RPM development at T/O - Constant Speed vs. Fixed Pitch

flyTotheSky

flyTotheSky

Well-Known Member
From my understanding we redline the tach in a constant speed a/c because the prop is in the most efficient angle for t/o (low blade angle/high rpm).

In a fixed pitch prop - we seldom, if ever redline the tach when we set full power for t/o. Is this because the prop might be a cruise prop? Or is there an additional reason for this?

Obviously we have environmental issues to consider, specifically air density and how that effects the prop.
 
flyTotheSky said:
From my understanding we redline the tach in a constant speed a/c because the prop is in the most efficient angle for t/o (low blade angle/high rpm).
Click to expand...

Prop efficiency isn't so much the issue as the total package. Thrust Horsepower is a function of propeller efficiency AND brake horsepower. Brake Horsepower is maximized at the highest RPM the engine can develop.

THP = Prop Efficiency * BHP

<<In a fixed pitch prop - we seldom, if ever redline the tach when we set full power for t/o. Is this because the prop might be a cruise prop? Or is there an additional reason for this?>>

A constant speed prop can maintain an optimal AOA throughout a wide range of airspeeds. A fixed pitch prop cannot. A typical prop is a compromise between an efficient climber and an efficient cruiser. At takeoff speeds, most props have too large of an AOA to allow the engine to develop full RPM. As airspeed comes up, the RPM will go up, because the AOA is reduced by the forward motion of the aircraft. If the propeller had a low enough blade angle, it could develop full RPM early in the takeoff, but would overspeed as the airspeed came up.
 
tgrayson,

Thanks again for the insight.

A typical prop is a compromise between an efficient climber and an efficient cruiser. At takeoff speeds, most props have too large of an AOA to allow the engine to develop full RPM. As airspeed comes up, the RPM will go up, because the AOA is reduced by the forward motion of the aircraft. If the propeller had a low enough blade angle, it could develop full RPM early in the takeoff, but would overspeed as the airspeed came up.
Click to expand...

This statement is referring to a fixed pitch prop, correct?
 
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