Helicopter tail rotor failure solution?

Question

Many thanks to all the responses to my previous posts. I have a followup question or two for engineers or helicopter buffs.
Would a small jet engine, mounted horizontally and suitably armored, in the tail provide enough thrust, be throttleable and be available to power up to full throttle in a matter of seconds to offset LTR. I've seen small jets offered by various companies for models that produce around 100 pounds of thrust and weigh only around 30-35 pounds. You could incorporate existing fuel lines from the main engine to run them and develop a system of control that is either determined by existing pedal inputs or a backup system of hand control or computerized input in conjunction with a gyroscopic mechanism as found on the wheeled single-person transport of the Segway and others. It appears that most LTR situations offer a measure of warning for a pilot to initiate the backup system. All responses welcome and thanks again.

I'm also wondering if a system has been investigated or could be developed using a counter-rotating mass in the tailboom or under the engine to offset the main rotor torque---somewhat akin to a gyroscope. Still working on the physics.

Answer 1

hello paul. i had no time to answer your previous questions.
Mounting the jet engine /regardless of its size/ would require reinforced construction of the tail boom. it would also reduce the helicopter pitch maneuverability, due to adding such a massive weight so far from the center of the gravity. the jet engine being a safety/backup unit would also mean a lot of dead weight.

in the final effect the jet engine instead of the prop would be a far more vulnerable unit than the tail rotor itself.

if the design of the tail rotor was actually that vulnerable, than all helicopters of today would follow the Kamov setup of counter rotating rotors to avoid the tail rotor.

summary: the setup designed by you might be possible, but certainly not efficient enough.

The critical tail rotor loss occurence - transmission shaft overtorque and interruption gives no warning at all nor does a destruction of rotor blades collided with an obstacle or shot away.

simple loss of control / tail rotor operating and generating force but uncontrolled/ is easily manageable - most of the helicopters are designed to maintain a certain pitch of blades that ensures the ballance of the main rotor torque during the landing and forward flight. it means, that even without the controls you are provided with an ammount of momentum to finish the short airplane landing /rolling landing/.


edit.. speaking of the blackhawk down movie you mentioned before... the RPG round would usually not hit the tail rotor, but more likely the cargo compartment and/or fuel tanks. the tail boom is simply to narrow to effectively aim at. using RPG 7 as a area suppresing shrapnel round is not a proper way of using that weapon. I dont say it /the movie scene/ cannot happen but it is rare.

Answer 2

The nice thing about LTE and settling-with-power is they provide lots of warning, and the circumstances are predictable. It's nice to see someone trying to come up with solutions to the issues, but the easiest, simplest, and most cost efficient I've ever seen is - don't get into them!
And if you do, know how to get out. I've been in both, far too many times, and although there is warning, there's never enough to be able to respond with anything more than a simple maneuver.

As far as the fixes for gyroscopic processes, the Chinook and Vertol counteract torque by placing counter-rotating rotor discs fore and aft on the airframe. Produces some interesting engineering problems with the mixing of controls, but takes away the anti-torque rotor. Course, I've never seen a Chinook fly very far with only one rotor turning. Kaman has tried with a couple of machines (the Husky and Kamax) to mount the blades side-by side and intermeshing. Interesting to look at, and quite effective, but expensive and complicated. Kamov (Russia) has several models that provide counter-rotating discs above one another, with a mast within a mast (much like the shafts within the turbine engine), but I've never been around them. Apparently once again effective, but complicated.

And that's what it all comes down to. The more complicated, the more expensive, and the less likely it will appear in the marketplace.

Answer 3

There is already a similar system called a N0TAR (no tail rotor) which replaces the tail rotor completely. It uses an engine driven fan to create thrust to counter act the main rotor by a combination of the coanda effect (for steady anti-torque) and direct thrust (controllable for yaw).

Yes there are light jets available. But the complexity would be huge, thus, it would be more likely to fail. Also it wouldn't be useful at low altitudes (by the time it started, the chopper would be toast, same with a regular tail rotor loss). The reaction time would have to be in the millisecond range to be useful for a low altitude tail rotor loss.

At cruise or high altitudes, the helicopter can autorotate (taking away virtually all of the torque) or it can maintain a forward velocity and use the vertical stabilizer to counteract torque (what it's designed for) and land like an airplane (not always pretty but more survivable that spinning into the ground).

Answer 4

There is an entire family of helicopters from McDonnell Douglas (now Boeing) that use what is known as NOTAR. Literally, No TAil Rotor. It diverts engine exhaust through ducts in the tail to counter rotor torque.

While a small turbine engine in the rear would do essentially the same thing, remember that turbines guzzle fuel and that, while throttleable, respond very slowly to rapid throttle inputs.

You also need to realize that having an additional throttle means one more thing for the pilot to have to use, especially when he already has his hands full with the cyclic and collective (unless you route the throttle control for the turbine through the rudder pedals)

Also, the standard procedure (as I remember it) during a LTR scenario is to chop throttle and auto rotate the helo to the ground as softly as possible. In that situation, the pilot isn't going to concern himself with starting another engine that during 99% of flights is just dead weight.

Answer 5

Whereas a tail rotor failure is a problem, it is only catastrophic in a few situations.
One is where you are hovering over uneven ground (Slope of over about 10 degrees) or over trees, etc. In these cases your going to bend some metal.
If a T/R failure is encountered in-flight it can be handled without incident. If in a hover (assuming your over some decent terrain) again no problem.
To add the extra weight to counter this type failure would run the cost out of sight. Remember WEIGHT is not a friend to helicopters, especially the lighter machines.
Of course all the above assumes the pilot has a good knowledge of T/R loss procedures and is quick.

Answer 6

I imagine anything is possible. however, you will run into center of gravity problems with the extra weight hanging on that far behind the datum line. thus requiring more weight in the nose of the aircraft.

further, if you have a t/r failure, what good will an engine do? most failures are mechanical in nature and will do no good to keep turning something that does'nt work. If you are refering to Loss of T/R thrust, then again the small engine will not work, because the blades are spinning just not producing lift.(stalled). All the pilot needs to do is get out of the situation the he is in, that is causing the loss of thrust. Often called LTE (Loss of T/R effectivness)

Answer 7

It wouldn't come up in time, mechanically complex too.

Try a hybrid rocket motor. It will produce high thrust for its size and can be throttled, very simple too. Couple that with a nozzle that can further adjust the effective thrust and you might be able to stabilize the helicopter for a few minutes.

Answer 8

That's an idea that would add to much engineering and cost. A controllable emergency solid rocket booster like a sparrow might be good.

Answer 9

I think you don't realize how many compromises are already built into aircraft.

Everything weighs something. Weight is one of the largest hurdles that aircraft deal with. There may be cheaper, simpler, easier way to deal with a relatively rare occurrence.

Answer 10

with any type of turbine enigine the time required for startup would be too much to extract a pilot from this type of failure....beyond that , the guy above me has pretty much got it covered....