26 July 2008

Look Ma, No Tail Rotor II !

(If you missed the first exciting installment of "Look Ma, no tail rotor", you can catch up with the rest of us here.)
I can never remember whether it's Newton's first, second, or twenty-third law...
"For every action there is an equal and opposite reaction." For conventional twin-engine airplane drivers, it's what makes takeoffs interesting because if you lose your "critical engine", Mr. Newton's law might make the last seconds of your life exciting.
Newton also keeps most helicopter pilots on their toes by constantly keeping them aware of that little turny-thingy on the back of the machine, the "anti-torque rotor".
The turning main rotor tries to twist the fuselage of the helicopter in the opposite direction. The tail rotor is used to stop or control the amount of "equal and opposing" force.

Tail rotors are dangerous.
Back when I was still flying Bell LongRangers, one of my main worries on scene flights was having someone walk into the tail rotor. The bottom of the tail rotor arc reached down to a level less than 4 feet above the ground, so even a child was at risk. I always tried to park the aircraft in such a way that the tail rotor was opposite the direction ground EMS personnel would approach the aircraft... Chaotic scenes with everyone running around willy-nilly were nerve-wracking.

You have to be more than 6 feet tall to be concerned about encountering the tail rotor on the BK117 I now fly, (assuming the aircraft is sitting on level ground). So I can now be a little more relaxed at accident scenes.
The other consideration about tail rotors is their vulnerability. They are quite fragile, and they are at the rear extreme of the helicopter, helping them to be a better lever and do the job they have to do. That position also makes it more likely the pilot will screw up and strike them against something in a hovering turn or extreme nose-high attitude. As we have seen before, a conventional helicopter suddenly losing its tail rotor is an interesting study in physics.
So for a number of reasons, efficiently doing away with the tail rotor is a good thing.

Sitting in my office the other day I heard what sounded like a truck going down the road with the tread of a tire coming off. When the sound didn't fade away into the distance I ran to see what was causing the noise and saw one of the above pictured aircraft land and taxi to the fuel pumps. It's a Kaman Aircraft company "K-Max". The two main rotors cancel out torque. Eliminating the tail rotor also increases the amount of power supplied to the main rotors... and since this machine is designed for lifting, having that extra power to lift is a great thing. And it's an efficient lifter... it can pick up its own weight. There is a penalty in the design... the pilot was flying his machine cross-country to do another job, and when I asked his cruise speed he replied, "Ninety knots".

Other machines use two main rotors to counteract torque...
here's an interesting shot of one you know. Here's another that has just started its U.S. military career.

Finally, there is one more example of a two main rotors to cancel torque shown here. This example is a Russian Kamov, with its coaxial rotor system.

Now, a mental exercise for ya...
Think of how hovering turns are accomplished in all these machines!



In the case of the UH-46-47 and Osprey (sp), I think the turns are made with the 2nd rotor/prop by powering and positioning. But the bird pictured above, I have no idea. Does it even have foot (rudder) controls??

This is an interesting question. I guess I could google it but then I wouldn't have to think, so I'll wait for your explanation.

Greybeard said...

You may wait a long time for my explanation, Dave, because I have no time in any of these aircraft and I'm as clueless as you.
I know in the case of the Ch-47 there are conventional helicopter controls, so anti-torque inputs are somehow introduced differentially into the main rotor system in such a way that the torque of the other rotor turns the machine. Next time I talk with someone who has flown a two main rotor system I'll pick his/her brain.

Hovering turns can be made around the aircraft nose, around the yaw axis, around the tail, or a permutation of any of those. How to do a "pedal" turn around the tail of a Chinook will be an interesting study!


Ty, GB. CH 47, not UH...ty, again.

Andrea Shea King said...

A fascinating history lesson. Amazing how many ways man can devise to keep himself aloft!

Ever think of teaching this stuff, GB? It seems to be an artform with you. I wouldn't miss any of your classes!


Greybeard said...

Ya know Andrea, I might actually be able to make a little money doin' that, right?! ;>)
Thanks for the grin.

cary said...

That is a good question, Greybeard. I knew that the twin rotor craft, like the Chinook and the Osprey, could vary the torque to make the turns. The Kamov and the K-Max, though? Well, along with TWD and Andrea, I will be anxiously awaiting your update...

Dan O. said...

When I was a volunteer firefighter I would often be tasked as LZ officer. As such I always instructed the firefighters helping me that it was our duty to maintain a secure LZ as it was an extended part of, although often remote to, our emergency scene. Meaning NO ONE, including EMS/Fire personnel and especially civilians, entered the grid marked with flares, until signaled by the pilot and then only a straight path to the side door.

This was as we had been instructed by the Metro Lifeflight (Cleveland) familiarity training we had every few years.

I can't imagine why anyone would be anywhere near the tail rotor, but then I have met people who I could believe would do it.

Anonymous said...

I'm interesting in learning how yaw is managed with the K-max. One of them was at my home airport recently and I got to see it in action and chatted with the pilot as well, but I didn't think to ask.

To my eye, its rotors are geared (?) together so I don't see how it could be yawed around. It was a really wonderful machine to watch takeoff...seen from below its like a stange insect.

nec Timide said...

Well since in order to not rotate the torque has be be precisely balanced between the two rotors, a system that could accomplish that would be able to slightly unbalance the torque to cause a yaw in one direction or the other. The arrangement of the rotor shafts is immaterial.

As far as intermeshing rotors, and probably for the Kamov as well, the most effective way to vary the torque on a rotor shaft is to alter the pitch of the blade(s).

See this this link. Which would allow for yaw control in the event of complete engine failure. I think the hardest part would be designing in the right control harmony so pilots of conventional helicopters could transition without too much difficulty.

Anonymous said...

The controls on a Chinook or a K-Max are exactly the same as on any other helicopter. The difference is made up in the control mechanism. If it wasn't nobody could every fly more than one.
The tandem rotor K-Max, is by Kaman Aerospace, the same people who made the HH-43 tandem rotor Husky for the Air Force. Flying a K-Max is downright spooky, it's like riding a bicycle. It's designed to have a good line of site down. When you are doing long line lifting, like logging, your vertical reference changes from the horizon to your line. The cockpit of the K-Man gives you an excellent look at your lifting line.