Translational lift and power

[Devil 49]

How much more efficient is a helicopter rotor at onset of ETL?
Is 10% more lifting power reasonable?
If I'm power limited to MTOW 10,000 lbs in still air, is there a chart, rule of thumb, to reasonably predict a maximum effort MTOW with 15 knots wind?

[Shawn Coyle]

If you're at 10,000 lbs, you'll be in a Part 29 certified helicopter, and you may well have some other limits to worry about before we get to the discussion of how much more lift you get at translation. Pay close attention to the limitations section of your flight manual.

Where are you starting from - a low hover at the height in the flight manual for IGE hovering? Or Out of Ground effect?

And what do you mean by a 'maximum effort takeoff'?

Why do you need to worry about using translational lift to get you off the ground / away from the ground? if you can't hover at the height in the IGE performance chart - should you be attempting to lift that load at all?

Sorry to be so negative, but there is a lot more to the legal aspects of performance than 'suck it and see'.

[K48]

Devil,

If you look at your typical power curve... probably you need (apprx) 2 to 3 times the power to hover than to cruise at the bottom of the U in the power curve (55 knots ish). For a rough percentage IGE and OGE are pretty close compared to the base of the power curve so "2-3 times" is good enough for casual conversation. Nearer the onset of ETL at 15 knots or so there will not be quite such a difference...

[SASless]

Shawn,

Perhaps Devs could have restated his question by asking what effect headwinds have upon takeoff performance and that would have made the question less prone to such concerns you suggested might exist.

Unfortunatley real life refuses to provide accurate weights for cargo....heights of barriers....and always up to spec engines...blades...and the like.

I am sure the test pilot school has courses covering rotor dynamics that would address the topic Dev raises....or would hope so anyway.

Perhaps we could get someone to quote Prouty or the old Sikorsky Blue Book to us.

[Shawn Coyle]

SASless:
Well stated. But the problem still remains.
I know of no civil flight manual that gives any benefit for headwind for takeoff performance - there may be some that allow it for Category A, but I believe you have to factor it by one-half even then.
And how do you measure the wind speed at the place you are in the helicopter? Even if ATC gives it to you, it will be measured somewhere a long way away from your current location.
Do oil rigs always have an anemometer that is telling you instantaneous wind?

More reasons why we need a good low airspeed indicating system!

[Devil 49]

Oops, poorly worded.
Here's the situation that brought the question to mind- Watching a utility guy check his aircraft's performance, post maintenance. The test was a long-line lift of X weight.
However, this test was accomplished with a good breeze, 10 knots gusting to 18, at the surface. At the top of the becky, the aircraft would have been above obstructions to wind. The pilot's no fool and knows his next lift might not be in a comparable breeze, and was wondering if there was any way to predictably compensate for the ops check being performed in ETL, beyond the principle that the rotor's more efficient in ETL and thus the bird's stronger.
I know of no rule of thumb, could find nothing comprehensible to a mere pilot. More efficient, powerful, whatever, in ETL is insufficient data, smacks of "ther be giants here".

[JimL]

Yes Shawn, most rigs in the North Sea provide instantaneous readings from an anemometer some from two vertically spaced (and even some remotely accessed).

Hover wind graphs are provided for most helicopters used in HEC Class D; they are also provided in the S61N Supplement for calculating offshore landing and take-off masses and for most helicopters when providing helideck procedures.

The latest Bell 412EP 'helideck' graphs provide wind accountability but limited to ensure take-off mass is not greater than AEO HOGE or second segment climb performance (i.e. PC2 performance).

Jim

[Shawn Coyle]

Nice to see that there are ways to take wind into account in the performance graphs of some helicopters. Wish it were so with all of them.

As for the specific problem mentioned - if there were good performance charts that gave power required to hover for different heights from IGE to OGE, and took wind into account as well, we'd all be a lot better off.
Keep asking for those charts!

[maxtork]

Hi All,

I may be a bit off base here so bear with me. Could you not use a rate of climb check in this case? I'm refering to the original posters situation not to the case of taking off from an oil rig etc.

If you can't account for the wind and it's effects on the machine then why not move through the air at a given rate and at a given power setting and altitude and see how fast the machine climbs. We use this often on the Astar when questioning the power check results. Where the power check tells you if the engine is producing enough torque on the drive shaft for given circumstances, the rate of climb tells you how fast the aircraft should climb at MCP and 55kts at a given AUM. In this case Devil 49's pilot could see how fast the ship climbed at a given "wind" speed of 55kts. As I said I know the 350 series has this chart availablein the RFM but I don't know if it exists in many others.

Just a thought

Max

[Shawn Coyle]

maxtork:
You're right that this should be checked with a rate of climb vs. airspeed chart. The problem is that there is no useful airspeed information in level flight at anything below 40 KIAS. With some of the rates of climb that are possible below 40KIAS, the angle of climb will be pretty steep, and the airflow hitting the pitot tube will not be optimum. This is before you consider the effect that sideslip will have. The sideslip is the most insidious variable in this case, because it affects the rate of climb, and unless you have a slipstring, or yaw vane, you'll never know what sideslip there is.