helicopter transmission

[radomir]

Good morning everybody.

I asked for help posting this message on the Rotorcraft Engineering three weeks ago but no replies. Therefore I decided to ask you for help. Here is the question:

"I have reacently run into a warning for a Sea King helicopter (twin engined aircraft) that single engine take off and climb can overtorque transmission. Me as a "dumb" mechanical engineer cannot understand that a single engine may produce more torque than two engines.

Probably we are not having the same overtorquing in mind. Definitely shafts cannot be overtorqued, I am absolutely certain about that. But maybe it is about gearing (tooth load is increased but number of load cycles is halved) and housing loads which may be increased because of utilizing engine contingency rating (max power, sometimes defined as 2.5 rating, not normally utilized when both engines operational) and maybe due to asymmetrical load case (when the both engine operational a couple is formed).

In that case it is unclear to me how the aircraft torquemeter can indicate the increased torque since I am led to believe that it is indicating mast torque (which is definitely lower in the single engine operating case, even on contingency rating, comparing to both engines running for example on 10 minute power rating)."

Please advise, I would be very grateful for the enlightening.

Best regards to you all

Rad

 

[NickB]

It's possible the input shafts to the transmission are not made to handle the torque required for full power take off and climb through a single shaft. When running on both engines, the torque applied to each shaft would be half.

 

[GregLocock]

I don't think that's the explanation, since the shafts would be sized to take full power from each engine. I suspect that you have a gearbox that looks like a differential on a car, working backwards, ie replace each wheel by an engine and the engine by the rotor. If one engine seizes the diff still transmits power, but at the cost of an enormous hubbub (technical term) inside the differential casing. Cheers

Greg Locock

 

[radomir]

Sir Gregory,

First of all thank you (both) for the response. You are absolutely wright regarding your analogy with a differential working backward. But that "hub-bub" of yours is that what I called housing (casing) load increase due to asymmetrical load or is it something else?

Best regards, Rad

 

[JayMaechtlen]

without understanding of the layout and design of the system, it is pretty hard to offer an informed opinion.

Is there somewhere we can get the info?

regards
Jay

 

[radomir]

The layout can be found in S61 manuals ("Sea King", an anscient Sikorsky helicopter, but not so anscient as myself; US Navy designation H3 or H53, CF designation CH124, the helicopter still transporting US president). Unfortunately I cannot append the scheme to this message so I'll try to explain.

Esentially the gear-train is rather simple and just to underline: input portion of it is symmetrical-mirrored, except for the direction of rotation. Two paralel input shafts rotating in the same direction ~19,000rpm max (1,500shp max each) are individually (spur gear pairs)reduced to 8100rpm. On the other ends of those two intermediate input shafts are so called "input frreewheel units" (the unit is a helical gear mounted on the interm. shaft by means of frewheel coupling, to prevent rotor seizure in case of one engine failure). Inbetween these two "input units", meshed with them (meshing points 180 deg. apart), is the main input helical gear reducing speed to 3,195rpm. On the other end of that main (single) input shaft there is a main input bevel pinion meshed with a main bevel gear (right angle bevel pair) rotating at 939rpm. The main bevel gear is coupled through a single stage planetary gear assembly to the rotor head rotating at 203rpm (total reduction ratio is 93.4).

The question raised is more to check my plain logic. Single engine operation at max rpm and 1,500shp means more load to transmission only in the input section (engine operating side) and to the casing because of non symmetrical load case. But definitely it is not a higher torque load to main shafts and gearing (bevel and planetary) comparing to both engine running at the same rpm with e.g. 1400shp each.

Best regards, Rad

 

[GregLocock]

OK, I was being cheeky.

Digression: In an automotive diff the 'hubbub' is created by the 4 bevel gears inside the crown wheel. If both halfshafts are turning these do nothing. If one halfshaft slows they begin to turn against each other. If one halfshaft stops completely then they are running at twice the overall speed of the diff, which since they are small, creates a fair amount of noise and friction. You could, of course design these small gears to take the full power and speed of the main gearset, but that would compromise the design of the diff the rest of the time, since it would be unnecessarily large and inefficient.

Things are both simpler and more complex in your case. I would guess that the problem is that the assymetric loading on the main input gear (8100->3195) will increase the loads on the bearing for that gear and the casing. In normal use that bearing would see no vertical load, suddenly it has to react twice the tooth force.




Cheers

Greg Locock

 

[Roncallo]

A single engine cannot produce more torque than a dual engine. However depending on the temperature and altitude the helicopter may be power limited which would occure at high altitudes and high temperature. In this case none of the engines would be able to produce enough torque to break things. Or at sea level on cool days the helicopter may be transmission limited because either engine could produce enough torque to break an input shaft tooth. This becomes critical when one engine fails in a dual engine helicopter, the pilot may have to ask the remaining engine to apply more torque than allowable from a single engine.

In your example it's not the main rotor shaft that is going to fail. It something on one of the engine inputs. You are applying 1500 hp to a shaft rated for 1400 hp. After you get past the input stage of the transmission there are no problems.

My opinions posted here are my own and do not represent the company I work for.
JR

 

[GraviMan]

Sounds like a lubrication problem, I've seen this on truck diffs before. I would imagine that the bevel gears are helical to keep weight down, possibly even in the "differential" assembly. Aero gears are different to auto gears since the fatigue load has to be full power, which is the normal operating power. The main problem with any gear is the fatigue on the tooth contact surfaces, especially if there is any hypoid action.

Trouble is the oil has to be sufficient viscosity to avoid tooth contact, but not so viscous as to absorb too much engine power. You will probably find there is a dedicated gearbox oil cooler. If one engine goes down the additional movement of the diff will cause gearbox oil temp to rise, until eventually it gets too thin to lubricate the tooth contact zone. The torque limit is likely to be to avoid tooth failure.

I'd be interested to know how close to the mark (or otherwise) this is.

Martin

 

[jabonet]

When a spur gear is working under load, you generate a high torque, and pretty big side load due to tooth contact angle, if you have the two driver gears 180 degrees apart, these forces counrbalance, disapear inside the driven gear and casing of the gearbox, and you end up with a nice 3000hp, 200 RPM output shaft.
But, when one engine fails, and I guess that you have some kind of a clutch to disengage the spur gear (a 93 to 1 reduction is not a good idea to drag around) the other spur gear generates half the torque, plus a big side load, perpendicular to the shaft axis, that has to be absorbed only by the bearings of the shafts, and maybe this is too much.

in other words, if you have a T bar for taking a wheel nut out, and you use both hands, the total output torque doesn´t have any side componets, but if one hand fail, you can breack the nut, sideways.

hope this helps.

Jabonet

 

[GraviMan]

Just read the rotorcraft forum response.

"what happens is you over torque the in put pinion gear is beyond its raiting. In twin engine mode you are deviding the power in to two seperate drive gears in the transmition. etc"

This sounds to me like it could well be tooth contact fatigue on the diff input pinion limiting the OEI (One engine Inop). I'm actually suprised that a larger gearset wasn't specified, but having seen the size heli gearboxes (have to be large for full power fatigue rating) I guess weight was the main concern.

Mart