Help with RPM & Airspeed

[Milton1995]

When i proceeded to perform slow flight manouvers today i completed the HASELL check, and with the carb heat out reduced power to 1700 RPM ( c152) I noticed that when the tas decreases so did my RPM, therefore i left it at about 1800 rpm and at around 55kts it had decreased to 1700RPM. I noticed this also happened whilst increasing RPM, i.e aiming for 2200RPM after recovery i would use 2100RPM to start and it would increase naturally. I look at the RPM/TAS graph with the wind component and saw the AOA increased when at slower speeds. Can anyone help explain this to me, thanks alot, Mark.

[gfunc]

The change in RPM you are seeing is due to the effect of the airflow turning the prop (windmilling) - the effect increases with speed. As you slow down you would expect the RPM to decrease from the value you set with the throttle and conversely as you speed up the RPM would inch upward on its own.

Sometimes it's quite surprising how big an effect this is - in the aircraft I regularly fly (Alpha 160 (Robin 2160)) you have to work rather hard to stop the engine redlining when starting an aerobatic figure. Similarly if the engine stops during a prolonged spin all you do is dive to 120kts and the engine fires back into life like magic!

Hope this helps,

Gareth.

[moreflaps]

Here's a way to understand the change in RPM: As you speed up the angle of attack of the blades decreases so there is less induced drag on the prop (and vice versa). Less drag means the engine speeds up.

Hope this helps.

[Genghis the Engineer]

When i proceeded to perform slow flight manouvers today i completed the HASELL check, and with the carb heat out reduced power to 1700 RPM ( c152) I noticed that when the tas decreases so did my RPM, therefore i left it at about 1800 rpm and at around 55kts it had decreased to 1700RPM. I noticed this also happened whilst increasing RPM, i.e aiming for 2200RPM after recovery i would use 2100RPM to start and it would increase naturally. I look at the RPM/TAS graph with the wind component and saw the AOA increased when at slower speeds. Can anyone help explain this to me, thanks alot, Mark.

Firstly, you have no TAS indicator, and no AoA indicator on a C152.

We can assume that IAS is about the same as CAS however for the present purpose, and actually this is fairly irrelevant.

The propeller is a thrust generator, but it's also a windmill. If you cut off fuel to the engine, and dive it, the propeller will still go around, and the faster you dive it, the faster it'll go around. This is how you re-start the engine if you have an engine failure and don't want to / can't use the starter.

You can add that effect to the fact that the propeller is driven by the engine. So, at the same power, the faster you fly, the higher RPM you'll get because the propeller and engine are directly linked.

You can put lots of maths on it, but it all basically comes back to that.

G