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Graviman
17th Nov 2006, 10:12
Helifolk,

My natural bias is towards powertrains, with heli powertrains being about the most interesting there is. I would like to cast the thread-net over the subject of engine choice for operation. In particular i would like to know which engines pilots prefer and why.

Any reason goes from good fuel burn to liking the noise it makes on a cold winters day. Since an engine is only as good as its interface, control preferences are fine too. :ok:

My own preference would be:
1. Dirt cheap (outlay and maintenance)
2. Ultra reliable (parts) (see 1)
3. Ultra durable (service life) (see 1)
4. Good fuel burn (BSFC or kg/NM) (see 1)
5. Good power /weight (>1 kW/kg)
6. Fast response (<0.5 sec transient, eg turbine lag)
7. Single engine control (no mixture, carb heat etc)

I've pretty much described future developments of the VW turbodiesel, but then my requirements are only for occasional recreational flying. I'm also a tight wad (aren't all engineers?). ;)

***Please note, this thread is not for the purpose of market research. The purpose is only to enlighten me as to what order to put the criterion affecting engine design for helicopters.***


Mart

ShyTorque
17th Nov 2006, 12:31
Criteria 1. There are at least two and hugely powerful so the failure of one of them doesn't take me to the scene of an accident shortly after. I want them big, man BIG!

Criteria 2. I want each to run on next to no fuel at all. I want them small, man, small!

Criteria 3. I don't want to have to lift the cowlings to check the oil levels (not that I EVER want to have to put any oil in them).

Who cares what they sound like? I don't want straight 6s or V8s, just a nice pair of turbines. ;)

Flingwing207
17th Nov 2006, 15:53
Has the VW diesel been tested in the aviation environment? Will it run at 90% power all day and for 1800 hours? Remember that auto engines only run at high power for about 10% of their lives, and usually for less than 10 seconds at a time.

Let us know how it all works out.

Graviman
17th Nov 2006, 22:37
Summing up Shytorque, powerful but you'd never know they were there. Any specific preference of engine? I'm guessing RR 250 C30P (206 powerplant).


Flingwing207, i use the VW turbodiesel as an example of the type of engine which would fit my (pretty undemanding) requirements. I am really more curious how much a factor the engine is to the pilot, or whether it is more a concern to the helicopter concept design team.

----

General facts about automotive diesels:

Truck turbodiesels operate at 90% for much of their lives and are designed to last >15'000 hours. Power-to-weight is however totally impractical for aero use. The latest VW turbodiesels offer ~100BHP/litre, so would easilly achieve 1800 hours (equivalent to ~100'000 miles), if 90% power was rated at say 50BHP/litre.

When laying out a piston engine the first consideration is piston speed. Once skirt loads are established, this very directly determines life of the engine. My belief is that emerging combustion technologies will reduce skirt loading while allowing higher rpms. There are also designs of piston (crosshead) and crank mechanism (epicycloidal) which may greatly reduce piston friction for improved BSP/Hr durability, allowing significantly higher turbo boost pressures. The aim for aero usage (cost less of a factor) would be >2kW/kg, which combined with improved fuel burn (kg/NM), puts it in competition with RR250 (4kW/kg).


Mart

ShyTorque
17th Nov 2006, 23:11
"Summing up Shytorque, powerful but you'd never know they were there. Any specific preference of engine? I'm guessing RR 250 C30P (206 powerplant)."

I was thinking of something a lot more modern and more powerful: Pratt & Whitney PW207Cs are nice, if a little thirsty. :)

Gomer Pylot
18th Nov 2006, 01:46
The RR250 series is very old technology, and I would never want one of them, or two of them. Turbomeca has some very nice engines - light and powerful. The PW207 might do the job, but it's still too early to tell.

Shawn Coyle
18th Nov 2006, 02:17
PT-6 of nearly any flavour would be nice. I just wish that P&W would put a DEC onto all models of them.
As for the RR 250 series - why not a DEC on all models of those - it would stop a lot of issues with hot starts, unexplained run-downs, and the like.
And why not a Wankel of some description for those who want pistons?

Graviman
19th Nov 2006, 08:09
So A109 jockey then, Shytorque? 3000 hours between services is pretty good.

Tech specs on Pratt & Whitney engines:
PT6B (http://www.pwc.ca/en/3_0/3_0_3/3_0_3_1_1.asp) and PW200 Family (http://www.pwc.ca/en/3_0/3_0_3/3_0_3_4_1.asp) from Pratt&Whitney Canada (http://www.pwc.ca/en/3_0/3_0_0.asp) :ok:


Any particular preference on Turbomeca Range (http://www.turbomeca.com/public/turbomeca_v2/html/en/produits/famille.php?aid=615&fid=500) Gomer Pylot? I'm guessing RTM322 (http://www.turbomeca.com/public/turbomeca_v2/html/en/produits/sous_famille_home.php?sfid=509&mid=615)


Shawn, electronic control is certainly cheap enough. There may be parts and certification considerations. Taken from RR 250 (http://www.rolls-royce.com/civil_aerospace/products/helicopters/model250/default.jsp):

The latest FADEC-equipped Model 250 Series IV turboshaft family (Model 250-C30/C40/C47) spans 650-715shp (815shp thermodynamic), and powers the Bell 407/430/206L and MDH MD530/600N.

The trouble with Wankels is the same as 2-strokes, in that the seals passing over ports need lubricating so emissions suffer. While for example Mazda RX-8 is a very good engine, turbocharging is the most effective way to improve power-to-weight. To my mind high boost 4-stroke is the way to go for small powerplants (so piston is almost a turbine comb chamber). The reason i prefer diesel is because the fuel may be injected during combustion, allowing high pressures to be achieved for a given structure. Diesel partload efficiency is also higher than gas since the engine runs "lean", so there is no need to choke.

Mart

Gomer Pylot
19th Nov 2006, 14:38
The only Turbomeca engines I have personal experience with are the Arriel series. Non-FADEC models can be somewhat exciting to start, but once they are running, they do well. We have seen excellent reliability, and very good fuel efficiency. The only problem with the PT6 is that it sucks down huge amounts of kerosene. It's also too big, but that's not a fatal flaw. Burning >30% more fuel than the Arriel is.

mini
19th Nov 2006, 22:56
As regards to diesel piston engine design factors, piston speed is high due to the requirement of high compression ratios - i.e longer stroke, and is not in itself an engine life limiter - take a look at the piston speeds on some of the HGV units. The real factor to focus on would be fuel injection control, older mechanical systems produced high detonation pressures on ignition - hence the knock and the need for substantially beefed up and thus heavier pistons etc to take the increased associated forces.

Modern electronically controlled systems have managed to reduce the ignition pressure wave substantially by carefully controlling fuel introduction to the combustion chamber - manageing flame propagation and reducing detonation, the resultant noise and hence reciprocating mass.

Give the automotive boys another ten years with Rudolph's design and we can finally kiss goodbye to petrol as a heat engine fuel.

Graviman
20th Nov 2006, 18:05
Non-FADEC models can be somewhat exciting to start, but once they are running, they do well.

Gomer, is this just managing the TET or is there another aspect to starting the Arriel (http://www.turbomeca.com/public/turbomeca_v2/html/en/produits/sous_famille_home.php?sfid=504&mid=615)? I've only got a small amount of piston experience (not that i wouldn't consider going further if cash wasn't so tight at the mo).

Any chance of outlining the difference in op between piston and turbine? I've been told that turbines are more-or-less single control. Have you ever found the TET response limitation to be a problem once airborn?

Give the automotive boys another ten years with Rudolph's design and we can finally kiss goodbye to petrol as a heat engine fuel.

I left Longbridge (UK) when it became clear i was never going to get the engine design experience i wanted - i'm sure a lot of talented engineers felt the same :mad: . There are some lines of research which look extremely promising at overcoming the combustion delay inherent to compression ignition. Once this is achieved 4kW/kg may be achievable from a small high boost turbodiesel. I am reduced now to trying to fit in a design study between other activities. :sad:

For sale: One country, slightly downtrodden, by lack government support for long term investments (like education). :yuk:

Mart

Gomer Pylot
20th Nov 2006, 21:57
Arriels tend to be sensitive to throttle inputs on the start. They can wind up very quickly, and need your full attention. The RR 250 series is pretty much the same. PT6 starts, OTOH, are generally uneventful. Once a turbine is running at flight speed, it seldom needs much management - you just adjust collective as necessary, and the fuel control takes care of the engine. You have to keep an eye on limits, and the first limit may be N1, temp, or something else, depending on the engine(s) and the environment.

Blackhawk9
20th Nov 2006, 22:44
Never been in a piston powered Helo and never intend to!
Now turbines ...... my opinion the CT7/T700 leaves everything else for dead exellent power to weight, good fuel burn and reliable (forget obout the couple of problems in the S-92) , fitted to more Helos than any other , S-70 series,AH-1 Cobra,AH-64,EH-101,S-92,Retro fit to B205/UH-1,B214-ST,YUH-61 , SH-2-G, etc, How many thousand helos is that.
Have worked on A/RR 250 series, T-53 ,T-55 ,CT/T58 ,Makila ,Arriel ,PT6T Twin Pac, and none of them come close.

Graviman
21st Nov 2006, 20:44
Interesting Gomer, the startup difference describes the advantage between having DEC and not having DEC well. I could see no reference of PT6 familly having DEC, but assume it is fitted. As you say DEC is fitted to later Arrial 2B (http://www.turbomeca.com/public/turbomeca_v2/html/en/produits/version.php?aid=630&sfid=504&mid=615), on latest Eurocopters:

It features a dual-channel Electronic Engine Control Unit (EECU) with manual backup.

Blackhawk9, with 3000 SHP available in the GE CT7/T700 (http://www.geae.com/engines/military/t700/index.html) i can see why you would want to avoid pistons! It looks like a thoroughbred developed from battle proven technology. Lycoming T-53 (http://www.aircav.com/huey/T53-13.html) and T-55 (http://www.chinook-helicopter.com/standards/areas/engine.html) are only available as spares now, so can't compete. GE T58 ( http://www.geae.com/engines/military/t58/index.html) is just hanging on, but probably showing its design age.

Couldn't see any distinguishing features for Turbomeca Makila (http://www.turbomeca.com/public/turbomeca_v2/html/en/produits/sous_famille_home.php?sfid=508&mid=615). Also not sure which ship PT6T Twinpack (http://www.pwc.ca/en/3_0/3_0_3/3_0_3_3_1.asp) is fitted to.

In general, i'm impressed (as usual) that helicopter pilots have such a profound knowledge of their aircraft. I guess the engine does maketh the machine...

Mart

Graviman
29th Nov 2006, 11:37
Helifolk,

One more query about turbines for helis:

Do non-DEC turbines (like RR350 or early Arriels) require any trimming on the collective correlator, or is N1/Nr auto controlled? Is any trimming required for fast yaws? I seem to remember that a free power turbine has a very "stubborn" torque vs RPM gradient to resist RPM changes.

Mart

Gomer Pylot
29th Nov 2006, 14:33
The PT6s I've flown have no DEC, they just start cooler and with more inertia than most.

Turbine engines have a governor which controls engine and rotor RPM. There may be some droop if you pull lots of collective quickly, but the rpm recovers quickly, typically within 3 seconds or less. Like a Ronco machine, you set it and forget it.

diethelm
29th Nov 2006, 14:54
One of the considerations may be cost to operate per hour per shaft horsepower. RR250 engines are an old design but they are fairly inexpensive to operate per hour compared to others as there are hundreds of shops who can rebuild and repair compared to other brands which have to go to factory repair and overhaul facilities. Also, the RR250 always leaks so the oil is always fresh...........:D