Learn something new everyday...

[UnAnswerd]

I thought I knew most of the absolute basics involved in flying. But today, I learned something entirely new. I was told that durring a X-country, you typically fly at full-throttle. I was pretty surprised. Even a textbook as good as Machado's hadn't made any mention of this. Durring those short flights to the practice area, I've always been told to cruise at 2350RPM in the Cherokee 140. Maybe I was wrong in assuming that 2350RPM was a "cruise" setting. Nevertheless, flying for long distances at full-throttle seems pretty intense. I guess those XC's are gonna be LOUD.

 

[vclean]

Who says you fly XCs full throttle? Full throttle gives you a few knots airspeed for a lot more gas. Full power for takeoff, aborted landing and some airwork, cruise power settings otherwise.

 

[Wasted]

I think you only got half the story there. You generally fly at 75% power in a typical general aviation trainer. However, to obtain 75% power at, say the 7500' cruise altitude you may use, you may have to place the throttle in the full open position.

When you go out to the practice area at your typical 3000' altitude, your 75% is represented by your pulling the throttle back to 2350rpms. At higher altitude, the derating of power from your engine will require wider and wider throttle settings to maintain that rpm.

 

[Yellow Snow]

Yep full throttle with no turbo at 7500 may only be 65% depending on rpm setting (constant speed prop). Big difference between full throttle and full power.

 

[Goose Egg]

You may have noticed this already, but in addition to what Wasted said, the throttle setting you'll need to maintain 2350 RPM at 75 kts and the throttle setting you'll need to maintain 2350 RPM at 100 kts at any given altitude are different. This is because of prop efficiency--a fixed pitch prop can only reach maximum efficiency at a certain range of airspeed.

The Power Available (PA) and Power Required (PR) curves come into play here, but the discussion is a little complex for me to describe without PA and PR curves to point at. Check it out in the Jepp Inst/Commercial Manual, Aerodynamics for Naval Aviators, Flight Theory for Pilots (by Dole), among other places.

-Goose

 

[UnAnswerd]

I think you only got half the story there. You generally fly at 75% power in a typical general aviation trainer. However, to obtain 75% power at, say the 7500' cruise altitude you may use, you may have to place the throttle in the full open position.

When you go out to the practice area at your typical 3000' altitude, your 75% is represented by your pulling the throttle back to 2350rpms. At higher altitude, the derating of power from your engine will require wider and wider throttle settings to maintain that rpm.

I see what you're saying. A naturally-aspirated engine will produce less power at higher altitudes unless the throttle is adjusted to compensate. In fact, at 300,000'MSL, I believe you will have 0% power at full-throttle.

The charts for the cherokee do indeed include 75% power when listing factors such as fuel consumption and TAS. Of course, just by looking at the charts, you can see that such figures are dependent on altitude. I believe these are the charts I will be using when I start to plan actual flights...

I think the reason I was surprised by the "full-throttle" comment, was because I failed to understand that 75% power does indeed occur at less than full-throttle when at 3000'MSL, which is about as high as I've ever been anyway...

Thanks for clearing it up

 

[Kream926]

i dont know if its already been posted but here goes

full throttle doesnt nessisarily mean full power. the throttle may physically be at the full forward position. i dont have that much time in a warrior but in the RG or the arrow, the throttle will be full up or in and only be getting 20-23" of MP.

you can always look in the POH cruise performance chart and see what that tells ya










tony c

im gettin some pop-corn ready

 

[NuGuy]

Obviously you guys never heard the term "renter's cruise"

Nu

 

[wmuflyguy]

yea no kidding, most the time you are paying by the hour, not by the gallon. Keep that sucker full open and get there as quick as possible

 

[Flyin Tony]

I never (most of the time) pull the power back in my plane well going on a XC. But I never see 25in MP.