Cruise RPM

Thanks Kevin, I am indeed in the UK so this explains the 4800rpm limit.

I've actually just ordered an E-Props prop which is what got me thinking about cruise RPM after reading their installation and setup instructions.

From what I understand this is not a technical restriction but a regulatory one. It was explained to me once by someone at the LAA, the requirement was something like: the aircraft must be able to fly at MAUM for a minimum of 2 hours with a 1 hour reserve. To achieve this, the maximum engine speed had to be restricted to redice the fuel consumption to be compliant.

I don't know the source of this requirement, it micght be in CAP482 BCAR section S or something similar.

Hi Kevin,

If your 912 is capable of delivering 5800 rpm (ROTAX Max for 5 minutes) it will also be able to run at anywhere up to 5500 rpm Rotax Max Continuous.

Your 4800 rpm is not an engineering Max continuous - it can only be PIC instigated/limited.

"It also lists the acceptable propeller types and specifies the pitch setting so there is no scope for adjusting it to obtain a higher WOT RPM"

This statement is in direct contradiction with  "2. Max engine RPM: 5800" which is Max WOT engine speed.

"5. Max airspeed flaps extended: 77 mph" has nothing to do with engine performance and likly aircraft performance.😈

Sean, I think you misunderstand the situation in the UK, you cannot just set your prop pitch to whatever you like, in my case the specified pitch results in a static rpm of 4800.

The limitations apply to the aircraft plus its engine, clearly some of them have no bearing on the engine, I just included them so that you had a sample of some of the other limitations that are specified for an aircraft operating on a permit to fly.

The max engine speed that is specified is not the WOT RPM in level flight. In my aircraft you have to put it in a dive to reach 5800rpm where you reach Vne at about 146mph.

In a level cruise you can indeed open the throttle so that the rpm increases beyond the max continuous of 4800. In my airframe WOT in level flight results in about 5300rpm. There is clearly nothing to stop pilots from doing this but they need to be aware that they will no longer be compliant with the limitations of their permit. There is no definition of 'continuous' in this context but I would take it to mean 'for prolonged periods'.

The 5800 maximum is the one imposed by the engine manufactuer and it is there to say that you should never exceed this no matter what you are doing.

I understand all of the technical reasons why it would be beneficial to have a finer pitch on the prop but that is just not allowed in our jurisdiction. There are potential consequences for not operating an aircraft as per its permit limitations such as suspension of the permit and invalidation of insurance.

I hope that clarifies the situation.

Hi Kevin,

Thank you for that explanation.

It's your business/country and I am sure you are aware that limiting your 912 to below 5200 rpm Static may have undesirable repercussions for engine service life and more importantly your life.

Your life because the engine, so configured, is unable to deliver Max Take Off Power, so extending Takeoff Ground Roll & reducing Climb Out performance - seems to me that your "Permit" providers are compromising your safety.

I would be intersted to know the origins and intention of this legislation, that has a potentially negative impact on pilot/passenger safety and shorten engine service life. This is not the Forum to do this.😈