# Can a freefalling helicopter start its engine, or otherwise save itself? **URL:** \ **Category:** Factual Questions\ **Created:** [April 16, 2013, 5:03pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850 "2013-04-16T17:03:52Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![wei\_ji](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wei\_ji](https://boards.straightdope.com/u/wei_ji)\ **Post date:** [April 16, 2013, 5:03pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/1 "2013-04-16T17:03:52Z")
In the James Bond movie, “Die Another Day,” Bond (pierce Brosnan) and CIA agent Jinx (Halle Berry) board an MD-600N NOTAR helicopter, which is then ejected from the rear bay door of a burning and disintegrating cargo plane. As the helicopter freefalls, Bond struggles to start the engine. He succeeds at the last moment and applies enough lift to arrest the freefall and fly away at ground skimming level. I know this is Hollywood, but is there any way a skilled pilot of such a helicopter freefalling from thousands of feet could use its controls to gain a measure of control in such a situation, start the engine, and either save the helicopter by the application of power or by the standard emergency technique of autorotation? Is there anything that might be done other than ride it in? ---
**Author:** ![sitchensis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sitchensis/32/4892_2.png) [@sitchensis](https://boards.straightdope.com/u/sitchensis)\ **Post date:** [April 16, 2013, 5:41pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/2 "2013-04-16T17:41:15Z")
[Autorotation](http://en.wikipedia.org/wiki/Autorotation_(helicopter))is what keeps a helicopter from dropping like a rock. At least it will give you something to read until the experts drop by 🙂 ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [April 16, 2013, 5:48pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/3 "2013-04-16T17:48:09Z")
Well, it is unlikely. Generally, there is a minimum rotor RPM at which it is possible to initiate an autorotation. Below that speed, aerodynamic forces are not strong enough to keep the blades from decelerating. While this differs for different helicopters, I am not aware of any helicopters for which it is zero. Starting the engine in uncontrolled free-fall, without autorotation, and then recovering with power is not theoretically impossible, but it would be exceedingly difficult to do, and most helicopter fuel systems probably will not work in an uncontrolled fall (they are designed to operate with the helicopter mostly upright and subject to approximately 1g). Additionally, the helicopter would likely far exceed it’s maximum rated airspeed in an uncontrolled fall before the engine could be started, which would mean various components could be damaged by overloading in trying to recover, perhaps irreparably. Finally, rotor blades are actually rather bendy. They only stay relatively straight in flight because of centripetal force when operating at the proper speed. When operating below that speed and carrying load, such as in a low-RPM autorotation or in trying to speed them using the engine in freefall, it is likely that they would flex enough to strike a part of the helicopter. ---
**Author:** ![Poysyn](https://avatars.discourse-cdn.com/v4/letter/p/f4b2a3/32.png) [@Poysyn](https://boards.straightdope.com/u/Poysyn)\ **Post date:** [April 16, 2013, 5:51pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/4 "2013-04-16T17:51:10Z")
I actually texted a pilot friend of mine about this scene - his response “The stunt is plausible in that you have a shot at recovering in a free fall - pilots need to be able to recover from a stall, after all” Does that help? ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [April 16, 2013, 5:57pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/5 "2013-04-16T17:57:28Z")
> [@Poysyn](#): > > I actually texted a pilot friend of mine about this scene - his response “The stunt is plausible in that you have a shot at recovering in a free fall - pilots need to be able to recover from a stall, after all” > > Does that help? I suspect he is missing the part about having the engine turned off and the blades not turning. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [April 16, 2013, 6:31pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/6 "2013-04-16T18:31:56Z")
You can auto-rotate some helicopters. If the engine starts up from it’s own starter motor you can recover the helicopter and fly away (although you are nuts if you don’t land as quickly and safely as possible). You usually can’t used the rotating blades to restart the engine because of the presence of a free-wheeling and/or automatically disengaging conventional clutch. The rotor has to be disconnected from the drive train in order to auto-rotate. Systems with a free-wheeling clutch allow the rotor to spin faster than the drive train and can’t start up the engine. Sometimes a conventional clutch can be manually engaged so an attempt to use the spinning rotor to start the engine could be made, but if it doesn’t work you drop like a rock. ---
**Author:** ![Blakeyrat](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@Blakeyrat](https://boards.straightdope.com/u/Blakeyrat)\ **Post date:** [April 16, 2013, 8:31pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/7 "2013-04-16T20:31:17Z")
They did it in Airwolf. But Airwolf’s a secret super-chopper. ---
**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\ **Post date:** [April 16, 2013, 9:35pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/8 "2013-04-16T21:35:11Z")
> [@Absolute](#): > > Finally, rotor blades are actually rather bendy. They only stay relatively straight in flight because of centripetal force when operating at the proper speed. When operating below that speed and carrying load, such as in a low-RPM autorotation or in trying to speed them using the engine in freefall, it is likely that they would flex enough to strike a part of the helicopter. This is the biggest problem. I work for NASA and the army doing helicopter research. When the blades are not spinning fast enough they will flap all over the place. There is almost no chance you would be able to get through the range of rpm where you have little centripetal force without smashing through your own tail. Speaking of the tail, I would suspect a NOTAR rotorcraft would have some serious issues maintaining stability while spinning up, but I do not know enough about how they respond at low power to say for sure. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [April 16, 2013, 9:54pm UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/9 "2013-04-16T21:54:38Z")
The NOTAR uses fairly typical rotor blades so flapping will probably make auto-rotation unlikely. It’s weight should be slightly more concentrated than a conventional tail-rotor configuration, so that would increase it’s falling stability if the blades are providing enough drag to keep it upright. That might make all the difference between a flaming fatal crash into the ground at a random attitude, and a flaming fatal crash into the ground with an upright attitude. ---
**Author:** ![rsat3acr](https://avatars.discourse-cdn.com/v4/letter/r/a3d4f5/32.png) [@rsat3acr](https://boards.straightdope.com/u/rsat3acr)\ **Post date:** [April 17, 2013, 12:02am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/10 "2013-04-17T00:02:56Z")
I flew UH-1’s in the Army for five years and my info is based on that. I’m not sure why some people think there won’t be enough blade RPM. If the helicopter is in forward flight and the engine stops the blades are still going full speed. In fact as you fall the rotor RPM increases. We did touchdown autorotations all the time. Engine wasn’t off, but the throttle was rolled all the way off. Autorotations from altitude we almost had to pull up on the collective to bleed off some RPMs on the way down. In a hovering autorotation if you pulled in to much collective you actually gained altitude and would then have a hard landing. Usual hover was 3 feet. Several times I saw choppers gain 4 or 5 feet and land hard. You only get one pull when you start at 3 feet. As for a restart, you’d have to be up fairly high to have the time needed. Helicopters glide about as well as a brick. We were told while in flight school that they had 2 instructors cut the engine off at fivethousand feet and attempt a restart and they didn’t have time to get it done even being ready for it. Since we usually under fivehundred feet I don’t think I’d doing anything but looking for a place to land, checking the rotor speed and making my mayday call. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [April 17, 2013, 12:09am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/11 "2013-04-17T00:09:23Z")
> [@rsat3acr](#): > > I flew UH-1’s in the Army for five years and my info is based on that. I’m not sure why some people think there won’t be enough blade RPM. If the helicopter is in forward flight and the engine stops the blades are still going full speed. In fact as you fall the rotor RPM increases. We did touchdown autorotations all the time. Engine wasn’t off, but the throttle was rolled all the way off. In the case in the OP the helicopter falls out of the back of a cargo plane, engine cold, rotor not spinning at all. So what people don’t believe possible is for the rotor to spin up to auto-rotation speed, or that the once the engine was started it could spin up the rotor to flight speed in a split second. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 17, 2013, 1:07am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/12 "2013-04-17T01:07:53Z")
I’m not sure how flexing blades would end up hitting part of the copter-- Wouldn’t they be flexing up, away from the rest of the vehicle? ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [April 17, 2013, 1:23am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/13 "2013-04-17T01:23:15Z")
> [@Chronos](#): > > I’m not sure how flexing blades would end up hitting part of the copter-- Wouldn’t they be flexing up, away from the rest of the vehicle? They could bounce up and down. But since it’s a NOTAR the chance of a blade strike is reduced. It’s probably the least concern in this situation. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 17, 2013, 1:28am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/14 "2013-04-17T01:28:31Z")
> [@rsat3acr](#): > > I flew UH-1’s in the Army for five years and my info is based on that. I’m not sure why some people think there won’t be enough blade RPM. If the helicopter is in forward flight and the engine stops the blades are still going full speed. In fact as you fall the rotor RPM increases. We did touchdown autorotations all the time. Engine wasn’t off, but the throttle was rolled all the way off. A UH-1 has a high-inertia rotor system. This allows a little more time to initiate autorotation. Once autorotation is entered, aerodynamic forces keep the blades turning. The inside portion of the blade is the ‘driving’ section; and the outside portion, which is giving you your lift so you don’t fall out of the sky, is the ‘driven’ section. That’s why you can increase your rotor RPM with the engine off; aerodynamics, not inertia. Smaller helicopters have low-inertia rotor systems, so you have to be pretty quick on the cyclic stick. > [@Chronos](#): > > I’m not sure how flexing blades would end up hitting part of the copter-- Wouldn’t they be flexing up, away from the rest of the vehicle? [Here’s what Robinson has to say about low rotor RPM](http://www.robinsonheli.com/srvclib/rchsn24.pdf). (Edited to fix pdf-to-html scan errors and to add emphasis to ‘doomed’ line, because I like that they used the word ‘doomed’ in an official document. 😉 ) > [@](#): > > R22 Helicopter Safety Notice SN-24 > > To: Date: 8 September 1986 All R22 Owners, Dealers and Pilot Operating Handbook Subscribers > > LOW RPM ROTOR STALL CAN BE FATAL > > Rotor stall due to low RPM is still involved in more helicopter accidents, both fatal and non-fatal, than any other contributing factor. Frequently misunderstood, rotor stall is not to be confused with retreating tip stall which occurs only at high forward speeds when stall occurs over a small portion ofthe retreating blade tip. Retreating tip stall causes vibration and control problems, but the rotor is still very capable of providing sufficient lift to supportthe weight of the helicopter. Retreating tip stall has not been a problem with the R22. > > Rotor stall, on the other hand, can occur at any airspeed and when it does, the rotor stops producing the lift required to support the helicopter and the aircraft literally falls out of the sky. Fortunately, rotor stall most often occurs close to the ground during take-off or landing and the helicopter only falls four or five feet. The helicopter is wrecked but the occupants survive. However, rotor stall also can and does occur at higher altitudes and when it happens at heights above 40 or 50 feet it is most likely to be fatal. > > Rotor stall is very similar to the stall of an airplane wing at low airspeeds. As the airspeed of an airplane gets lower and lower, the nose-up angle or angle-of-attack of the wing must be higher and higher for the wing to produce the lift required to support the weight of the airplane. At a critical angle, (around 15 degrees or so) the airflow over the wing will separate and stall causing a sudden loss of lift and a very large increase in drag. The pilot recovers by adding power and diving the airplane to recover the lost airspeed > > The same thing happens during rotor stall with a helicopter except it occurs due to low rotor RPM instead of low airspeed. As the RPM of the rotor gets lower and lower, the nose-up angle-of-attack of the rotor blades must be higher and higher to generate the lift required to support the weight of the helicopter. Even if the collective is not raised by the pilot to provide the higher blade angle, the helicopter will start to descend until the upward movement of air through the rotor provides the necessary increase in blade angle-of-attack. Again at a critical angle, as with the airplane wing, the blade airfoil will stall, resulting in a sudden loss of lift and a large increase in drag. The increased drag on the blades acts like a huge rotor brake causing rotor RPM to quickly decrease even more, further increasing the rotor stall. As the helicopter begins to fall, the upward rushing air continues to increase the angle-of-attack on the slowly rotating blades making recovery virtually impossible even with full down collective. > > When the rotor stalls it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher airflow on the advancing blade than on the retreating blade. This causes the retreating blade to stall first allowing it lo dive as it goes aft while the advancing blade is still climbing as it goes forward. The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disc sometimes referred to as ‘rotor blow-back’. Also, as the helicopter begins to fall, the upward flow of air under the tail surfaces tends to pitch the aircraft nose-down. These two effects, combined with aft cyclic by the pilot attempting to keep the nose from dropping will frequently allow the rotor blades to blow back and chop off the tailboom as the stalled helicopter falls. Due to the magnitude of the forces invoked and the flexibility of rotor blades, hub flapping stops will not prevent the boom chop. \*\*The resulting boom chop, however, is somewhat academic. as the aircraft and its occupants are already doomed by the stalled rotor before the chop occurs. \*\* > > To prevent rotor stall and its catastrophic results the pilot must always do whatever is required to maintain a safe rotor RPM. It must take precedence over all other considerations, even if it means landing short in a swamp instead of trying to stretch your glide to the dry road beyond. > > Remember the power output of the engine is proporticnal to RPM and when the RPM is low you have less power available from the engine with which to regain the lost RPM. The power-on low RPM recovery procedure of simultaneously rolling on throttle while lowering collective must be practiced until it becomes an automatic reaction to any indication of low RPM. Low airspeeds combined with high sink rates must always be avoided and full collective must never be pulled until the helicopter is within one foot of the ground. The applicable part as to why they hit the boom is ‘This causes the retreating blade to stall first allowing it lo dive as it goes aft while the advancing blade is still climbing as it goes forward.’ ---
**Author:** ![Grateful-UnDead](https://avatars.discourse-cdn.com/v4/letter/g/ccd318/32.png) [@Grateful-UnDead](https://boards.straightdope.com/u/Grateful-UnDead)\ **Post date:** [April 17, 2013, 2:35am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/15 "2013-04-17T02:35:44Z")
> [@rsat3acr](#): > > As for a restart, you’d have to be up fairly high to have the time needed. Helicopters glide about as well as a brick. We were told while in flight school that they had 2 instructors cut the engine off at fivethousand feet and attempt a restart and they didn’t have time to get it done even being ready for it. Since we usually under fivehundred feet I don’t think I’d doing anything but looking for a place to land, checking the rotor speed and making my mayday call. So, what happened? Did you have two dead instructors, or what? ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [April 17, 2013, 2:43am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/16 "2013-04-17T02:43:55Z")
> [@Grateful-UnDead](#): > > So, what happened? Did you have two dead instructors, or what? He’s saying they didn’t have time before they auto-rated to a landing to get the engine started, he’s not suggesting they crashed. ---
**Author:** ![Grateful-UnDead](https://avatars.discourse-cdn.com/v4/letter/g/ccd318/32.png) [@Grateful-UnDead](https://boards.straightdope.com/u/Grateful-UnDead)\ **Post date:** [April 17, 2013, 2:47am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/17 "2013-04-17T02:47:51Z")
> [@Richard\_Pearse](#): > > He’s saying they didn’t have time before they auto-rated to a landing to get the engine started, he’s not suggesting they crashed. OK. Got it. ---
**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\ **Post date:** [April 17, 2013, 3:50am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/18 "2013-04-17T03:50:46Z")
> [@Chronos](#): > > I’m not sure how flexing blades would end up hitting part of the copter-- Wouldn’t they be flexing up, away from the rest of the vehicle? Mostly, but it is a very unsteady and messy aerodynamic environment, especially the section above the tail, which is quite wide on NOTAR rotorcraft. Even if they are not impacting immediately, the crazy aerodynamics as you spin up the rotor in this environment almost certainly would cause a strike. > [@Johnny\_L.A](#): > > [Here’s what Robinson has to say about low rotor RPM](http://www.robinsonheli.com/srvclib/rchsn24.pdf). (Edited to fix pdf-to-html scan errors and to add emphasis to ‘doomed’ line, because I like that they used the word ‘doomed’ in an official document. 😉 ) > > The applicable part as to why they hit the boom is ‘This causes the retreating blade to stall first allowing it lo dive as it goes aft while the advancing blade is still climbing as it goes forward.’ Even without stall a strike could happen. In testing slowed-rotor compound helicopter designs (for reduced drag at higher forward speeds) we see that the 2/rev flapping goes to hell. You reach a point where you are bouncing off of the droop stops before you get near stalling the blade. The math says it should just get worse as the advance ratio goes up. Assuming you are not falling straight down with the flow perpendicular to the disk, this is the situation you are in when starting up the rotor in the OP. ---
**Author:** ![Ranger\_Jeff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranger_jeff/32/2065_2.png) [@Ranger\_Jeff](https://boards.straightdope.com/u/Ranger_Jeff)\ **Post date:** [April 17, 2013, 5:19am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/19 "2013-04-17T05:19:44Z")
If I recall the movie cited correctly, the rotor blades are in a stowed position above and parallel to the tail boom. I guess you could start the engine with the rotor like that, but how many helicopters have a feature allowing a pilot to deploy and lock the rotor blades from the cockpit. Cause, autorotation or powered flight isn’t going to happen unless the blades are out. ---
**Author:** ![rsat3acr](https://avatars.discourse-cdn.com/v4/letter/r/a3d4f5/32.png) [@rsat3acr](https://boards.straightdope.com/u/rsat3acr)\ **Post date:** [April 17, 2013, 11:21am UTC](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850/20 "2013-04-17T11:21:03Z")
> [@Richard\_Pearse](#): > > He’s saying they didn’t have time before they auto-rated to a landing to get the engine started, he’s not suggesting they crashed. correct [Next page](https://boards.straightdope.com/t/can-a-freefalling-helicopter-start-its-engine-or-otherwise-save-itself/655850.md?page=2)