prop controls and fuel

Recently testing how prop controls work to try to understand what prop does. To my understanding for constant speed propeller its supposed to just change the propeller pitch to maintain your chosen RPM. However, I found that pulling rearward your prop controls also seems to decrease GPH. Why is this the case? what has the pitch of the propeller have to do with how much fuel is used? I thought that only your throttle and mixture would effect GPH. also when taxiing should one push the prop forward or keep it back?

would appreciate any clarification on this. thanks

As a general rule I keep the prop pitch full forward after engine start, for taxiing and takeoff. After gear up and flaps to 0, I begin to adjust the pitch for proper RPM into the green zone, normally around 2500 RPM. For descent and landing I lower it to around 2000 RPM.

For default aircraft and some of the 3rd party freeware I have found that this adjustment is not critical but on aircraft like the "Super Conny" it's do or die!

The throttle and prop control together, along with the mixture control will control the power setting, with reduced RPM also being reduced power, thus the lower fuel flow (GPH). For instance, according to my 1974 Bonanza F-33A manual, 23.9" MP and 2500 RPM give 75% power, while 23.4" MP and 2300 RPM give 65% power, reducing fuel flow from 15.4 GPH to 13.3 GPH -- this is on an ISA -20ºC day at sea level. ISA day means a standard atmosphere, that is 59º F (15º C), 29.92" Hg, 0% humidity at sea level, with a 3.5º F (2º C) drop for each 1,000 feet of altitude.

Note that on the 55% power chart the same -20º C day shows 23.1" MP and 2100 RPM, and 45% shows 20.1" at 2100 RPM.

In a far from complete analogy, a lower RPM is a bit like a higher gear in a car, able to transmit less power to the prop/road.

Thanks very much,

but now im confused about something else. When I increase the prop (higher rpm) there is a an increase in gph but no increase in MP. So how exactly is engine power and GPH increased when a higher RPM is selected? Is it an increase in rate of the fuel/air going that is being changed (and if so why doesn't MP change) or is it an increase in the mixture setting so that there is more fuel compared to air?

When you increase the PROP control, you increase the propellor's speed, which makes the engine work harder to turn it faster, thus increasing fuel used, without changing MP or MIXTURE. The overall throttle settings stay the same but the engine has to burn more fuel to turn the prop faster. Similar to pedaling a bicycle, in that if you go up a hill, with no change of the gearing OR speed, you will work harder, burning more energy, with no change in you internal throttle, so to speak. You pedal at the same speed, but you have to work harder to accomplish the same effect. OR, if you go up a gear, while staying the same speed, on level ground, the tires will turn faster (increased PROP rpms), the pedals go the same speed (engine rpm, MP, and MIX) but you will be working harder, burning more energy (Higher GPH).

Hope all this helps some. Usually people balance the three adjustments, PROP, MP and MIX to get the best possible effect on the plane, whether it's max cruise range, most efficient, highest speed, whatever, for a particular altitude, temperature, humidity, etc.

All tied together. It all affects the engine, and how it burns the fuel.

Just as both pitch and power control both altitude and airspeed. Again, it's all tied together. ANYthing you do in an aircraft affects the entire system!

Goood luck and great flights!

Pat☺

As Pat says, there won't be a change in MP from changing the prop RPM, nor should there be -- if it changed, then there'd not be much need for the prop control, since you can change the MP with the throttle.

Think of the prop control as a transmission control. On a car, in first gear you can rev the engine high (high MP) while the wheels (prop) are not turning very fast. When you go to 2nd gear, now the wheels (prop) will turn faster for the same amount of throttle (MP) applied. In 3rd gear, same deal, the wheels (prop) will go faster (RPM) at the same amount of throttle (MP) applied. But in each of the higher gears the engine works harder for the same amount of throttle (MP), allowing more speed over the road, but making it much more difficult to start the car moving, especially up hill, which is the basic reason cars have gears.

So the engine/prop are more efficient for takeoff (low speed) at a very fine pitch (high RPM), allowing the prop to take a LOT of small bites of the air for the most efficient use of the engine power. Once you're in a climb, you're at a faster speed, so you'd like to have the prop take somewhat larger bites of the air on each revolution (make the prop pitch more coarse), again for maximum efficiency, but you're still not ready for really large bites. BUT, when reaching cruise speed, now you want to further reduce the prop RPM (highest gear) for better efficiency, though you'll have trouble getting TOP cruise speed if you reduce the RPM too much, as if you had a fifth overdrive gear in your car (or tried going up a steep hill in overdrive), instead of only one or two.

Hope this helps a bit.