The Supermarine Sea Otter's Scissors

[JDK]

Another photo from the personal archives. This is the prototype Supermarine Sea Otter taxiing out from Supermarine's slipway.



The Sea Otter was the replacement design for the Supermarine Walrus (the Walrus looked very similar but with a pusher prop set up) and while almost every detail of the Sea Otter was different - arguably 'improved' - the overall concept was the same, and thus the aeroplane offered very little actual improvement. With the advent of W.W.II and Supermarine having other things to do, the Sea Otter was only built in small numbers, and never actually managed to replace the Walrus in service.

But the interesting thing here is the Sea Otter's unusual - I think unique - prop arrangement. After the two-blade prop proved inadequate, it was fitted with a four blade airscrew, because the clearance aboard HM Ships wasn't high enough (in hangar deck or cruiser and battleship aircraft compartments) to allow for the normal 90 degree 'cross' setup, it had the two two blade props set at 35 degrees, often known as a 'scissor', due to the resemblance to an open pair of scissors. Production Sea Otters had three blade props and also rarely had to operate aboard ship, so it became a forgotten modification...

One of those little oddities. Does anyone know of this being used on other aircraft? Like the one-blade counterweight propeller, these remind you that what's familiar isn't always the only way of doing things!

Last edited by JDK on Sun Jun 20, 2010 9:09 pm, edited 1 time in total.

[The Inspector]

I wonder about the harmonics with that arrangement-

[RMAllnutt]

Wow... never seen a prop like that before... very cool! Thanks for posting.

Cheers,
Richard

[Zachary Weibel]

is it 2 props
although theyd be spinning opposite one another

[JDK]

is it 2 props
although theyd be spinning opposite one another

Hi Zachary,
Good question! No it's two two-blade props bolted together to make a four-bladed one, that rotated as a single unit. There were contraprops before this, most famously on the Macchi Mc 72, but this is doing a different job.

http://en.wikipedia.org/wiki/Macchi_M.C.72

I wonder about the harmonics with that arrangement-

As far as I've been able to establish, there were no problems with the prop type - except (perhaps) the perennial reluctance to fly anything that just looks scarily different.

There's very few clear pictures of the arrangement, another reason it's often overlooked.

Regards,

[K5DH]

Automobile cooling fans are often four-bladed "scissor" arrangements like this. I've seen other strange blade setups on car fans as well: five blades at odd spacing on some small-block Fords and big-block Chevies, for example. (our '71 Corvette originally had such a fan, which I've changed to a seven-bladed unit, with even spacing, to improve the big-block's weak cooling abilities).

[Aeronut]

Obviously far too modern for this forum but....... AH64 Apache tail rotor.

[JDK]

Automobile cooling fans are often four-bladed "scissor" arrangements like this. I've seen other strange blade setups on car fans as well: five blades at odd spacing on some small-block Fords and big-block Chevies, for example. (our '71 Corvette originally had such a fan, which I've changed to a seven-bladed unit, with even spacing, to improve the big-block's weak cooling abilities).

Obviously far too modern for this forum but....... AH64 Apache tail rotor.

Thanks for the input chaps, two areas I'd never even thought of looking!

Any explanations as to why in either case?

TIA,

[AndyG]

I recall reading an article regarding several Hughs helicopters which had been hushed for special forces insertion purposes in Vietnam. The 4 blade tail rotors were similary distributed and the reason stated was that they were much quieter than 90° spacing.

[JDK]

Interesting. Any more?

[Steveo05196]

i worked on apache helicopters for 4 years in the army. i was told that having the tail rotors configured that way was quieter. but i dont know if there is any truth behind that but it sounds good. the seemed to work well configured that way we never had any issues with them other than the iraq sand chewing them up but you cant avoid that.

[JDK]

Thanks Steve and Andy. It's interesting once you start looking, isn't it?

I've just done a bit of looking online, and there seems to be two answers, or a two part answer...

The tail rotor has a "scissors" configuration, with the blades arranged at alternating 55 and 125 degree intervals, also to reduce rotor noise.

http://www.vectorsite.net/avah64.html

So far so good, and this 'noise' statement is repeated on most sources. But:

When Hughes Tool Co., Aircraft Division experimented with the OH-6A to make it
very quiet, the engineers knew that a big source of noise was the tail rotor.
To make the tail rotor quieter, they decided to increase the blade area and
decrease the rpm. The most convenient (i.e., least costly) method was to
simply add another set of teetering blades to the tail rotor.

In addition to the addition of more blades, Hughes engineers experimented with
blade phasing to decrease the noise level as much as possible. As I remember,
the phasing they decided upon was about 45 deg.

When the AH-64 Apache came along, the Hughes engineers incorporated what they
had learned from the OH-6A development work. The same basic design for the
tail rotor of the quiet OH-6A was incorporated into the Apache design; two
pairs of teetering rotor blades. The same concept was used on the 4-bladed
tail rotor option on the MD 500D, E, and FF series of helicopters.

The one additional requirement for the Apache was to fit into a C-130 transport
without disassembling the rotors. In order to meet this requirement, the tail
rotor blade spacing was changed to the 30 deg spacing you found. The noise
signature suffered a litttle bit, but the transportaion requirement was met.

http://www.airtalk.org/ah64-tail-rotor-vt35959.html

OK, so it was 'noise' advantage first, then a shipping advantage (which, interestingly, would match the Sea Otter's requirement. The more things change, the more they stay the same?)!

But then, in the same discussion:

The Apache tail rotor is, as you note, actually a pair of stacked tow-bladed
teetering rotors. There have been claims that the unusual blade spacing
you're asking about was intended to reduce noise. While the spacing may
give some noise benefit, the real reason for the orientation is simple: it
was how the blades would fit. If you look at an Apache tail closely you
will see that if you tried to orient the outboard rotor at 90 degs from the
inboard rotor, the pitch links for the outboard rotor would interfere with
the inboard blades. (The interference geometry is largely a result of the
skewed teeter hinge, which is needed to get the desired pitch-flap coupling.
This pitch-flap coupling reduces the flapping of the tail rotor in rotor
flight.)

So in this case, it's just so two sets of blades will simply fit together.

Anyone able to add to or correct any of the above, please?

Regarding the automotive car cooling fans, does anyone have any comment on those? Again, I'd be interested.

Cheers,

[John Dupre]

I think the skewed or scissors propeller blade arrangement certainly dates back to WW1. I know I have seen pics of DeHavilland types with it.

[JDK]

I think the skewed or scissors propeller blade arrangement certainly dates back to WW1. I know I have seen pics of DeHavilland types with it.

Interesting, can you be more precise? Like where, or what, or when? I don't recall any, but that doesn't prove anything! Even a type would help.

Meanwhile, looking around, I found another example, the Tupolev A-3 Aerosled:


Details here (but frustratingly, not about the prop!)

http://www.gizmag.com/go/6433/

[John Dupre]

Well I thought I had seen photos of skewed four blade props in WW1 but a couple of hours research in my own books and on the web don't turn up any. I think that I might have been confused by the F.E. 8 propeller which as a weird profile that from an angle could look skewed.......