PDA

View Full Version : A question for helicopter gurus


Dave Incognito
11th Jul 2001, 10:45
How does airspeed affect helicopter climb performance?

Just from a very basic physics guestimate, having a lateral velocity of zero would seem to give the best climb performance due to the lift vector acting at 90 degrees to the horizontal. Therefore any lateral velocity would (by this reasoning) result in diminished climb rate.

However, helicopters generally seem to have a noticeable forward velocity when on final approach, during the initial climb-out and during an auto-rotation. If something went wrong during there phases of flight, I assume you would want the smallest ground speed possible. A low/nill ground speed would mean that lift does not have to be 'allocated' towards slowing the aircraft down prior to touchdown (assuming nill wind).

So does a higher airspeed make for easier handling etc. or is my initial guestimate simply wrong (I think it’s more likely to be the latter... :rolleyes: ) ?

Sorry if this has been covered before but without the search function it’s a tad hard to find out.

Thanks in advance, Dave.

[ 11 July 2001: Message edited by: Dave Incognito ]

ft
11th Jul 2001, 13:35
I am no expert by any means but until they get here... ;)

You will actually GAIN lift when you build some forward velocity. A really heavy helo migh actually only be able to leave the ground through using a take off roll. This is since the airmass available for the rotor to accelerate will increase. Instead of the volume of air being rotor area times rotor downwash velocity it is rotor area times the resultant between downwash velocity and forward velocity... er... a picture would really help here I guess. :)

During an autorotation, 60 kts or so of forward speed is essential since you actually flare before landing (or impact...), using the airspeed to kill the sinkrate. Low and slow is a death zone where autorotation is impossible. Having some airspeed built up before leaving the altitude where falling straight down is survivable if the engine decides to call it a day is desirable.

You'll also want some forward speed on approach to avoid creating vortex rings, basically a ring of rotating air around the rotor system giving the rotor very little chance of accelerating the air further to create lift. I'm not sure about the english term but a direct translation from Swedish would be "sink-through". Very dangerous phenomenon.

Cheers,
/ft

Edited due to incoherent grammar caused by serious lack of caffeine while posting.

[ 11 July 2001: Message edited by: ft ]

Tinstaafl
11th Jul 2001, 16:17
During take-off there are combinations of height/airspeed that don't provide sufficient stored energy, or time, to recover if an engine should fail.

On a graph of height vs IAS the no-go area is a bulge on the vertical axis, starting at some smallish height/0 kts, expanding to the right & upwards then finally contracting to disappear as sufficient height is gained.

There is a a second bulge on the horizontal/IAS axis that starts at some knots then, expands and then contracts & disappears once past a certain critical speed.

The upshot is that there is a 'window' of heights vs. speeds in which it is safe to operate ie to take-off, accelerate & climb.

For landing, a vertical descent can cause the helicopter to descend through its own downwash. If you imagine how much loss of efficiency a fan or propellor would experience if it was moving backwards with the airstream it was trying to accelerate then that is similar to what the rotor blades would experience.

It can become severe enough that no 'new' air is being accelerated through the rotors. Instead, air that has just passed through the rotors curls up & around the rotor tips in a vortex & then feeds through the rotors again.

This results in a massive loss of lift producing ability for the rotor disk.

[ 11 July 2001: Message edited by: Tinstaafl ]

martinf
11th Jul 2001, 16:49
Can I be naive for a second - what is autorotation - have heard the phrase but not sure what it is? Please enlighten me. :confused:

ft
11th Jul 2001, 20:02
Potato Head,
autorotation is what you do when the engine quits on you. You basically enter a controlled descent, much like gliding with a fixed wing aircraft, where the air streaming upwards through the center of the rotor disch turns the rotor enough to provide some lift.

The easiest way to picture how it works is by drawing cross sections of the rotor blades and the freestream velocities in relation to the blades. A constant component upwards and a small horizontal component from the rotation at the blade roots but a large one at the tips. At the blade roots, the lift vector (perpendicular to the freestream velocity) will give a larger forward component than the rearward component of the drag vector (parallell to the freestream velocity) making the rotor turn.

When the engine dies, you push the collective lever down (setting the rotor to "fine pitch") and push the nose down. As you approach the ground you pull the nose up and pull the collective back up. Ideally, you want your skids/wheels to touch down with zero IAS and with the rotor coming to a stop. The energy stored in your forward momentum and in the rotational momentum of the rotor disc is what slows down your descent.

Cheers,
/ft

t'aint natural
11th Jul 2001, 21:25
Fairly simple analogy: a helicopter climbing vertically is like a man climbing a steep sand dune, and slipping back down a ways with every step.
A helicopter with some forward velocity is like the same man climbing a solid hillside, making more ground for the same energy input.
In a vertical climb the air meeting the rotor has already been accelerated to a degree, whereas in forward flight the degree of downward acceleration is less. Result: More bang for the buck.

The Nr Fairy
11th Jul 2001, 21:26
As Tinstaafl says, certain combinations of height and airspeed result in too little enegry in the system as a whole ( combined kinetic and potential ) to make a safe flare and have enough left for the cushion at the bottom of an autorotation.

Single engined take off profiles follow a narrow corridor outside the high/slow and low/fast portions of the avoid curve. In an R22, this is basically an acceleration at 5' to 10' agl, rotate at 40kts, then climb away at 60kts.

Towering ( vertical ) takeoffs are available in certain circumstances - especially if you're in such a confined area that you've no room to accelerate. However, an engine failure will mean some seriously quick reactions and even that won't guarantee a successful outcome.

As for the best ROC speed, imagine an upside down bell-shaped curve where the faster you go, up to a certain speed, you need less power for straight and level. Above that point you need to add power to stay straight and level. 53kts in an R22 is the bottom of the curve, and therefore where most power is left for climbing.

Ask on the Rotorheads forum if you want more - there's far more knowledgeable people than me there.

[Edited for spellin]

[ 12 July 2001: Message edited by: The Nr Fairy ]

Dave Incognito
12th Jul 2001, 14:31
Thanks for all the responses, much appreciated.

Have a good one, Dave.

4dogs
12th Jul 2001, 21:08
Dave,

Your assessment of the physics would be correct if the lift vector was a constant. It isn't.

Generally, the hover is the most power hungry mode of flight. The wonderful world of RW aerodynamics enjoys dramatic increases in rotor thrust with forward speed, almost identical with that of aeroplanes.

The power required vs airspeed diagram looks very much like the diagram you are more familiar with. It matters not that the causes are different, the outcomes are largely the same. One accelerates for take-off to maximise the reduction in power required - things really start to happen around 10-15kt, max angle is generally around 35-45kt, max rate is around 55-74kt and above that you really start to pay for the privilege of going (relatively) fast.

While on the subject of power required charts, the multi-engine issue is exactly the same as for your twin aeroplane: lose one engine (twin, not EH101) and you lose between 75-95% of your performance.

Droopy
12th Jul 2001, 22:26
Your question was specifically about speed affecting climb performance so this is how I explain it in uncomplicated terms to non-flyers; they seem to be happy:-

In a still air hover, nil airspeed, the rotor disc is working very inefficiently as it sucks air down from above; this is caused by induced drag [same idea as needing coarse pitch at speed with a VP prop].

As the disc flies forward faster - just like a frisbee - it becomes markedly more efficient. This process starts with a "step" of reduced drag at a point known as translational lift and then continues progressively. This also applies hovering into a reasonable wind.

As the airspeed builds, however, drag on the airframe itself increases and the power required just to go faster increases.

The net result is very high power required to hover with a low power requirement in the middle speed range and high power again at the top end of the range. The typical "efficient" speed range for an aircraft capable of 120kt will be in the order of 20kt - 80kt and the most efficient will be 55-70kt depending on type.

Clearly if you want to climb at the best rate then you accelerate to it at low level and then climb, as a power loss at 50-150ft or thereabouts with no speed would result in no time to set up a stable autorotation before you hit the ground.

It's rather a long subject but that's it in a nutshell.