Understanding drag?

[AdamFrisch]

I struggle to get my head around a subject. Drag versus efficiency. Let me explain.

We all know that the bigger the propeller, the more efficient the propulsion is at subsonic speeds. Or the less horsepower you need. I will ignore the weight of the bigger prop for now. But obviously, the drag from a bigger propeller is higher. However, as long as the prop is powering the craft forward, can this drag be ignored, i.e. will it only come in to play when it's windmilling? If not, when does the size become greater than the efficiencies from increasing diameter (ignoring weight once again)?

Reason I'm asking is that fuel efficiency seems to be very linked to fanjet size/prop size. That's why they keep getting bigger and bigger. In helicopters, it's a well known fact that the bigger the rotor is, the less hp's it need to hover or takeoff. But at some point the drag increase must 'catch' up with the oversize, unless that can be ignored as mentioned above.

Thank you.

[Mark 1]

Propulsive Efficiency (http://en.wikipedia.org/wiki/Propulsive_efficiency)

Thrust is approximately the change in relative momentum of the incoming to outgoing air (M*(V1-V0)) but work is approximately the change in relative kinetic energy of the incoming to outgoing air (0.5*M*(V1^2-V0^2))

So increasing the mass flow and reducing the relative velocity produces the same thrust with less work. It also produces that thrust more quietly as an added bonus.

[zerozero]

That's what I was gonna say.

:hmm:

[Oktas8]

The efficiency of a propeller (or jet engine, or whatever) is a measure of how much thrust is produced, compared with how much power was generated by the engine in order to generate that thrust. "We generated 90 thrust horsepower in the climb, by running the engine at its rated power of 180 horsepower. So in the climb, our propeller is 50% efficient."

Now: drag. Each individual propeller blade (or fan blade, or whatever) suffers drag as it whirls around through the air. If you want more efficiency at low speed, you'll get a larger diameter fan as Mark1 said. But then the tips of each blade are moving quite fast at the rated RPM. If you try to go fast through the air, increasing RPM to do so, your blade tips will suffer very high drag in a rotational sense, leaving them unable to turn fast enough to generate thrust. So engine efficiency decreases - same power in, less thrust out.

Note that the thrust is axial: in the line of flight. But the drag in this sense is tangential: in the plane of rotation of the fan. Don't fall into the trap of trying to compare axial forces (thrust of a propeller, drag if it's windmilling) with tangential forces (torque of the engine).

Hope that helps.

[cockney steve]

Saw the title and thought it was about men in dresses :O

[asc12]

Another way to think about Mark 1's post is this:

You need to accelerate a mass of air backwards to generate a force to move your engine forwards.

You can accelerate a small amount of air by a lot, or a large amount of air by a little to get the same force. But the fuel flow is related to the kinetic energy of the air-- the amount of air times how much we accelerate it squared. So let's do a small acceleration on a big amount of air.

That's why big props are more efficient. Big props spinning fast get higher tip drag, for sure, but that's tangential drag (harder on the engines) and not axial drag (slower airplane), which is what octas8 just said, too.

If you have a big, flat propeller that's not spinning in flight, sure-- you'll have a lot of axial drag. But I don't think that's terribly relevant to the dynamics of the prop in flight.

[barit1]

The illustration I used to show: Compare the helicopter to an afterburning jet. The Helo generates enough lift to levitate the machine plus payload directly, using a pretty small engine. The rotor downwash velocity is low. But its Vmax is very limited.

The A/B jet can go like hell, because its exhaust velocity is astronomical; but it burns HUGE quantities of combustibles, and low-speed efficiency is terrible.

And all the other varieties of powerplants lie somewhere in between.

[AdamFrisch]

Thanks. But gearing can take care of tip speed drag. A prop doesn't have to spin faster when it gets bigger. Slow it down, increase angle of attack on blades slightly and it will move as much air mass as it did before, without tips getting supersonic and adding drag. So that can't be the only penalty - there must be something else.

[asc12]

I think I'm still confused about the question.

Are you thinking about the induced drag from the blades moving through the air and generating lift along their length?

Both lift and drag will increase with a longer blade. Although, I wonder how the l/d ratio changes along the length of a blade, which may be at the crux of the question. You said

But at some point the drag increase must 'catch' up with the oversize...

But the drag will only catch up to the lift if the l/d ratio changes for some reason-- perhaps because the tip velocity is so much higher in a large-bladed prop? I don't know that l/d does change, but I think that drag by itself isn't as important to consider as the ratio when thinking about the prop efficiency.

[awblain]

The "tangential drag" of the propeller/fan still needs engine power to overcome, and so that hits efficiency - more fuel needs to be burned to produce given thrust than for a propeller/fan that imparts less kinetic energy to its spiralling airflow in noise, turbulence and bulk spiral motion.

A smooth aft-directed jet in the wake of the propeller would be ideal, in which case the low-delta-v, high-mass flow is the unique most efficient answer.