Increasing performance through power

[Kingfish]

Hey all,

 

I'm a turboprop guy, but have a question for those knowledgeable in helicopters. If you take a helicopter and hang more powerful engines on it, you increase performance - but how? Does the enhanced machine generate greater lift/performance by higher rotor rpm, or is there another explanation? Far as I know rotor rpm is a pretty narrow band, and an increase just puts tip speed that much closer to mach and loss of efficiency.

[vortamock]

Oooh, I would love to know the answer to this question too!

 

I always kinda thought it would allow you to overcome the extra drag and pull more blade pitch while maintaining rotor rpm. My problem with this thought was that you can apparently only pull so much pitch before

 

A: stalling the blade or

B: hitting a mechanical limit¿

 

Then I thought... well... You would be able to have larger rotor blades. I dunno how customizable helicopters are though. I'm guessing that's probably a highly regulated aspect of them. Probably not changeable by an owner or operator.

 

So that leaves me wondering, like in the example of the mosquito, what exactly is the effect of having more power? Obviously more weight capacity and higher speeds based on the spec sheets, but how exactly?

[terminal_velo]

Long story short, you'll be able to operate at higher angles of attack because you have the power to overcome the drag produced by those higher AoAs.

[Eric Hunt]

Rotor RPM stay constant, at (usually) 100% of the design RPM. Some (like the S76B) operate at 107%. Kinky.

 

As the AoA increases, to either climb higher or carry more weight, the drag increases, so the extra engine power is still able to keep the RRPM constant. But as power goes up, so does fuel burn, so you need to carry more fuel to stay up for the same time, more weight needs more power, blah blah. It's a vicious design circle, and each helicopter type is a compromise of engine size, desired performance, and speed and range.

[akscott60]

You can pull more pitch and not droop the rotor.

 

Just imagine what kind of power my CH47F has with 10,000hp. Its eye watering.

[Guest pokey]

The transmission is only rated for a certain power also, so? Lets say your transmission is rated for 500 hp, and you now put in a 1,000 hp engine, you are still limited to the 500, but? your hot day and high altitude performance be much more fun with the bigger engine.

[Kingfish]

Thanks guys; the explanation is much appreciated.

[Kingfish]

You can pull more pitch and not droop the rotor.

 

Just imagine what kind of power my CH47F has with 10,000hp. Its eye watering.

 

Or the 22,500shp on the CH-53K my company is testing

[PondJumper]

Not to mention with more power comes more torque. With that comes the need for a new tail rotor, new drive shafts, etc.

[vortamock]

Awesome sauce! Now we know with certainty.

 

Isn't the CH 47 the fastest (top speed) in the (US Army) fleet? Since it's pulling mega power and retreating blade stall proly isn't much of an issue?

[akscott60]

Awesome sauce! Now we know with certainty.

 

Isn't the CH 47 the fastest (top speed) in the (US Army) fleet? Since it's pulling mega power and retreating blade stall proly isn't much of an issue?

 

Well, tandem rotor helicopters have a different set of limitations. Aerodynamic principles such as ETL and Transverse flow have essentially zero effect. I have a gauge in my cockpit called a "Cruise Guide Indicator." It shows real time stress of the dynamic components of my rotor system. When the needle starts pointing in the yellow band and red band, I need to decrease rotor loads. So other than strict -10 airspeed limitations or PPC calculations, the cruise guide indictor is the "real" limit of forward speed.

 

I have routinely flown greater than 150 kts in the CH47.

 

The real strength of the CH47 in speed is the ability to fly fast while heavily loaded and at altitude.

[Wally]

 

Well, tandem rotor helicopters have a different set of limitations. Aerodynamic principles such as ETL and Transverse flow have essentially zero effect.

...

 

I'll bet you meant "LTE"...

[akscott60]

Eh, I meant you dont feel ETL or Transverse flow thru the aircraft other than a reduction in TQ required for a desired climb.

 

LTE is difficult without a tail rotor....haha

[Guest pokey]

tandem rotors have always intrigued me, i built an r/c ch47 about 30 years ago, never flew it tho. And oh how i love the old HUP retrievers ! My friend restored an old retriever he found in (either switzerland or sweden), brought it back to the states, completely went thru it, beauty of a ship when he was done! sad,, he hit some powerlines on way to airshow with it in Ca and that was the end of him, couple others and the retriever. He had one funny story about a French pilot that he talked to about flying the old HUP, to quote the French pilot about the early development of the Sperry autopilot "the thing works great on the ground, but turn it on in flight?====suicide!"

 

How does the ch47 fly without the autopilot engaged? sideways??

 

If you notice the old HUP's "barn door" rear vertical stabs, that was B4 the Sperry system & and attempt to make the puppy fly nose 1st.

[akscott60]

Our DAFCS makes the aircraft very easy to fly. Pitch hold, airspeed hold, bank angle hold, etc etc. All automatic.

Turn it off, and, well, she wants to fly sideways on her side, like a turboprop twin. Haha.

[Guest pokey]

. Pitch hold, airspeed hold, bank angle hold, etc etc.

 

that is very interesting, so? you can hold a different nose pitch, and still keep different airspeed? So it must use differential collective along with the tilting for/aft of the swash plates?

 

the ole "pitch controls airspeed, throttle controls altitude" is out the window? very cool !

 

BTW? that is the way i set up the control system in my r/c '47, just never got to try it out.

 

one thing i always wanted to do also? get 2 hughes 300's, remove the tailboom from one & bolt it to the other, problem was getting the 2nd transmission to spin backwards. and yes ! i mean real 300's,, not r/c models.

 

ever see that comedian ? Ghallagar?? he had stuffed poodle with 2 heads and said "ya ain't gonna sneak up on THIS puppy"

[vortamock]

Those CH47's are stange and interesting aircraft indeed.

[akscott60]

I will start a new thread.

[Jaybee]

In regards to the original question - changing nothing else and adding a little power essentially just raises your altitude ceiling. It's why many Bell 47 operators would swap their 200 hp Franklin engines out for the Lycoming engines ranging from 220 - 280 hp.

[Joe_P148]

Hey all,

 

I'm a turboprop guy, but have a question for those knowledgeable in helicopters. If you take a helicopter and hang more powerful engines on it, you increase performance - but how? Does the enhanced machine generate greater lift/performance by higher rotor rpm, or is there another explanation? Far as I know rotor rpm is a pretty narrow band, and an increase just puts tip speed that much closer to mach and loss of efficiency.

So basicly, the higher HP engine is putting out more SHP. Probably burning more gas to do so, but that's another issue.

 

Typically, a higher performing engines high speed shafts is spinning faster, all of that is stepped down by gearboxes to keep the rotor spinning at 100%. In modern / advanced helicopters the governed / ECU / FADEC does this for us, we demand more power and the ECU dumps fuel to keep the rotor speed at 100.

 

So as you go up in altitude, the engine has to work harder due to the increase in pitch required. Thinner air. Also, the compressor section of the turbine engine is working harder as well, thin air effects those blades as well. Your higher performing engine can output more HP at higher altitudes, essentially, by spinning faster.