Comparing cubic wing load and power loading.

[Gray Harlequin]

I am not sure that WCL and watts/lb can be correlated in any way. I do have some confusion regarding the watts/lb rule of thumb. Take this example on 3S:

Emax 2204 -> 166 W 498 g thrust
Emax 2213 -> 106 W 670 g thrust

In this case, should you choose the motor based on thrust or the watts/lb rule?

 

[JasonK]

I am not sure that WCL and watts/lb can be correlated in any way. I do have some confusion regarding the watts/lb rule of thumb. Take this example on 3S:

Emax 2204 -> 166 W 498 g thrust
Emax 2213 -> 106 W 670 g thrust

In this case, should you choose the motor based on thrust or the watts/lb rule?

there isn't a watts to static thrust conversion equation. - those are probably spinning different props and different speeds. if I was to guess the first one has a higher pitch speed with those numbers, so has a lower static thrust (how much plane can 'hang' from the prop) but could get the same plane to a higher top speed (as thrust drops out as the speed approaches the pitch speed)

IE EDFs have low static thrust for their power draw, but can get a plane moving rather fast because they don't loose much thrust as the plane goes faster and faster as their pitch speed is so high.

 

[telnar1236]

I am not sure that WCL and watts/lb can be correlated in any way. I do have some confusion regarding the watts/lb rule of thumb. Take this example on 3S:

Emax 2204 -> 166 W 498 g thrust
Emax 2213 -> 106 W 670 g thrust

In this case, should you choose the motor based on thrust or the watts/lb rule?

You need to consider both. Your TWR needs to exceed your D/L ratio and you need excess power to accelerate or climb. Different systems have different efficiencies at different speeds. Broadly the faster your efflux speed is (the smaller the prop area for the same thrust) the less efficient the system will be but the slower the thrust will fall off with speed. There are a lot of other factors too (higher voltage is more efficient, prop airfoils have different efficiencies, the airframe aerodynamics change the efficiency of the power system somewhat) but they are less significant and harder to quantify. You need enough thrust when slow to overcome your larger drag coefficient and enough power to keep flying. That is why some EDF jets need to be kept fast and cannot climb when slow even though they are not below their stall speed; the lift to drag ratio is not high enough a small angles of attack. If you want to look more into this you can research velocity instability.

This website offers good estimates for power loading for a variety of planes. The estimates are on the lower end, especially with modern power systems so you can definitely go higher without having way too much power. The Watts Per Pound Rule for Electric RC Planes (rc-airplane-world.com)

 

[quorneng]

My own view is that this thread rather confirms that "plane performance" is subject to a great many variables many of which are due to its aerodynamic and structural configuration as well as power and weight.
As many of these variables are counter productive there is no simple formula to determine what is "the best".
All you can do is use judgment based on knowledge and experience to get close to a required performance specification.

 

[Piotrsko]

My own view is that this thread rather confirms that "plane performance" is subject to a great many variables many of which are due to its aerodynamic and structural configuration as well as power and weight.
As many of these variables are counter productive there is no simple formula to determine what is "the best".
All you can do is use judgment based on knowledge and experience to get close to a required performance specification.

Only thing I would add is user experiences. @quorneng probably out flies me by a factor of ten, I out fly newbies by a similar ratio. What is a hotrod for him is a gofast handful for me and a one shot crash test for someone just starting out.

 

[Cabreraman]

I have this train of thought I am currently following and I thought maybe some others might want to weigh in on it. It's a theoretical and number crunching problem.

As many know, wing loading is a terrible way to compare how aircraft of different sizes will fly. To compare models of different size we use cubic wing loading, which gives us a number we can compare pretty universally across all sizes of aircraft. They fall somewhere in the following categories:
Slow-flyers and Gliders: under 4
Trainers and Parkflyers: 5 to 7
Aerobatics: 7 to 10
Scale: 10 to 15
Warbirds and Racers: 15 and over

When sizing motors we have compare watts/pound depending on the type of flying you want to do. The categories are something like the following, depending on who you ask:
50 -70W/lb. …....Trainers and slow flyers
70-100 W/lb. .....Basic aero/sport flying
125 W/lb. …........Precision aerobatics
150+ W/lb. …...…Extreme 3D performance

My current line of thinking is: why do we compare power to weight directly? Would there be any benefit to comparing power to cubic wing load?
I crunched a few numbers to start a comparison. All data for the given aircraft are from Wikipedia based on rated power and maximum weights, so they don't necessarily reflect actual use cases of a given aircraft.

- A full scale Cessna 172R has about 160hp(119312W) and a gross weight of 2450lbs, for a power to weight of ~49W/lb. At gross weight a 172R would have a cubing wing loading(WCL) of 17.1.
- A Zivko Edge 540 has 310hp(231167W)and a max aerobatic weight of 1550lbs, for a power to weight of 149 W/lb. At its aerobatic weight the Zivko Edge 540 has a WCL of 22.7.
- A J-3 Cub has 65hp (48470.5W) and a max weight if 1220lbs for a power to weight of 39.7W/lb. Its WCL at max gross is 8.2.

At what point does a plane become completely unflyable? I'm building a funjet style plane with a 15kw (yes 15kw) motor and the weight should be around 3 to 3,5 kg. It will be airdropped from another plane and the goal is to fly really fast and then belly-land without braking. What is the maximum amount of cubic wing loading I should aim for?

 

[JasonK]

At what point does a plane become completely unflyable? I'm building a funjet style plane with a 15kw (yes 15kw) motor and the weight should be around 3 to 3,5 kg. It will be airdropped from another plane and the goal is to fly really fast and then belly-land without braking. What is the maximum amount of wing loading I should aim for?

- what is your wing area?
- what do you want your stall speed to be?
This will let you put in some stuff to get an approx stall speed, given a known wing area and desired stall speed, you should be able to come up with a "max weight", but also consider what it will take to maintain a speed above that stall speed and control flying it at those speeds.

Stall Speed Calculator

Calculate Plane's Stall Speed.

rcplanes.online

 

[Cabreraman]

- what is your wing area?
- what do you want your stall speed to be?
This will let you put in some stuff to get an approx stall speed, given a known wing area and desired stall speed, you should be able to come up with a "max weight", but also consider what it will take to maintain a speed above that stall speed and control flying it at those speeds.

Stall Speed Calculator

Calculate Plane's Stall Speed.

rcplanes.online

I want to ideally land at less than 100kph and a around 20dm2 wing area.

 

[JasonK]

I want to ideally land at less than 100kph and a around 20dm2 wing area.

use that tool then to get some ideas on what it could be: