Jet-A gasguy From United States of America, joined Dec 2000, 266 posts, RR: 2 Reply 1, posted (11 years 10 months 1 week 2 days 17 hours ago) and read 12071 times:
When a helicopter loses an engine, it doesn't fall to the ground like a rock. It enters into what is called an "Autorotation". That is where the pilot reduces the pitch of the rotor blades thus creating a windmilling effect as air blows up thru the rotor blades and the aircraft loses altitude. Forward speed must be maintained in order to maintain rotor speed and momentum. In the Jetranger, you want to keep forward airspeed at about 69 KIAS(knots indicated airspeed) The aircraft, although falling quickly, is completely controllable by the pilot. In other words, he can turn left or right to search for a good touchdown spot. Then when the aircraft is over the touchdown area, the pilot pulls back on the cyclic to decrease forward speed while simultaneously increasing collective to slow the descent. This done properly, will allow a gentle touchdown with no damage whatsoever to the aircraft or it occupants. Of course pilots don't always get it right and thus we sometimes have damage or injuries. I've been in an autoration before and it was really uneventful(thanks to the efforts of the highly skilled pilot!) My bigger concern would be a loss of the tail rotor! I've seen video footage of helicopters that have lost their tail rotors, SCARY!
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Twotterwrench From , joined Dec 1969, posts, RR: Reply 4, posted (11 years 10 months 4 days 6 hours ago) and read 11939 times:
In some heli's, loss of tail rotor can be survived by pitching forward on the cyclic and reducing rotor head torque (bottom the collective). The loss of torque coupled with the forward momentum tends to even out the loss of rotational control. you are going to hit hard, but it can be survivable.
Soku39 From United States of America, joined Nov 2000, 1797 posts, RR: 10 Reply 7, posted (11 years 10 months 16 hours ago) and read 11870 times:
I think each engine has its respective rotor so if you loose the tail rotor's engine I don't think you can redirect power from one of the other engines to the tail rotor one. Thats my guess but I am sure all copters have different emergency procedures thou.
Soku39 From United States of America, joined Nov 2000, 1797 posts, RR: 10 Reply 8, posted (11 years 10 months 16 hours ago) and read 11866 times:
I think each engine has its respective rotor so if you loose the tail rotor's engine I don't think you can redirect power from one of the other engines to the tail rotor one. Thats my guess but I am sure all copters have different emergency procedures.
Twotterwrench From , joined Dec 1969, posts, RR: Reply 11, posted (11 years 10 months 13 hours ago) and read 11865 times:
Most helicopters (I am talking about your typical JetRanger, A-Star, etc corporate type, not military) with twin engines have what is called a twin pac configuration. You have two engines coupled through a common transmission. The transmission divides the powder between the main rotor and the tail rotor. The tail rotor and Main rotor are alway directly coupled together so that the speed of the tail rotor is maintained in the proper proportion to the speed of the main. Loss of tail rotor control is usually caused by failure of the tail rotor gear box or failure of the long shaft going back to the tail rotor gear box. It has nothing to do with engine failure. If you have engine failure, you loose power to both rotors, even with two engines. the transmission is designed so that catastrophic failure of one engine will not, in most cases, prevent the other engine from providing power to the rotors. Is that a little clearer?
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 13, posted (6 years 5 months 2 weeks 21 hours ago) and read 11773 times:
First of all, most larger helicopters have more than one engine.
Quoting Soku39 (Reply 8): I think each engine has its respective rotor so if you loose the tail rotor's engine I don't think you can redirect power from one of the other engines to the tail rotor one. Thats my guess but I am sure all copters have different emergency procedures.
No. The tail rotor is powered by the same engine(s) as the main.
As has been mentioned, autorotation is a completely viable procedure. Rotorheads practice this just like fixed wing pilots practice engine out situations.
In other words, complete power loss in a heli is not necessarily the end of the road.
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As you can see, the engine is standing on its accessory section with the "prop" shaft up. This drives a transmission directly above it hidden behind the fuel tank. The upper part of the transmission has two (I believe) sets of planetary reduction gears to reduce from engine RPM (which I don't recall) to rotor RPM of about 360 or so. (If you fly the -47 please correct me)
There is also a higher-speed driveshaft out the back of the transmission for the anti-torque (tail) rotor. You can see the shaft passing through hanger bearings along the top of the tailboom. It angles up the little pylon and makes a 90° to the tail rotor.
So long as the engine is turning, it will drive the transmission and the transmission will drive the main rotor through the "mast" and the tailrotor through this long driveshaft. Later single engine designs are pretty similar but have the mechanism faired in, hidden away.
If the engine quits, the engine will automatically uncouple from the transmission (sprag clutch or centrifugal clutch) the autorotation maneuver described in reply #1 will backdrive the transmission and through it, the tail rotor driveshaft. This helicopter in autorotation, without power can do anything but hold altitude or climb. It is fully maneuverable in the descent but descend it will.
Multi-engine, single-rotor designs like the Bell 212 and even CH-53 and CH-54 have this basic layout except their engines feed the transmission through separate drive quills. In some cases twin engine was a retrofit and may use a combining gearbox to allow both engines to drive one transmission.
Tandem rotor helicopters, or any layout with two "main" rotors will have those rotors shafted together. If they did not, and the RPM dropped ever so slightly you would die - right at that place and at that time. The rotors mesh and must be geared to pass blade-to-gap and not blade-to-blade.
The old Piasecki H-21 "flying banana" serves to illustrate this well enough:
The engine was back near the tail, you can see the exhaust sticking out the side at the back end of it. This was a single-row radial mounted conventionally - with the "prop" shaft forward. From there a driveshaft ran to a little gearbox overhead just above the middle of the cargo door. From this gearbox a driveshaft ran at an angle up to the rear rotor. You can see this driveshaft in the uncowled area going up the tail. The shaft to the forward rotor is under the cowling along the top of the forward fuselage. There is a transmission under each rotor making the final RPM reduction.
Other tandem rotors like the Chinook are substantially the same, differing only in the point in the drivetrain where the engine torque is input. On the Chinook there is a combining gearbox back between the engines.
The important part however is that the rotors are splined together and cannot be run at differing speeds.
So, in a multiengine helicopter if you lose one engine you only lose that part of your power. If you have a heavy slingload on a mountain peak at the moment you will probably have to punch off the load or lose the whole aircraft. If you lose one of two engines in a tandem rotor helicopter the remaining engine continues to drive both rotors.
True for the Osprey too. Picture an engine failure in that thing if the prop/rotors weren't shafted together!
When the pilot pulls collective pitch at the bottom of the autorotation to cushion the touchdown he is using the kinetic energy stored in the spinning rotors to arrest the descent rate. This is why second generation helicopters have weights in the blade tips - more energy stored. If you squander that blade energy before landing it is gone.
If you do not put the collective down quickly enough the rotor RPM will decay very rapidly on some models, (shudder I just had an OH-23B flashback) and you are seriously hosed. Where you might survive the resultant hard landing, the spinning rotors are also your only means of control. That last bit is important. On some helos with quite a lot of blade inertia pilots have been known to pull a tiny bit of pitch early, to "carry" them past some trees or powerlines but it is a very risky move and often does not work. I've lost a friend this very way.
Gee, I don't think I've used the phrase "sprag clutch" in thirty years.
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