Grob Tutor question

[Riskman]

I've mentioned elsewhere that I'm an instructor with the Air Cadets hence the question.

Part of the syllabus covers the Grob Tutor and the engine controls are described as Throttle, Mixture and RPM.

Throttle and mixture I'm happy with but the book doesn't describe what exactly the RPM control does. I thought the throttles did that ie push forward = increased rpm so I assume it's something to do with the prop.

Does the RPM control enable the pilot to adjust the prop rpm through a gearing mechanism? Is this the modern equivalent to coarse/fine pitch?

Thanks in advance

[Fg Off Max Stout]

You are thinking of traditional fixed pitch props. The Grob has a constant speed / variable pitch prop. Basically...

The RPM control sets the desired rotational speed of the engine and propellor. The prop is in most cases bolted directly to the engine and hence they turn at the same speed. A constant speed unit (CSU) adjusts the pitch of the propellor blades, changing the thrust produced and torque required, to maintain a constant RPM.

The throttle simply controls the power output of the engine.

So, open the throttle, more power is developed by the engine, the prop RPM will try to increase, but the CSU will coarsen the pitch of the blades, producing more thrust and requiring greater torque absorbing the increased engine power, thus maintaining RPM at the selected value. Consequently in simple terms throttle controls thrust, RPM controls well, er, RPM.

To take it to the next level of geekiness, a variable pitch / constant speed prop is more efficient because it allows the prop blades to operate close to their optimum angle of attack over most of the airspeed range and the engine can be made to run at its most efficient RPM despite changes in aircraft speed and power output. The CSU works mechanically using centrifugal weights opposing oil pressure and spring force. As I said, in most cases the prop is bolted directly to the engine and they turn at the same RPM. In some aircraft the prop may be geared so that its RPM is directly proportional and in a single fixed ratio to that of the engine. You will not find gearing as on cars with several selectable ratios. The CSU / variable pitch prop has a performance effect in some ways equivalent to a car gearbox, but achieves the result in a diferent way, as described above.

[abbotyobs]

I have a different Grob Tutor question, if perhaps someone could help.
I am current on the Grob Tutor as an AEF pilot.
Anyone know a Grob Tutor instructor who is also a CAA examiner since I would like to get a SEP rating added to my licence.

Thanks very much in advance.

I need a SEP added to my fATPL.(Only have MEIR, not MEP, did it in euroland not UK!)

[stevef]

Good explanation here:
http://answers.yahoo.com/question/index?qid=20070713185435AAdZbJ6&show=7

[Jucky]

I believe some of the Civilian Instructors at 727 NAS at Yeovilton are CAA Examiners.

[abbotyobs]

Jucky,
Thanks very much, will try and call them next week.
Regards
Abbotyobs

[Riskman]

To Flg Off Max Stout, thanks very much for giving an answer at exactly the right level for me, and to Stevef, thanks for the link which also covered a supplementary question on manifold pressure.

Cheers

Riskman