Maximizing efficiency at cruise

[razor02097]

It IS going to be very specialized, because you're designing and optimizing for a specific cruise-condition. This, by definition, means that all other aspects of the flight envelope will be worse - because you've designed it to be most efficient at cruise. This isn't a BAD thing, but you need to be aware of it. A cruise-optimized aircraft will climb slowly, probably not be incredibly fast overall, and will not be aerobatic (not that you were wanting that, I'm just making a point).

My wife's Toyota Echo is cruise-optimized, but it doesn't "get up 'n go", nor does it have the payload capability of my truck...then again the truck gets 1/4 the mileage of the Echo... Different vehicles designed for different jobs.

It's true, Prop Diameter is more effectively tied to its efficiency - given a certain airspeed - however, given a fixed max diameter, there will be an optimum prop-pitch that is most efficient - and it won't be the 'greatest' or the 'least' - it will be the pitch that best matches with the speed at which your airframe achieves its greatest L/D (lift/drag ratio).

Calculate or estimate the speed at which your airframe's L/D is maximized (altitude also affects this, it's important), once you know "D" (drag) you know how much thrust you need to make at cruise. Use that thrust-estimate to choose a prop such that it is most efficient at producing that amount of thrust, no more and no less.

agree with the above. I would think the airframe would have a lot more to do with the cruise efficiency at a certain speed than the prop diameter... OP is the airframe you are using optimized for a 70kmh cruise speed?

 

[Ocean]

It IS going to be very specialized, because you're designing and optimizing for a specific cruise-condition. This, by definition, means that all other aspects of the flight envelope will be worse - because you've designed it to be most efficient at cruise. This isn't a BAD thing, but you need to be aware of it. A cruise-optimized aircraft will climb slowly, probably not be incredibly fast overall, and will not be aerobatic (not that you were wanting that, I'm just making a point).

My wife's Toyota Echo is cruise-optimized, but it doesn't "get up 'n go", nor does it have the payload capability of my truck...then again the truck gets 1/4 the mileage of the Echo... Different vehicles designed for different jobs.

It's true, Prop Diameter is more effectively tied to its efficiency - given a certain airspeed - however, given a fixed max diameter, there will be an optimum prop-pitch that is most efficient - and it won't be the 'greatest' or the 'least' - it will be the pitch that best matches with the speed at which your airframe achieves its greatest L/D (lift/drag ratio).

Calculate or estimate the speed at which your airframe's L/D is maximized (altitude also affects this, it's important), once you know "D" (drag) you know how much thrust you need to make at cruise. Use that thrust-estimate to choose a prop such that it is most efficient at producing that amount of thrust, no more and no less.

Thank you, I understood the fact that a larger diameter is more efficient but I worded it poorly (sorry if anyone got confused).

I understand this is a specialized, it is designed for long flight times with AP and SAR capabilities.

Would it be correct to assume that a propellor with a smaller diameter but higher pitch (a pitch that better matched my cruise speed, 42 km/h) would be more efficient that a propellor with a larger diameter but smaller pitch (that did not my match cruise speed better).

 

[Ocean]

agree with the above. I would think the airframe would have a lot more to do with the cruise efficiency at a certain speed than the prop diameter... OP is the airframe you are using optimized for a 70kmh cruise speed?

40-70km/h, most likely 42 km/h.

 

[HawkMan]

Thank you, I understood the fact that a larger diameter is more efficient but I worded it poorly (sorry if anyone got confused).

I understand this is a specialized, it is designed for long flight times with AP and SAR capabilities.

Would it be correct to assume that a propellor with a smaller diameter but higher pitch (a pitch that better matched my cruise speed, 42 km/h) would be more efficient that a propellor with a larger diameter but smaller pitch (that did not my match cruise speed better).

First you get the biggest size prop you can based on plane and motor, then you match pitch to prop size, motor speed and plane optimal cruise speed.

Yes a small prop with a correct pitch will be more efficient than a large prop with a bad pitch, but a large prop with the correct pitch will be most efficient.

 

[quorneng]

Ocean
Just a point but the optimum cruise speed is determined by the aerodynamic characteristics of the airframe and the planes weight not by the pilot.

 

[Ocean]

Ocean
Just a point but the optimum cruise speed is determined by the aerodynamic characteristics of the airframe and the planes weight not by the pilot.

I appreciate this.

The planes "Best Loiter Speed" is 42km/h according to the designers of the plane here. So I am going to presume this is also the best cruise speed regardless of weight (not 100% 'correct' I know, but there is only so much you can do before you end up with differences of a fraction of an amp )