Comparing Bell 206 and R44, they are very similar in size, but R44 mast is about 1 foot taller (tail is about 1 foot longer).

 

 

As you can see from the replies above there are a number of advantages to mast height. Blade clearance was also listed. The increase in rotor diameter is not surprising for small helicopters with low power-to-weight ratios. In this case, you want low disc loading. The way to do that is to increase rotor diameter for a given gross weight; consequently, you have to provide increased clearance between the blades and airframe.

 

A few advantages of low disc loads are:

 

Low power required in hover

Low induced velocities

Low autorotative rates of descent

 

The R22 disc load is only around 2.7 (gross wt/rotor disc area)

Edited by iChris
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Frank Robinson says the tall mast allows him to use larger diameter rotors, which is more efficient. He didn't mention it is because of CG spread.

Well, I was asking Frank at Heli-Expo, and that was his specific answer - to make the changing CG easier to manage. Straight from the horse's mouth. No other reason stated.

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  • 2 months later...

A bunch of good responses here. The type of rotor system, fully articulated, teetering, or bearingless/rigid is a big factor on the deflection angle of the blades and thus the clearance required. The type of rotor system has a huge role in the allowable CG range. Further, a longer mast is more drag. And, in some very small aircraft, a short mast would mean very little rotor to human head clearance.

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And, in some very small aircraft, a short mast would mean very little rotor to human head clearance.

 

Ever seen the Brantly close up? Mama! I sat in one, making helicopter noises, and it would seem pretty in-teresting to have that disc buzzing... inches above your cranium.

 

So why didn't they make that mast longer? Seems spooky short. I know it's an old design, but somebody bought the type certificate a while back.

 

Never mind "suicide doors", the Brantly's got 'em all beat with "suicide blades"? Or am I being unkind? Maybe some Brantley buff is gonna squash me...They too have a fan club & following..

 

Interesting link here to Brantly info

 

:P

Edited by Francis Meyrick
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FWIW, the prototype of the H60 originally had a shorter mast than when it went into production; one of the requirement of the bid was for minimal maintenance to load into a C130 or C141. However, Prouty's book mentions undesirable aerodynamic vibrations with the short mast so a 15 inch extension was installed which solved the vibration problems.

 

Prototype vice Production

post-46443-0-60671400-1398999885_thumb.jpg

post-46443-0-93628800-1398999992_thumb.jpg

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