powered control vs fixed wing passive control

[slowrotor]

Helicopters have what I call "powered controls", meaning the control surfaces are powered by the engine, giving the helicopter full control at low speed.
Airplanes have movable controls (elevator, rudder and aileron) that only function above a certain forward speed. Airplanes, when landing, need to increase the approach speed in strong turbulence to avoid a loss of control.

Is it acceptable for helicopters to reduce the approach speed in strong turbulence because of the "powered control"?

If the answer is yes, this would be a sustantial advantage for helicopters, but I have not seen this idea stated in any of my helicopter books. Is this well known?

[Matthew Parsons]

I think the ability to land and immediately get out of the turbulence is the helicopters overriding advantage.

Manoeuvring speeds and design speeds for gusts are well defined for fixed wing aircraft in the airworthiness requirements, but are not for rotorcraft. For helicopters, it seems it is more a common sense approach. If you get airspeed variation during turbulence that drops your IAS to the back side of the power curve, control will be more difficult. Without other information, take your L/Dmax speed, add 1/2 the gust spread, round up to an easy number to read, and that would be a good speed to maintain.

There might be information obtained from design limitations or flight test results that indicate a better speed or speed range to use, so go with your aircraft manual if it has that information.

Also, remember that the faster you go the sooner you'll be out of the turbulence.

Matthew.

[slowrotor]

Matthew,
Thanks for your input. I understand you would increase your approach speed.

But if a strong downdraft was encountered at the threshhold, would not a slower penetration of the downdraft be preferred?

I certainly have very little helicopter experience, so I thank you for your input.
slowrotor

[helmet fire]

Though I understand why you like to think of the systems in terms of "powered" and "passive", I think that is where your question actually has its origins. In fact, the control systems are both "powered" but just to different speeds. The rotor blade is merely a wing that rotates to get its airflow, rather than fixed wing that relies on fuselage speed for its airflow, thus the rotor blade generates lift independantly (but not in isolation from) its fuselage, but the wing cannot.

Gust spreads in turbulance can exceed (but rarely do) 30kts. Given a typical small aircraft approach speed of 80 kias to keep the wing flying with good controlability, a gust spread of 30kts can take the speed down to 50 kts and thus cause stall: hence the need for an increased approach speed in tubulance. The typical helicopter blade is travelling at over 300 kts (tip speeds higher, root speeds lower). Thus a 30 kt deviation of gust spread has much less effect, and will not produce a blade stall. Hence approach speeds are relatively irrelevant other than for comfort.

Comfort is a factor because of the other difference of the rotor disk system over the fixed wing: whilst the 30kt gust might reduce airflow over both wings of a fixed wing, it will actually only reduce on one side of the disk (advancing) and increase on the other (retreating). Thus whilst the fixed wing may suffer a considerable loss of lift from a 30kt windspeed reduction (velocity is squared in the lift equation) the rotor disk can flap to equality to somewhat balance loss on the advancing against gain on the retreating to produce a system that may or may not reflect an overall loss of lift. That rebalce is associated with a lumpy ride in the machine.

Whilst the above is true at higher speeds, at very low speeds (below 20kias), the helicopter becomes vulnerable to gust spreads. A 30kt spread is more than capable of oscillating the rotor system in and out of translational lift and thus causing both large fluctuations in overal lift, and attendant power changes from the pilot attempting to maintain a constant angle. If you are short of power (heavy, high, hot) then you become very vulnerable to the gust spread exposing the lack of power margin in this way. At 35kts on approach, you would be unlikely to be close to the ground, thus a gust reducing wind by 30 kts would put you in a situation suddenly requiring OGE power. On the face of it then, it would make sense to keep approach speeds high for helicopters BUT helicopters generally have to slow to zero (ish) for landing and cannot simply whack on a few knots to shield the risks of gust spread - and high spproach speeds will also require greater power margins too.

Therefore, helicopter or fixed wing, large gust spreads can really hurt close to the ground. The fixed wing can increase approach speed to compensate, but the helicopter would be best advised to increase power margins rather than speeds - at least to include OGE plus a bit for gust spreads over 15kts. Using Mathew's rule of thumb for the speed is a great idea if first you determine that have the power to cope.

[ShyTorque]

Slowrotor,

In turbulence a helicopter pilot really has to worry about slightly different things to a fixed wing pilot. A helicopter isn't affected so much by windshear and won't stall by temporary loss of airspeed in the fixed wing sense. Bear in mind that along the length of a blade and as it travels around the "rotor disc", there is a whole range of airspeeds in any case, even without gusting winds.

It's often more a case of finding a speed where the ups and downs don't cause the passengers to feel ill.

Another consideration is Vne. Modern helicopters cruise close to their Vne so it is prudent to slow down to allow a margin for gusts that would send the IAS too high. On the final approach to land, it's really just a case of finding a comfortable speed and a sensible ground speed. Most helicopter landings take place at very low airspeeds, in any case.

For commercial flights for twin engined helicopters, certainly in UK, it is a requirement to operate in accordance with an approach profile (IAS, heights of decision point etc, as laid down by the manufacturer).

BTW, of course, landing immediately to avoid gusting winds isn't always an option if on an IFR/IMC flight in weather to minima, requiring an instrument approach!

[IFMU]

Matthew,
But if a strong downdraft was encountered at the threshhold, would not a slower penetration of the downdraft be preferred?
slowrotor
Slowrotor,

No. Because as any glider pilot would tell you, the less time you spend in sink the less altitude you will lose. Glider pilots fly fast in sink, slow in lift. So if you were in sink that was worse than your ability to climb, get through it fast before the earth rises to smite thee!

-- IFMU

[slowrotor]

IFMU,
Thanks for reminding me about gliders... the glider pilot has a nifty control (dive brake/spoiler) that he usually sets at half so that the glide can be instantly adjusted up or down without changing angle of attack or power of course (no engine). The spoiler is a handy control that airplanes lack.

The helicopter has a handy control called the collective to adjust approach path in up or down drafts that the airplane lacks.

So I think gliders and helicopters have a big advantage in approach path control.

I am a glider pilot. My ship (a motorglider) can come down at about 2000fpm with full spoiler. So with half spoiler on a normal approach I have access to about 1000fpm of up lift or 1000fpm of down force instantly. The dive brake control works a lot like a helicopter collective. On the other hand a airplane pilot is trained to control descent with power. Adding power is not fast enough to correct for a downdraft close to touchdown, the spoiler is much more effective.
I would think a helicopter pilot would arrest a descent in a downdraft with collective (assuming a reserve is available) and that would be an advantage over using an airplane to get into a short strip with turbulence.

[IFMU]

Slowrotor,

Airplane pilots and helicopter pilots are trained that adjusting power is one way to change glide angle. But, not the only way. In FW I was taught power off approaches. Changing glide angle meant either slipping or speeding up, or slowing down. In FW, generally if you were slow, and the approach was getting too steep, you would speed up. When I learned to fly helicopters, I was taught to speed up in an auto if it looked like I was going to land short. After I became a glider pilot, it became more apparent why this is so. If you are stuck in a downdraft, you want to spend the least amount of time there. It's kind of like fighting with your wife- you are best off if you can take your licks and get the heck out of there. Any prolonged fight just gets you in more trouble and with less options. So, the less time you spend in the downdraft, the less altitude you lose from it. But, it is a little counter intuative, because while you are speeding up your drag goes up, and you are actually going down faster. But overall you will lose less altitude by spending a short time going down at a horrendous rate, compared to a long time going down at a bad rate. And, when you are through the sink you reconvert the airspeed to altitude.

I have a glider too, mine has no motor. I don't know how fast it goes down with full spoilers. Pretty good though.

-- IFMU