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I had the opportunity to go to one of the Smithsonian Air and Space museums today and they had a Kaman K-225 on display. For those who don't know, the K-225 is a dual side-by-side helicopter design with an open cockpit:

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Source: Own Work

The blades look to be very basic wooden design without any kind of variable pitch on the rotor head. What I found very odd were the "planes" on the rotors:

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Source: Own Work

Each rotor seems to have one of these "planes" on it located about 2/3rds of the way from the root. Here is a close look:

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Source: Own Work

I wish I could have gotten a better picture but it looks like there is a rod going from the main blade to the smaller "wing" on the device. The rotors seem to be solid, so I'm wondering...

What are the purpose of these "planes" and how do they work (I'm assuming to change the AoA of the blade)?

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    $\begingroup$ What an amazing museum that place is! $\endgroup$ – Ralph J Mar 5 '18 at 3:46
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    $\begingroup$ @RalphJ In the center photo you can see a the tip-jet helicopter (next to the K-225), the Concorde in the distance, and the Virgin Galactic hanging from the ceiling. A little further back is Bob Hoovers Shrike Commander and several "only surviving samples" of Japanese and German WW-II aircraft. That's just one wing. They also have the Space Shuttle Discovery, an SR-71, F-22, F-14, F-4, F-86... the list goes on. Very interesting place. $\endgroup$ – Ron Beyer Mar 5 '18 at 5:07
  • $\begingroup$ Sorry for asking this, but is "dual side by side helicopter" the correct definition? 'cause it sounds like it is two helicopters connected together, not simply an heli with two rotors O_o $\endgroup$ – motoDrizzt Mar 5 '18 at 12:19
  • $\begingroup$ Yeah, a "synchropter" or a helicopter with intermeshing rotors would be a better descriptor (en.wikipedia.org/wiki/Intermeshing_rotors). $\endgroup$ – Marius Mar 5 '18 at 13:59
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These are a form of servo flap, commonly found on Kaman designs, but also on at least one early helicopter (namely, a coaxial rotor aircraft built by d'Ascanio (shown below):

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As opposed to controlling blade pitch from the blade root using pitch links (which are, essentially, pushrods), as in a typical rotor head, the servo flaps allow for pitch to be controlled by a control surface on the blade (similar to how controls work on a fixed-wing aircraft). The yellow "wing" is the flap, and the black item, I'd imagine, contains a hinge and actuator rod to pitch the flap up and down, generating a force at a small moment arm behind the blade, which then alter's the blade's pitch as desired. Instead of using an actuator to twist the blade, you're using aerodynamics to do a lot of the work for you, similar to how a fixed-wing aircraft works (except, instead of changing the rotor's effective angle of attack, you're twisting it instead).

Hence, if you look closely at the first picture you posted, you'll noticed that there is no swashplate, no pitch links, and really nothing reaching up the rotor mast from the helicopter except for a structural brace. This system does away with pitch links and their associated actuator systems and can decrease total system weight. In addition, as the flaps are significantly more streamlined than a typical pitch link, there is a possibility for drag savings as well. Compare the following two rotors heads -- the first is a semi-rigid 2-bladed rotor from a Bell 230, and the latter is a typical Kaman rotor head. The Kaman head is a picture of simplicity when you realize the only bits sticking out in the wind, beside the blade grips, are bump stops.

Bell 230 Bell 230

Kaman H-43 Kaman H-43

However, there's no free lunch. The price you pay for all this is that, instead of an actuator somewhere relatively "easy" (on the body of the helicopter), you have to put an actuator (albeit a much smaller one than would be required to drive traditional pitch links) out on the rotor blade and feed both control and power out to it. Control and power isn't too bad -- it's already done for anti-icing gear to some extent -- but the actuator can be tricky, particularly, I would think, in terms of tuning blade dynamics. Kaman (who built the K-225 and the highly successful KMAX, which also utilizes servo flaps) found a way to do this that works great, but it hasn't really made a splash in any other manufacturer's helicopters (to my knowledge). Rumor has it that Sikorsky and Northrop put together an entry for DARPA's Unmanned Combat Armed Rotorcraft program that had servo flaps on it, but that program got cut in 2004...so who knows? In the interim, swashplates have been the control method of choice for upwards of 80 years and work well for those who don't have experience with flaps.

Sources: https://www.helis.com/howflies/servo.php and rotor dynamics class.

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