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Wondering about aircraft propeller sizes

#1
Hi! I'm glad to finally make an account after watching Flite Test for quite a few years. Today I'd like to ask about propeller efficiency and how it correlates with aircraft range.

So, from what I've gathered from data, it seems larger props unanimously produce more thrust for a given amount of power. This is because they move more air at a slower velocity than smaller propellers do. Similarly, for producing a fixed amount of thrust, it seems two propellers of the same diameter will more efficiently complete the task than one on its own, again due to lower RPM. I have a few basic questions pertaining to this assumption (or what appears to be a fact) and I'm hoping the community can be helpful in resolving the mystery in my mind!

1. Helicopter rotors are enormous compared to airplane propellers. From what I understand, they produce a ton of thrust for not much power. Why aren't we attaching helicopter-rotor-sized props onto our aircraft for better fuel efficiency? One could cite ground clearance and weight savings, but for the ground clearance issue you could use a tilting system to keep the prop away from the ground and use a secondary prop just for takeoffs and landings. As for weight, I understand this can make a great difference in aircraft performance; regardless, if weight of a propeller is that significant, what kind of function can be used to find the largest feasible propeller for an aircraft at a given cruise speed, altitude, payload, etc.?

2. How does static performance differ from dynamic performance? This is referring to a propeller pulling air through while it is at rest versus when it is flying through the air at speed- I imagine that the difference grows as you add more blades to a propeller, though that's just a guess. Is there data on this out there that is easily accessible?

3. Why aren't propellers long and skinny like glider wings (high aspect ratio)? Aerodynamics seem to favor long, thin wings with high aspect ratios when it comes to aerodynamic efficiency. Why use a propeller with wider blades at the root if that lowers the aspect ratio? Wouldn't it decrease performance? And if it is contrary to what I'm thinking (that propellers SHOULD have long thin blades with a high aspect ratio) then why is it so? It seems to contradict very important principles of aerodynamics. That said, I'm just an enthusiast, and though I am an engineering student, I haven't taken any aerodynamics classes.

I will probably think of more which I will post later, but these are what are on my mind at the moment. Thank you ahead of time for your patience with any ignorance I display. I am looking forward to making friends with the folks here in the Flite Test forum!

EDIT:

Just thought of another question. Do you think it'd be worth looking into the effect of prop-span to wing-span ratio on aerodynamic efficiency? It may have already been looked at but I figured I'd ask anyway. Thanks again!
 
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Merv

Well-known member
#2
Static performance will always differ from dynamic thrust. First a few basics, regarding amp draw, larger diameter, higher pitch & higher RPM all draw more amps than smaller diameter, shallower pitch & slower rpm. Let’s consider 2 setups. The first a 1000kv motor with a 10x4 prop, pulling 30 amps. The second a 1,400kv motor with a 7x6 prop, again pulling 30 amps. The larger diameter lower pitch lower rpm prop will produce more thrust than smaller diameter higher pitch higher rpm prop on a test stand. However in the air, the smaller diameter prop will go faster. The larger prop will move more volume of air at a slower speed. The smaller prop will move less volume of air at a higher speed.

It comes down to how do you want to fly. If you want to go fast, spin a smaller diameter, high pitch prop very fast. If you want to 3D, spin a larger diameter, shallow pitch prop slowly.

Helicopters use most of their energy to produce lift at a relatively low air speed.

Take a look the F1D rubber power planes, they are using your idea, a very large diameter prop at low airspeed
 

Merv

Well-known member
#3
There is no such thing as the perfect air plane, that is the plane that can excel at everything. Everything is a compromise. Do you want to go fast, carry a heavy load, fly for a long time or something else?
 
#4
There is no such thing as the perfect air plane, that is the plane that can excel at everything. Everything is a compromise. Do you want to go fast, carry a heavy load, fly for a long time or something else?
I want to fly 225 miles cross country at 60mph, with a max gross takeoff weight of 500lbs. Now what? We’re flying in a straight line here.
 
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Hoomi

Well-known member
#6
A helicopter rotor is not a propeller, per se. It's a rotary wing. The concept of the rotor isn't to produce thrust, but rather, to use the rotary motion to provide sufficient airflow over the wings to produce lift. By manipulating where in the rotation lift is greater or lesser, the helicopter either hovers, or moves in a given direction. While it may seem like semantics, it is an important distinction.
 

Chuppster

Well-known member
#7
1. Why aren't we attaching helicopter-rotor-sized props onto our aircraft for better fuel efficiency?

Weight savings and ground clearance are large factors here. In a lot of your statements, you seem to be ignoring propeller pitch. I try to think of propellers as gearboxes, where you can have torque or you can have speed. Ideally you want to mix the torque and speed to get your work done. Full scale airplanes have variable pitch to help them out, but we are stuck with fix pitch props. Keep in mind, if you have all the torque (diameter) in the world but no speed (pitch), you won't get anywhere. Conversely, if you have all speed and no torque that doesn't do anything either. Choosing propellers is a bit of an art and I suggest you play around with options (keep a watt-meter handy!) and see what you can learn!

2. How does static performance differ from dynamic performance?

This is largely dependent on the propeller, but in general the larger your pitch the more you make thrust at speed. You can have a propeller with a huge diameter and a tiny pitch and make tons of static thrust, but not be able to take off due to the fact that you run out of thrust at 10 mph. KV plays into this. A 2000kv motor can do great with a 3-pitch prop, but if you put that 3-pitch prop on a 1000kv motor it won't fly very fast, if at all.

3. Why aren't propellers long and skinny like glider wings (high aspect ratio)?

If you look at an APC prop, it actually is. Specifically the outer 3rd. You may ask why the entire propeller is not skinny, and that has to do with the relative speed of the propeller. The inside of the prop has a much shorter path to get around, but it spins at the same RPM as the outer portion. Therefore in order for it to have as great of an effect it needs to be thicker and more steeply pitched to move air.