Turbine Helo Generator Question

[BlenderPilot]

I was starting a Bell 212 the other day, I had already started the first engine and set the generator to the "on" position in order to make a generator assisted start of the second engine, then as I engaged the starter on the second engine I noticed a 40 or 50 degree ITT temp rise on the engine already running, It was then that it came to me that this was caused by the extra load on the working generator, which in turn caused the engine to want to slow below Flight Idle, as a result the engine needs more fuel, and thus produces a ITT rise in order to maintain Flight Idle.

I commonly operate at or above 7,500 FTAMSL, and I am constantly very severly power limited, and was thinking how much more power would be available for T/O if I waited for the generator load (amps) to decrease before I took off right after a battery start. Especially say in Jet Rangers in which the load can go from say 70% to about 20% by just waiting a couple of minutes after the battery start.

Actually the other day I had a hung start in the JR, after switching the generator on at 70% N1, I returned to flight idle (61%N1) and the engine started to slow down below flight idle and the engine out audio/light came on due to the high generator load I had to switch the generator "Off" for the engine to return to normal.

Does anybody have any idea just how much horse power is required to keep those generators turning with a high load?

Has anyone noticed a difference in available power with different generator loads?

I have known of some pilots who switch the generator off before T/O in order to have more power, and then once in flight lower their N1 and reengage the generator, as not to break the shaft, what do you think about this.

Just please remember at 8,445 FTAMSL with 28 C, every single horsepower counts!

Opinions anyone?

[rotormatic]

With the Jetranger, there is a reason the flight manual states the generator should be turned on after the N1 is past 70%.....below that speed, there is not enough fuel scheduled in the fuel control to keep the engine at the minimum N1 speed for idle...(59%)...with a high generator load....

The philosophy is that after engine start, and you go flying, the battery will be charged, and when you land, there will be a lite load on the generator.

Any excessive electrical load will effect the TOT reading in the Jetranger... the starter/generator is driven by the N1 geartrain... this load will not be shown on your torque meter...(N2 system).

An excessive load on the generator will be like flying at a higher altitude...it affects the compressor drive train load (N1).... and will result in a higher TOT reading during operation...

[Nick Lappos]

Blender Pilot,
This effect is part of the design of the machine, as every accessory robs power, of course. The gens load down the gas producer, and raise the temperature and N1 when they draw big loads to recharge the battery. We account for this in the power available charts, usually assuming max generator load (or telling you in the flight manual what load to check for performance). I recall the B-222 manual having you check generator amps and adjusting to attain the scheduled OEI climb capability.

The power loss is quite predictable. For a 28 volt dc system, 100 amps means that the aircraft is giving 28 x 100 = 2800 watts to the electrical system. That is 2.8 kilowatts, about 3.75 horsepower. (A Kw is worth 1.34 HorsePower). In terms of Hover performance, if the gen is drawing 100 amps, you could lose about 34 pounds of total hover capability (at 9 pounds per horsepower).

The effect looks worse during the start for two reasons: The starter can draw 300 or 400 amps for the first couple of seconds(600 for an S-76) and because the engine at idle is barely able to run itself, so the draw of the gen drags it down badly. At 85% N1, the effect is much smaller, because the engine is much more efficient.

I wouldn't turn off the gen to save its shaft, that shaft is quite strong, way beyond the steady load it sees as it runs under high load. The cycles due to the power switching are what design it, not the steady torque. Making takeoffs with the electrical system crippled by your actions is a good way to invite all those experts to the investigation. It is one thing to have a system involuntarily fail, another for the PIC to elect to turn it off. An FAA accident inspector has a rubber stamp in his kit for just such actions, they make his decision making much easier in accidents (not more accurate, just easier!)

Comanche has an SPU (supplimentary power unit - full time APU). It runs critical accessories all the time, saving engine power for driving the rotor.

Nick

[Rob_L]

Working up at Dodoma in Tanzania in the mid 80's with a BO 105 we had a problem on start up. No 1 engine started fine, when starting 2 no 1 stalled each time no 2 starter button was pressed. After some had scratching we started no2 first then no 1 successfully. Flying back down to sea level at Dar es Salaam the problem went away for about a week the reacurred. Rather baffled I was having a coffee with the Police helicopter flight when there chief eng Dietmar Burger asked how the engines were coping with the errosive atmosphere given that we didn't have sand filters. The penny dropped. a quick inspection of the no 1 compressor revealed serious errosion of the plastic lining.

The damage to the compressor had reduced the stall margin to the point where loading the generator to start the second engine was sufficient to cause a stall. Only a few rpm in it but enough.

[HeliMark]

Nick, I am one of those pilots that every so often have to turn the generator off inorder to be able to takeoff. The 500E I fly is almost always within a couple hundred pounds of gross, and during the summer I am usually operating at 5000 or above with temps of 30+ on searches. Not as bad as BlenderPilot, but certainly feel it. It can give me an extra 10-20 degree's TOT, which is at times is all I need inorder to stay within my transient time limits.

You are right in that the electrical system is "crippled", but when the battery should be almost full (one should wait a little time inorder to recharge the battery some), and should be able to handle whatever electrical needs are for the 30 sec's or so before you turn the generator back on.

Obviously I do not advocate this for normal everyday flying to get that extra power, and as twins go, I have only flown the H-3 and would obviously not do that in it. But with that short of a time, a battery that could handle the EPO, and meaning that one can get off the ground, I do not see that problem with it.

[IHL]

Blender Pilot:

Though it is a different category ; when starting a Beechcraft King Air (with PT-6 engines)it is recommended that prior to starting the 2nd engine the running engine be set to high idle(70%) n1 and the load meter be below 50%. This is done to preclude exceeding the temperature limits on the running engine.