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I've read enough crash reports to see that helicopters discovering too late in takeoff that they lack the power to fly out of ground-effect is a serious problem.

I am wondering why the following isn't standard takeoff practice: Establish positive climb rate out of ground effect before beginning any translational flight. Failure to climb out of ground effect would be cause for immediate ATO, and there would be this nice landing zone to abort into!

(Only in exceptional emergencies, like when a mountain takeoff gives the rotor-craft a clear glide-path into denser air, would it be reasonable to ignore this procedure.)

(I note that I am assuming that translational flight out of ground effect is always more demanding than hovering. I vaguely remember that this is not the case, but I don't remember why, and it seems counterintuitive since translational motion always establishes a retreating blade with lower lift that then needs a higher angle-of-attack and more power to maintain lift. So if the helicopter can't climb under full power with no translation, it would not be able to once it tries moving forward.)

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  • $\begingroup$ I don't think I can attempt a full answer on this, but I believe that the reason they translate to forward flight has to do with the potential of overheating the engine during hovering. Forward motion gets air flowing over the engine, helping it to control temperature. $\endgroup$ – Howard Miller Oct 27 '15 at 20:12
  • $\begingroup$ I'm a bit late to the party, but there is some related information here. $\endgroup$ – FreeMan Mar 22 '16 at 16:18
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Forward flight is much more efficient than hovering. As airspeed builds, lift increases from "translational" lift as the air moves more horizontally over the disc. Since the relative airflow is more horizontal, the angle of attack for a given pitch angle is increased.

The vortices and turbulence move behind and down from the helicopter so undisturbed air is drawn over the disc to be accelerated downwards to produce lift. Lift is more efficiently produced by accelerating a large mass by a small amount rather than a small mass by a large amount.

It is more efficient to remain in ground effect and slowly increase speed to gain translational lift than to increase power to HOGE (Hover Out of Ground Effect) and attempt to translate.

The problems you refer to are where an unwary pilot attempts to transition from HOGE whilst allowing the aircraft to climb or attempting to maintain height. Without enough power, the loss of ground effect before the benefits of translational lift are gained will result in a rapid descent back to earth (as vortex ring state or settling with power is pretty much impossible to avoid) or you overpitch and drag the RRPM (rotor RPM) down so low that you cannot produce lift or even worse, stall the blades.

Bottom line, it takes more power to establish positive climb out of ground effect than to gain translational lift in ground effect.

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  • $\begingroup$ Thanks, I had forgotten about the benefits of translating the disc into "smooth" air. However, at some larger translational airspeed the translational lift does begin to fall as increasing (retreating) areas of the disc are unable to maintain positive relative airspeed, right? Or is that always outside of the flight envelope of a helicopter? $\endgroup$ – feetwet Oct 27 '15 at 13:16
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    $\begingroup$ The benefits of translational lift do drop off but not in a linear fashion. Remember that as the blades flap up (advancing side) and down (retreating side) the angle of attack also changes. The blade(s) flapping up experience a reducing angle of attack because the relative airflow is more from above and the blade(s) flapping down experience the opposite. Therefore, there is a natural tendency for the advancing blade to generate less lift and the retreating blade to generate more. Dysermmetry of lift becomes a problem as VNE is exceeded. $\endgroup$ – Simon Oct 27 '15 at 13:33
  • $\begingroup$ @Simon two things you might want to addd: 1. just pulling into a hover requires a fair amount of power, and will help the pilot determine whether she has enough power to take off (2) there's always the running take-off $\endgroup$ – rbp Nov 21 '15 at 22:21
  • $\begingroup$ You state that: 'As airspeed builds, lift increases from "translational" lift as the air moves more horizontally over the disc. Since the relative airflow is more horizontal, the angle of attack for a given pitch angle is increased.' Well, the opposite is true: for a given pitch, the more parallel the relative airflow is to the rotor disk, the less the AoA... $\endgroup$ – xxavier Aug 28 '18 at 5:29
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A helicopter takeoff profile is governed by having enough margin to autorotate to a safe landing in event of engine failure, not by more efficient flight due to ground effect. Margin consists of both forward speed and altitude.

Helicopter manufacturers publish altitude vs airspeed charts which prohibit operation above a survivable altitude until sufficient airspeed is achieved. Height without speed, as you suggest for use in a HOGE profile, results in high speed vertical impact on engine failure. Staying in ground proximity until there is sufficient forward velocity to initiate autorotation means vertical impact speed stays low with loss of power.

It just happens to be that a survivable altitude is also in ground effect, so efficiency is a side benefit of a typical takeoff profile rather than a driver.

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