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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:

enter image description here
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:

enter image description here
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:

enter image description here
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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  • $\begingroup$ 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):

enter image description here

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 droop 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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    $\begingroup$ I believe you are correct! Correction made. Thanks! $\endgroup$
    – Marius
    Jul 27 at 2:55
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Kaman have a swashplate and blade pitch control links on their helicopters just like any other. The blade pitch control links are inside the hollow rotor shaft - the swashplates are inside the helicopter at the bottom of the hollow rotor shafts.

Also instead of directly controlling blade pitch by using the pitch control links to twist the root of the blades like other helicopters each blade pitch control link connects to a small torque shaft inside each blade which drives those small flaps located near the end of blades (servo flaps) the small flaps motion twists the flexible helicopter blades to change their blade pitch.

This servo flap technique requires less force to control pitch than other helicopters. Most other helicopters require powerful hydraulic servos to drive the swash plate position - on the Kaman the servo flaps amplify the control force so they don't need much force to control the swash plate.

I noticed some people think the Kaman dual intermeshing blade helicopter has no swashplate or control links to the rotor blades - The reason there is a misunderstanding that it has no swashplate or blade pitch control links is because the swashplate is not located near the top of the rotorshaft like traditional helicopters where it is easy to see and the pitch control links are hidden inside the hollow rotor shaft. Their understanding of how this helicopter works is deficient but unfortunately this incorrect assumption is passed about.

https://patents.google.com/patent/US3217809

The Kaman patent to which I link shows diagrams and describes the swashplate control and shows the hollow rotor shaft and so on - the patent shows only a single rotor helicopter but the dual intermeshing design uses the same hollow shaft etc. The control linkage mechanism which controls the swash plate or plates is called the azimuth mechanism.

The Kaman design is extremely robust and low maintenance and the latest Kaman K-MAX twin inter meshing design is popular for logging and fire fighting among other applications. The dual rotor design has outstanding fuel efficiency, outstanding payload capacity and makes less noise as well.

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  • $\begingroup$ Something you may want to know: Kaman called their version of the swashplate the azimuth on the Sea Sprite. (The accident where the azimuth came apart and led to the two bolts being added to it happened at our squadron). To say that the servo flaps "control the swashplate" sound off to me; you might want to rephrase that. As to not needing hydraulics to fly them (example, the Sea Sprite); the tail was a bit harder to control, but the aircraft was very flyable boost off. Have not flown the Kmax, can't comment on that one. (Wasn't my down vote) $\endgroup$ Jul 22 at 12:59
  • $\begingroup$ The swash plate controls the servo flaps which in turn controls the blade pitch by twisting the blade - i never said the servo flaps control the swash plate. ! $\endgroup$ Jul 23 at 3:27
  • $\begingroup$ note: It's Kaman, not kamen, named for the company's founder, Charles Kaman. $\endgroup$ Jul 23 at 11:01

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