TAS & Pitch

[chris]

Hey guys,

I have heard contradictory statements on the relation between your prop pitch (fine vs. coarse) and TAS, and I would like some of your opinions.

I always thought that with a finer setting (high RPM), you will have a higher fuel consumption, higher temperatures for CHT, and a higher TAS. Is this the case?

Also, can someone explain when and why prop efficiency is greatest- low, or high RPM?

I understand that with a coarse setting (low RPM), the prop's AOA is greater and thus, the props take a "bigger" bite out of the air, implying greater distance travelled per revolution (i.e. pitch). I always thought this implied a higher TAS, but this is contradictory to the first statement I made. Am I misinterpreting something?

Any help is greatly appreciated.

Chris.

 

[E_Dawg]

With a lower pitch (higher RPM) your CHTs will be lower. This is because the engine pressures (and hence temps) are reduced.

If you have high pitch (low RPM) and high MP CHTs will be high because there will be more fuel / air burning inside the cylinder for a longer period of time (raises pressures).
This is the basis of the whole 'pull the throttle back before you pull the prop back' - however on the vast majority of engines you can't hurt anything unless you are already at a very high power setting and actually try to mess things up.

Efficiency does not = higher speed.

The highest speed you will get is at high RPM and full throttle... but at a very high fuel burn cost. When you pull the prop control back you may lose a small amount of speed but you gain a lot of fuel efficiency.

A good book that talks about all of this is 'Flying High Performance Singles and Twins' by John Ekalbar (much more useful than Deakin's articles IMHO... but it talks about the same kinds of things)

 

[PhotoPilot]

First, fuel comsumption is tied to more factors than pitch and RPM. It's also related to throttle setting and mixture. Properly using the prop control should allow you to obtain the most fuel and power efficient situation for each operation undertaken, but there's no simple, best pitch for optimum fuel efficiency.

Now, as for TAS and prop efficiency being greater at high or low pitch settings, it's all very realtive. Remember that your prop blades are airfoils. The greater your airspeed, the lower the angle of attack of the blades due to the change in relative wind. The variable pitch allows you to increase the blade angle to maintain the most efficient pitch angle (angle between chordline and relative wind) as the relative wind increases. Using these controls properly allows a pilot to find the best efficiency (ratio of thrust hp to engine brake hp) for many situations, including high and low rpm.

What this means is that a low-pitch setting allows an aircraft to accellerate more quickly at slower airspeeds (like starting from a stationary position on the runway). After accellerating and beginning a climb out, the prop is adjusted to a slightly coarser setting to adjust for the change in relative wind while continuing at climb speed. When a cruising altitude is reached, the plane accelerates to cruise speed and the prop is adjusted to the most efficient pitch - again to compensate for the increased relative wind. On approach, the prop is again adjusted to the low-pitch setting so that acceleration and climb performance are maximized in the event of a go-around.

If the pitch angle is too high, the airfoil stalls. If it's too low, then the airfoil is less efficient than it could be. Just like a wing, the blade is best utilized at a certain angle of attack. The variation in pitch allows this angle to be maintained through a great range of speeds, throttle settings, and relative wind conditions.

Clear as mud? I'm a new pilot and am not skilled as quick, concise explanations yet. I can fly 'em, and I understand 'em, but need some work at describing 'em!

-PhotoPilot

 

[SteveC]

Looks like a great discussion for the Technical Talk forum.

 

[ananoman]

[ QUOTE ]
Hey guys,

I have heard contradictory statements on the relation between your prop pitch (fine vs. coarse) and TAS, and I would like some of your opinions.

I always thought that with a finer setting (high RPM), you will have a higher fuel consumption, higher temperatures for CHT, and a higher TAS. Is this the case?

Also, can someone explain when and why prop efficiency is greatest- low, or high RPM?

I understand that with a coarse setting (low RPM), the prop's AOA is greater and thus, the props take a "bigger" bite out of the air, implying greater distance travelled per revolution (i.e. pitch). I always thought this implied a higher TAS, but this is contradictory to the first statement I made. Am I misinterpreting something?

Any help is greatly appreciated.

Chris.

[/ QUOTE ]

It is probably easiest to understand this by discussing airplanes with fixed pitch props.

You have probably noticed that when you add full throttle to take off, you only get arount 2300-2400 rpm, yet your airplane may make full power at 2700 rpm. In a Piper Cadet rated at 160hp, you only get arount 145hp on take off. Rate of Climb is directly related to available horsepower. So less available horsepower = lower rate of climb.

It would be possible to reduce take off ground roll and increase the rate of climb by installing a propellor with a lower pitch. This would increase the available horespower by allowing the engine to reach full rated power in a Vy climb. The trade off is that once you reached cruising altitude, you would not be able to cruise at a high power setting, because the engine will overspeed. This results in the pilot having to reduce the throttle setting. This understandably results in a lower cruise speed.

We could fix this by installing a propellor with a higher pitch. This would solve our cruise problem. We can now cruise at a high power setting without overspeeding the engine. The only problem is that we cannot make full rated power on takeoff, causing longer ground roll and reduced rate of climb....

This is really no different than a car with a 1 speed transmission. Do you want a low gear for acceleration, or a high gear so you can drive more than 30 mph without redlining the engine?

As a result of this, most airplanes with fixed pitch props have a prop that is not optimized for climb or cruise. It is a compromise that allows decent climb performance without excessively hurting cruise speed.

There are a few aircraft that are fitted with pure climb props, but these are usually Piper Cubs, or other small STOL aircraft. The cruise penalty is around 10 kts. I do not know of any airplanes off the top of my head that have cruise props.

The adjustable pitch prop solves our delemma, by giving us a low pitch prop / high rpm for takeoff and a high pitch prop / low rpm for cruise. Since pilots are inherently lazy, the constant speed prop is even better, since we do not have to manually adjust the prop pitch every time we change our power or airspeed. The governor does it for us.

However the constant speed prop disguises what is really going on. It is true that at full throttle and high rpm an aircraft with a constant speed prop will have its highest TAS, but the angle of the prop blades at full power and cruise is greater than their angle at takeoff. As our airspeed increases, the governor increases the angle of the propellor blades to keep the prop from overspeeding.

Overall the efficiency is greatest at a low prop rpm. It is more efficient to run the engine (recip) at a higher MAP and lower rpm, and it is more efficient to turn the prop at a low rpm. In general, the most efficient way to create thrust is to move as much air as possible, as little as possible. At the speeds we operate at, propellors are more efficient than turbofans, which are in turn more efficient than the pure turbojet.

Again, look at your car for an example. It is often possible to cruise at 65 mph in several gears. Which is most efficient? Top gear, which is usually overdrive (transmission output shaft turns faster than input shaft) is most efficient.

 

[E_Dawg]

Ha, I once flew a 152 with a cruise prop on it...

Climbed at 400fpm at sea level on a cold day if I was lucky.