Rotax CSU Operation

[Lantirn]

Flying a Katana DV20, with CSU, but I think the question applies to all CSU pistons.

Regarding its operation,

I know that in a climb we should put the RPM at MAX, or at max continuous RPM, and then full throttle.

In a descent, I am wondering which is the correct way to fly it.

1)Increase the RPM Max (or near max), and then reduce throttle MP as required?

2)Or first reduce throttle MP as required and then increase RPM? (or leave the RPM as required depending on what descent you want)

Tought by an instructor to do the first way, allthough in my understanding of engines, my way was firstly reduce the throttle MP as required (for a fast cruise descent with 500fpm lets say half the MP), and leave the RPM in the cruise setting, in which after some time before landing I bring the RPM to MAX.

Instructor told me that in the 2nd way the engine "cannot handle" this RPM, and that I should allways bring RPM Max or near max and after that adjust the throttle MP as required.

Flight manual is not as clear as I wanted to.

Can anyone clarify please?

[barit1]

Not specific to the Rotax, here is several decades of general practice:

1) At top of descent, reduce throttle slowly but leave prop as-is (cruise rpm).

2) In the traffic pattern, along with landing checklist (flaps, undercarriage,...) set prop to high rpm. This will increase drag, giving a steeper approach angle, AND give you faster response in event of a go-around.

As I stated, this is a generic procedure. What does Rotax say about it? I should think if they discourage the above, would have publicized it.

[Mach E Avelli]

At top of climb reduce MP first to about cruise value, set RPM to cruise, then re-adjust MP to more accurate cruise value (it will have changed with the RPM reduction). At top of descent, the common practice is to leave RPM at cruise and reduce MP. On final approach, increase RPM to maximum (in case of a go-around). Experienced pilots will tend to do this fairly late on approach to reduce noise and drag, and with some geared engines to minimise time with reverse loading on the reduction gears.

Having said all that, I am not familiar with the Rotax, other than I understand that it runs a reduction gearbox which may have some unique loading or RPM limitations. Your instructor may have a technical reason for going to max RPM prior to descent. Ask around.

Also, is the Rotax set-up a true constant speed unit or just an in-flight adjustable pitch prop? Not that it would make much difference to the above-described technique unless there are specific RPM bands to avoid.

[hvogt]

With the previous two posts in mind, you will certainly find the manual much clearer than before:4.4.10. Descent

[...]
2. Throttle as required
3. Propeller Speed Control Lever 1900-2400 RPM
[...]

NOTE

To achieve a fast descent:
Propeller Speed Control Lever 2400 RPM
Throttle IDLE

[Volume]

In the traffic pattern, along with landing checklist ... set prop to high rpm. This will increase drag, giving a steeper approach angle,It is not a wise idea to use the propeller as a brake on the Rotax. If possible, avoid driving the Rotax by the propeller. Rotax drives back the oil from the crankcase to the reservoir by blow by gasses. Blow by only happens if the average pressure inside the combustion chamber is higher than in the crankcase, hence the engine drives the propeller. Driving a Rotax by the propeller will increase the oil level in the crankcase, cause the crankshaft to touch the oil, creating air bubbles in the oil. Air bubbles in the oil will kill the hydraulic valve lifters (at least in the long term).
It is fine to set the CSU to max RPM once you have reduced speed in the pattern, but leave it at cruise RPM during decend and deceleration.

[Lantirn]

Thank you for your replies.

In further searching, I found that if the engine is propeller moved, you kill the oil piston rings because of lack of the pressure in cylinder, causing the rings to move temporarily from their original position.

[Mach E Avelli]

So your instructor has it wrong. Does he own an engine overhaul business, perchance?