E6B Max cruise VS Manual RPM configuration

[Lepape2]

I just put together a chart I made offline to visually put a discrepancy I noticed in many airplanes. It seems that there is two kind of engine behavior when it comes to different RPM settings at different speed and different altitude. Somehow, the E6B max cruise setting in ANY plane does not give you the best speed for minimum fuel consumption. I find that reducing RPM ONLY is the way to go.

Based on these numbers, it looks like for the last 3 airplanes listed, when you reduce RPM with pitch control, it gets stuck at a certain value when the prop starts windmilling due to the aircraft speed. When the aircraft speed decreases, the RPM gets slower and slower (less windmilling) until your are too slow to call it a sustainable cruise setting. As the RPM keeps decreasing, so does the engine Manifold Air Pressure - Even at maximum throttle, the engine cannot keep up with the prop feathered.

The most interesting values are those regarding the first 5 aircrafts listed. It seems not only that the prop windmills (RPM gets stuck) very near military IAS as the RPM is decreased and aircraft drag is equal to thrust, but also that as the prop is becoming more feathered (can be seen by the fuel burn decreasing as the pitch control is moved) the engine RPM still keeps its value and does not slow down the airplane anymore . In this case, I risk myself to conclude that:

"On some aircrafts, true prop pitch DOES NOT affect the aircraft speed(thrust) but only RPM does; and also that INDICATED RPM is only showing (or stable on) the windmill margin threshold but still NOT affecting the airplane thrust behavior when true prop pitch is changed under such margin".

Because I can't see a completely feathered aircraft maintain such fast airspeeds and RPMs. It bothers me very much and drives me insane.

The windmilling feathered prop helps the engine maintain MAP and RPM, that I understand. But it looks like no matter the prop pitch position once it ranges below windmilling speed (Indicated RPM is stuck), the (first 5 listed) aircraft sustained speed DOES NOT CHANGE. This is why I conclude that engine thrust is not dependent on true prop pitch but on indicated RPM - which should actually be dependent on prop pitch, engine MAP (is not entirely dependent on throttle lever), and relative air speed. Indicated RPM looks accurate.

For the last 3 airplanes listed (P47-190-La7), the windmilling indicated RPM threshold is not high enough to maintain sustained airspeed, and therefore MAP (more evident by looking at fuel burn). Seems like the prop pitch has a MUCH STRONGER effect aerodynamically for these planes.

Either I am being a complete idiot and am missing something basic or this doesn't make sense.   I would prefer to be an idiot in this case and think it has something to do with something else in the game.

Discuss

The numbers have been taken at SEA LEVEL (500ft) with 50% fuel except for P51D and A6M2 (25%)
*Mistake in this chart: Throttle (Read) should be MAP (Read)

[hitech]

You can not manually feather any of the props AH.Most multi engines will auto feather if you stop them. All planes have a minimum rpm settings.

HiTech

[lulu]

Welcome Lepape2!

 

[Lepape2]

So each single engine aircrafts have different maximum/minimum pitch values but not low enough to be considered feathered and this is what determines the windmill threshold of each aircrafts if I understand correctly. Each aircraft, depending on these values, can therefore retain a fast airspeed for a given windmilling pitch value or be unsuccessful in retaining sustained optimal cruise speeds.

It still doesn't explain why the Max Cruise Settings in the E6B does not actually give the best cruise (which is the lowest fuel consumption for highest possible speed)

I'm really confused 

http://www.eaa1000.av.org/technicl/perfspds/perfspds.htm#Optimum_Cruising_Speed

[Baumer]

Lepape,

In general if you look at the Max Cruise Value listed on the E6B then cross reference it to the actual aircraft manual you'd see why. For example if you look at this chart for a P-51D;



As you see "Maximum Cruise" is the settings for best speed with the best fuel consumption. However, if you need to get maximum distance for the minimum fuel the settings would be under "Minimum Specific Consumption" and this data is not shown on the E6B.

The only other issue that comes into this is that we have no control over the mixture settings. So looking at the chart it would have you set to Auto-Lean and we can't do that (but given how this game is played it's not a significant issue).

If you read most of the US manuals you will find 3 settings that are the most important.

1.) best speed
2.) best range
3.) Most time aloft

And the settings will be different for each

<S> Baumer