Out on hover power
 

This is about a production helicopter whose case of 'Hover OGE power being outside limits' found line pilots struggling. The hover power measurement for this twin engined helicopter is in terms of torque, which was out by 4-5% on each engine (A total of 10% of MORE power was required to hover than the stated performance).

The established deficiency in performance in NIL wind conditions was corresponding to a weight of approximately 300 kg at sea level, which for 5.5 ton Dhruv helicopter is significant.

Following were checked and found correct:-

1. Power Assurance requirements for ascertaining correct power output from each engine.
2. Empty weight of the helicopter through re-weighing.
3. Torque calibration.
4. Fuel and OAT gauge calibration.


Just that it doesn't go unsaid, this was a unique case in itself.

Having finished with the above checks without any worthwhile results, wouldn't it be prudent to check back the rigging angles of main rotor blades, which may be out of limits and thereby spoiling the L/D ratio of the rotor disc as a whole.

I opine that the additional drag as a result of the 'spoiled' L/D ratio due to the incorrect rigging angles may be resulting in higher torque values to provide the same rotor thrust, and therefore the helicopter is hovering at the given all up mass with higher torque values.

This opinion is also in light of the this helicopter undergone MGB change immediately before the problem surfaced. Though, the only factor which kept me wondering was whether a marginal variation outside the OEM stated rigging angles would result in a Hover OGE power variation corresponding to 300kgs?


Requesting for subject opinion and any suggestion to resolve the issue. Thanks

Bleed air? Have you got a blow off valve or anti ice system robbing you of performance? I know of one engine where if you inhibit the engine anti ice it actually comes on anyway. G

Rotor rpm?

This may help - not entirely related, but perhaps useful.
Check the blades are clean.
This will test to see if the torquemeters are reading the same - put one engine at idle, and pull up to light on the skids and note torque on 'good' engine. Then leaving the collective at the same position, bring engine that was at idle to normal position, and reduce formerly 'good' engine to idle. Should have the same torque.

And there are two ways to do hover out of ground effect - 'hover up', or 'fly in'.
'Fly in' is done by transitioning to the hover from forward flight at a height well above the break line from in-ground-effect to out of ground effect (try 150' AGL).
The 'hover up' method starts from IGE and attempts to climb up to OGE.
My experience is that there is probably at least 5-7% difference in power required to hover - with the 'fly in' requiring less power.
I've never been able to determine why there would be difference, but there is.

Shawn

That's my next test pilot continuation training sortie profile sorted then!

G

Rigging shouldn't affect power required to hover, but it can change the control position required to generate a given torque.

Has anyone done a comparison of post engine shutdown rotor run down times against another example of the type? The gearbox change may be significant in that it's internal drag may be higher than it should be.

I sometimes shut off the engines and observe the rotor rundown from the cockpit with my headset off, without using the rotor brake. Not required by the manufacturer but I feel it's worth doing on occasions.

Another clue is at what N1 the rotors start turning on startup. However, I feel this is less reliable because the oil isn't up to working temperature.q

If this aircraft achieved the stated performance prior to the gearbox change and nothing else was changed, then the gearbox is certainly suspect. But if the replacement gearbox has enough internal drag (friction) to produce an additional 5% torque required per engine, I would think the amount of heat generated would be enormous.

That sounds a lot like the check we do where we test what the residual torque on each engine is. After the first landing of the day each engine is returned to idle in turn and the torque of that engine is recorded. Anything above 5% is bad and might link in to your loss of performance.