Static RPM

[lukyduky81]

Hi guys, sorry if i'm publishing this topic in the wrong place ,
I have a question concerning a static RPM during the roll on takeoff, in the Cessna172 the static arc range is 2280 to 2400 RPM, i want to know what causes the difference of RPM in flight resulting of rising to even 2700 RPM?
Thanks

[evansb]

Other than the obvious throttle effect, I suspect "inertial engine load tachometer cable stretch". Install a new tach cable.

[+TSRA]

Hi lukyduky81,

I've inserted text from "The General Aircraft Technical Knowledge, Third Edition" published in 2000 by The Aviation Theory Centre, New Zealand, Page 8-2 (Propellers). I've also put proper APA citation below to cover myself even further:

Another disadvantage of the fixed-pitch propeller is that, at a given throttle setting, changes in airspeed have an effect on engine rpm. With a decrease in airspeed, the propeller angle of attack is increased. Accordingly, propeller torque also increases resulting in a drop in rpm. Conversely, if the airspeed is increased with a constant throttle setting, the propeller angle of attack and torque reduces, resulting in an increase in rpm. In most cases where a steep nose-down attitude is adopted, the throttle will have to be fully closed to prevent an engine overspeed.

Hopefully that clears things up. IIRC all C172's have fixed pitch props but the same principle applies to Variable Pitch Props where the throttle and pitch are maintained. For those though the CSU will adjust for the increase/decrease to maintain rpm. So to the pilot it doesn't seem like the principles apply, but they do.

If that doesn't clear it up its triangle of velocities time!

Citation

Wagtendonk, W., Boys, S., Wagtendonk, J. (2000, January). The Commercial Pilot's Series: General Aircraft Technical Knowledge. Third Edition. Aviation Theory Centre (NZ) Ltd. Tauranga, New Zealand

[lukyduky81]

Thanks a lot for both of you,
+TSRA, i noticed you always give me a great and straight answer
Thx a lot once again

[+TSRA]

Haha...I find the discussion here is often more fruitful and professional than on the other Canadian website, even if that one does get way more traffic.

Thanks lukyduky81, I try my best to give the right answer.

[Mostly Harmless]

Altitude. While you will get 2400 RPM out of a C-172 at sea level, you won't get 2400 RPM out of that engine at an airport located at 4000 feet asl because the engine is normally aspirated (not turbo/supercharged) The information is in the C-172 POH if you dig deep enough... therefore it is a range of acceptable power depending on altitude.

Sorry, upon rereading your question, the speed is what causes the rest of that difference in RPM, as stated by +TSRA

[Pilot DAR]

Luky, Consider that when you open the throttle on a fixed pitch prop, stationary on the ground, the blades are fully stalled - they are churning up the air, while hardly creating thrust. As the plane accelerates, the blade angle more and more matches the angle of the air entering the prop disc. This is like the plane recovering from a stall.

If you think of how much power it takes to fly slow flight right near the stall, compared to how fast the plane will fly with the wing not at all stalled, it's about the same thing. If you've got the prop blades in slow flight, nearly stalled (or fully so) it will take lots of power, so you won't get the static RPM. Once the prop, or the wing recovers, and is less stalled, the drag reduces, and it will speed up on the same power...

[iflyforpie]

Typically you will get a normal static RPM at higher altitudes if you lean the mixture out. While the engine has less power, there is also less air resistance on the propeller.

[fujii]

The OP asked about static RPM during the roll on takeoff. The static RPM is obtained when engine is stationary on the ground, once the aircraft rolls it's no longer static RPM.

[m39462]

Fujii, not exactly. He asked if the static range only goes up to 2400, then how can you get up to 2700 in flight? A question +TSRA answered, followed by sporadic answers to questions that lukyduky did not ask!

[iflyforpie]

fujii

My statement was referring to static (on the ground) RPMs at high DAs.... like if you were taking off out of Leadville CO (>10,000 ASL) on a hot day.

[lukyduky81]

Originally Posted by Pilot DAR

Luky, Consider that when you open the throttle on a fixed pitch prop, stationary on the ground, the blades are fully stalled - they are churning up the air, while hardly creating thrust. As the plane accelerates, the blade angle more and more matches the angle of the air entering the prop disc. This is like the plane recovering from a stall.

If you think of how much power it takes to fly slow flight right near the stall, compared to how fast the plane will fly with the wing not at all stalled, it's about the same thing. If you've got the prop blades in slow flight, nearly stalled (or fully so) it will take lots of power, so you won't get the static RPM. Once the prop, or the wing recovers, and is less stalled, the drag reduces, and it will speed up on the same power...

Thanks a lot for this simple answer

[lukyduky81]

Thanks a lot guys i got it