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How do you meter power in a turbo recip?


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I was sitting here thinking about power gauges and different helicopters. Turbines have a torque meter, reciprocating engines have inches of vacuum or manifold pressure. Then I wondered what gauge a turbo charged recip engine would have. I imagine the boost would vary by altitude and engine speed (although fairly constant).I have heard the turbo is set up to boost more at high altitude and less at low altitude to keep engine performance somewhat constant regardless of altitude. This makes me think the boost would vary quite a bit. Do they just have a boost gauge in place of the manifold pressure or is there some sort of torque meter?

 

-Thanks

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This is just my theory on it, so take it for what it's worth.

 

Seems like if they set it up to provide more boost at higher altitudes, and less when lower, the pressure being forced in would be the same. So a MP gauge would still work, the values would just be higher.

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just manifold pressure gauge... I don't think any turbo or supercharged aircraft has ever had a boost gauge (usually in psi) It is always manifold pressure just correlated to the amount of manifold pressure useable over standard pressure.

 

29.92 manifold = 14.5 psi

so if you're used to automotive "boost" gauges you might recognize 6psi or 10 or 12psi boost. That would correspond to 42.04, 50.1, or 54.16 inches manifold respectively.

Most aviation applications use relatively little boost (to prolong wear, high cylinder temps, and a wide detonation margin.

The Enstrom F28C for example uses 36.5" to produce 205 HP

The later F and FX use 39" to produce 225 hp.

The Scotts Bell 47 G3B1 uses 32.8"

Aviation is usually turbocharching to keep from losing power as we ascend the atmosphere... NOT to produce more power... but that is a generalization.

I believe some of the P51's at Reno are boosted over 100"mp!

 

The boost more at high alt. vs low is a more detailed explanation of what is going on... No, more boost is not created at high altitude.

1. The turbo has an easy time at sea level making whatever map is requested.

2. As the aircraft ascends the wastgate closes more to maintain the required "boost", so the turbo is working harder but not "boosting" more.

3. As altitude increases and turbo works harder, more heat is created and inlet air temp increases... in a non intercooled installation Inlet air temp may approach 200 degress F in the mid to high teens in a mildly boosted application.

Edited by apiaguy
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Thanks for the info. I've only flown the R22, R44, 300, and 47...never anything turbo or turbine and it hit me today to wonder what the gauge would be when turbocharged. I looked at gauge clusters online and some of them didn't even have a manifold pressure gauge so I was a little mystified. Man I'd love to fly that Bell 47 B31. I took my private checkride in a 1952 Bell 47 D1 model. Awesome aircraft, but a little more power would be cool too.

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The Enstrom which is turbocharged has only the manifold pressure gauge. The turbo increases airflow into the engine which increases manifold pressure. As I understand it, the Bell BH47G3-B2 was turbocharged and had a different gauge, although I have never seen one.

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Hi

 

The PWR is measured with a manifold pressure instrument, same as in your normally aspirated engine.

 

Have a look at our normally aspirated engine MP when the engine is off. What does it show? pretty close the same as your altimeter setting;-) About 29.92at SL. When you start your engine it generates suction, thus also called suction engine. The throttle is controlling airflow in the manifold, restricting the airflow into the engine. When its fully open the MP reaches its max. This is less than your Altimeter Setting because of losses due internal friction in the gaspath, airfilter etc....

 

Now you take a turbocharged engine. What does the MP show when the engine is off? Yep, the same Altimeter Setting eg. 29,92 at SL. Now crank on the beast and look what happens, yep, the throttle resticts airflow and the MP drops.

 

Here comes the difference. When you open the throttle the MP can reach higher values than the current athmospheric pressure, because the Turbo pushes air into the manifold. A supercharged engine can have max MP of 35 inches, so thats abot 10 inches more than your robbie has throttle wide open.

 

You just have to be a bit more carefull with engine control, big movements in throttle cause big changes in MP and may cause massive overboost. Same when decending with constant throttle setting. Denser air will increase your MP the lower you get and may lead to overboost. Also quick decents in idle are to avoid, this cools the really hot turbocharger and may cause shockcooling which cracks in case as it puts huge thermal load on the engineparts.

 

Turbos do fail. Means massive PWR loss, instantly. This is why they dont suit for Helicopter ops. Rather bigger bore than complex tech.

 

Lars

 

 

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Here comes the difference. When you open the throttle the MP can reach higher values than the current athmospheric pressure, because the Turbo pushes air into the manifold. A supercharged engine can have max MP of 35 inches, so thats abot 10 inches more than your robbie has throttle wide open.

 

You just have to be a bit more carefull with engine control, big movements in throttle cause big changes in MP and may cause massive overboost. Same when decending with constant throttle setting. Denser air will increase your MP the lower you get and may lead to overboost. Also quick decents in idle are to avoid, this cools the really hot turbocharger and may cause shockcooling which cracks in case as it puts huge thermal load on the engineparts.

 

Turbos do fail. Means massive PWR loss, instantly. This is why they dont suit for Helicopter ops. Rather bigger bore than complex tech.

 

Lars

 

The Enstrom 28F and 280FX go to 39 inches. And I have operated engines that go to 55 inches of manifold. Plus I have heard that the engines at the Reno air races go well above 70 inches.

 

Shock cooling is an issue. Usually more of a problem for the cylinders than the case. Unless you do a lot of autos, it shouldn't be too much of a problem in helicopters as the pilot will normally keep the power up.

 

One of the biggest issues for pilots during power changes is 'turbo lag' or 'boot strapping'. What happens as the pilot increases power, the increased exhaust, spins the turbo faster, which increases the manifold, which increases the power, which causes the pilot to reduce power, which decreases exhaust, which slows the turbo, which decreases the manifold pressure, and so on. The best way to handle that is to make slow power changes and stop the power change at a point just prior to your desired power setting. Each machine is different and it will take some learning on your part.

 

Yes you can overboost the engine if you are not careful. None of the turbocharged helicopters that I know of have an automatic wastegate to protect the engine. Turbocharged engines take some planning to operate properly, just like the helicopter itself.

 

Since every turbocharged helicopter I have heard of is a single engine machine, wouldn't a partial power loss be better than a total engine loss? Turbocharged helicopters are a specialty type helicopter. They work well in high and hot areas. Cases in point, are the Enstrom F28F and 280FX. They are still marketed and sell steadily. The Bell BH47G3-B2 is a highly desirable helicopter. Robinson and Schweitzer use a flat-rated. It works well, to a certain altitude. Above that the turbocharged helicopters are ahead of the game. And I wouldn't consider a turbocharger complex tech. They use them in Detroit cars and have for years. Plus it is much simpler than a supercharger.

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Hi

 

Never flown a turbo heli but many, many fixed wings. I just tried to explain simply how power is measured in turbocharged engines. I know that you know how this really works:-)

 

Did many moons ago para ops with c206/c207 and there the shockcooling is an issue. Sometimes its the rear cylinders, sometimes the case of turbo. The engines with intercoolers seemed to be most vulnerable. Depends how the airflow under the cowling is ducted, use of cowlflaps etc..

 

I basically agree with everything exept the reliability of turbocharged compared to normally aspirated.

 

TC will always have more issues than an nomally aspirated engine.

 

Higher MP means higher mean pressure on piston if same power is generated. This puts more stress on crank, pin, case, bearings etc

TC runs hot. Red hot. Has to be cooled with oil, will put a lot of thermal stress on engine.

More moving parts means more possible failures.

I dont even go into the possibilities of engine missmanagement...

 

If you take an normally aspirated engine with 300HP and and turbo with 300HP the difference will be that the first will have more displacement. Also less economy and will be more reliable. In a single engine aircraft this would be my pick. Flatrating is nothing else than carrying a bigger engine than necessary to have some spare when operating hot/high. Engine displacement vs. MP generated by TC. There is no substitute for bigger pistons:-)

 

The partial EF will usually be due to the seizure of the turbine in the TC. This means very restricted airflow into the engine. I had it in a C206 and it went down like a brick. Not my choice in a single engine heli.

 

Ps. The 70inch MP engine at Reno air race is a great piece of engineering, has awesome sound but is not really reliable :-) But i still would chop off a finger just to fly an hour the Bearcat or the Mustang.

 

 

Blue skies

 

Lars

 

 

 

 

 

 

The Enstrom 28F and 280FX go to 39 inches. And I have operated engines that go to 55 inches of manifold. Plus I have heard that the engines at the Reno air races go well above 70 inches.

 

Shock cooling is an issue. Usually more of a problem for the cylinders than the case. Unless you do a lot of autos, it shouldn't be too much of a problem in helicopters as the pilot will normally keep the power up.

 

One of the biggest issues for pilots during power changes is 'turbo lag' or 'boot strapping'. What happens as the pilot increases power, the increased exhaust, spins the turbo faster, which increases the manifold, which increases the power, which causes the pilot to reduce power, which decreases exhaust, which slows the turbo, which decreases the manifold pressure, and so on. The best way to handle that is to make slow power changes and stop the power change at a point just prior to your desired power setting. Each machine is different and it will take some learning on your part.

 

Yes you can overboost the engine if you are not careful. None of the turbocharged helicopters that I know of have an automatic wastegate to protect the engine. Turbocharged engines take some planning to operate properly, just like the helicopter itself.

 

Since every turbocharged helicopter I have heard of is a single engine machine, wouldn't a partial power loss be better than a total engine loss? Turbocharged helicopters are a specialty type helicopter. They work well in high and hot areas. Cases in point, are the Enstrom F28F and 280FX. They are still marketed and sell steadily. The Bell BH47G3-B2 is a highly desirable helicopter. Robinson and Schweitzer use a flat-rated. It works well, to a certain altitude. Above that the turbocharged helicopters are ahead of the game. And I wouldn't consider a turbocharger complex tech. They use them in Detroit cars and have for years. Plus it is much simpler than a supercharger.

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