914F RPM - MP settings

Look in the 914 Operators Manual, Section 5,  Performance Data, 

Hi Bill,

Thanks for your reply.   I was aware of the various curves in the operating manual but, when you look at the performance graphs Figure 1 9HP VS rpm) and the Pitch Propellor chart, they don't agree very well partifularluy at 55%;  eg;

at 4300 rpm, the pitch prop chart shows 55% and 28" Hg MP, but the performance graphs show 46% and  about 27.7 " MP at 4300 rpm

at 4800 rpm, the pitch prop chart shows 65% and & 29" butvthe perf graphs show 65% and 30.3 in

The 5000 and 5500 rpm conditions agree

I presume it is OK to interpolate linearly between the data points - that is accurate enough?

Also, I have a very efficient aircraft and would like to know the correct settings RPM and MP at LOWER power settings down to 30% if possible.

Regards,

Angus Fleming

The engine charts assume a Wide Open Throttle at Sea level and a varying load to slow (Lug) the engine to various RPMs on a dynomometer.

The prop numbers assume a Fixed Pitch prop and a varying throttle.  

Hard to compare the two.

If you have a Ground Adjustable Prop, adjust it to produce ~5800 rpm at some point in the flight.

The rest of the powere setting will fall into place.

A WOT climb at ~5800 at Vy is to be recommended.

If you have an In-Flight Adjustable Prop,  in cruise, set the governor to ~5500 rpm and adjust the Throttle/MAP to produce the power you need to get the results you want.

It is not clear what you are going to do with the information you are asking for.  ???

Always prefer keeping the RPM's UP and the MAP Down.

You might want to review This Thread.

Hi Bill,

Thanks for your comments so far – this is getting even more interesting than I thought and revealing that I am not the only one not sure about a few things.   But first the full story and my basic reasons;

1. I am planning a long distance non-stop flight for which I am marginal on fuel and daylight (or plain old stamina).
2. My aircraft (one-off Magnum VLA) has a 914F and electric CS prop, has lots of fuel capacity and is very efficient – I only need around 40% power to cruise at around 115 kts (to be checked) and so far, prior to detailed testing reckon for my flight that 120-130 kts (55-65%) will be best. I can cruise at just under 160 kts at 5500 rpm and 35” – maybe slightly higher V at 5500 and higher MP.
3. So, what I am looking for is data that will allow me to work out the best power setting for the flight, or at different stages at varying altitudes to avoid airspace. Then, for each power setting there is an ideal RPM and MP and consequent fuel consumption.   From the threads that you have supplied it is apparent that there may be some room to run at higher prop pitch settings for given RPM but of course slightly higher MP’s.   So, what increase in MP, if any, is permissible and what then is the penalty for running continuously at permissible higher MP and the effect on fuel consumption?
4. Given this data I then plan to compile (and test) a chart to enable me to select the correct RPM and MP to cruise at a given speed, with altitude correction if necessary and know the fuel consumption for those settings. Feed all that into Sky Demon and the possibilities and options become relatively easy.

Hope you can help further.

Regards,

Angus Fleming

The charts only take engine performance on a test stand into account.

You will also need to factor the Aircraft, Weather and the Atmosphere into the equation.

The most fuel efficient flight will be at Vy where the Induced Drag losses equal the Parasitic Losses and so total losses are at a minimum.

Vy is Minimum Power (and Fuel) required for level flight.

This is not the speed to use if getting somewhere is high on the priority list.

What you are looking for would be in the aircraft operating manual for your aircraft.

If this is a One-Off machine, this probably means that it is something you would have needed to produce yourself by flying at multiple power setting, and recording the resutls.

It looks like you have a Powered Sailplane, so energy managment sould not be a stranger to you.

Flying into a headwind probably will mean going near max speed to minimize the time fighting the headwind.

A tailwind would be a completely different story.

All of this will be based on your density altitude which can be considerably different from the altimeter at times.

Engine Power and Fuel consumption are closely linked.

What your airframe does with that power will not be found in the Roxax literature.

With a variable pitch prop, you will want to cruise in the 5200-5500 rpm range even if you only need 30% power and 15" MAP.

Fuel consumption is proportional to POWER,  Not RPM.

Planning a exhausting marathon flight that anticipates marginal fuel and fading daylight is not a good plan.