xendelaar

Experiments with several different rotors to determine Lift to weight ratio

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Posted (edited)

TLDR

I did some tests with various rotors to determine which yields the highest lift/weight ratio for heavy payloads on either Kerbin or Eve. In the static tests I’ve performed on the ground of Kerbin and Eve, I noticed that the large helicopter Type S blades provide the most lift per tonnes rotors+motor .

In my flawed tests, 8 rotors per motor work best on Kerbin and 4 rotors per motor work best on Eve.
I was also able to roughly determine the maximum mass of a craft at which it would still fly. 

 

Long story

I’ve been running tests with various sizes of propellers to determine the a correlation between vertical lift and mass. I’m especially looking for optimum mass to lift ratio’s for heavy crafts and trying to find a way to calculate the amount of propellers I need to get something in the air.

I’ve been trying to approach it scientifically but eventually just tried some stuff at random.

Literature study

None.. just felt like messing around with the propellers. There are probably a bunch of better articles describing how to calculate the amount of lift on a given planet at a certain altitude.

My initiial guess is that the lift is calculated something like:

Lift = Function [ rotating speed, angle of attack, air density, rotor blade  type]

 

Experimental setup

I performed two experiments:

1.       On Kerbin , using a rigid clamped setup  with 3 heavy motors in series. I tested 3 types of rotors with 8, 4 or 2 blades per motor

2.       On Eve, using a massive s4 fuel tank setup ( 1,200 tonnes) with 4 motors. Testing 2 rotors, with either 8, 6, 4 or 2 blades per motor

Results

The current measurements have been done under static conditions on kerbin and Eve. Below you find the results.

Propeller type

Amount of props on motor

Total weight of props (ton)

Weight Motor

(ton)

Max RPM

optimal Angle of attack

Planet

Lift (kN/prop)

Total lift

(kN)

Lift/total weight motor + rotors

(kN/ton)

Heli S-Type

8

1.44

2,2

440

5

Kerbin

270

2160

593

Heli S- type

4

0.72

2.2

445

8

Kerbin

335

1340

459

Heli S- type

2

0.36

2.2

450

8

Kerbin

346

692

270

Propeller S type

8

0.96

2,2

460

82

Kerbin

34

272

86

R-25 fan

8

0.96

2,2

460

84

Kerbin

37

296

94

Heli S-type

8

1.44

2,2

253

5

Eve

403

3224

886

R-25-fan

8

0.96

2,2

450

84

Eve

163

1304

413

Heli S-type

6

1.08

2,2

253

3

Eve

500

3000+

915

Heli S-type

4

0.72

2,2

418

3

Eve

700

2800

959

Heli S-type

2

0.36

2,2

450

3

Eve

1100

2200

859

 

 

I changed the angle of attack to find the optimum angle at which most lift was created. From these short tests, I found that the Heli S-type blades performed the best when you look at lift per weight ratio.

I also tested if less rotors would yield in better performance on either Kerbin or Eve. From what I’ve measured I could state that:

1.       On kerbin 8 Type S Helicopter blades  performed better than less blades

2.       On Eve, the optimum lift/ton motor+rotors weight seems to be  4 rotos per large motor

 

Discussion

One of the things I wanted to find out If I could calculate the maximum weight a certain setup would be able to lift.

From what I’ve conducted. It’s fairly easy to calculate:

m  = max mass (in tonnes) = F * G 
F = Lift (kN)
G= gravitational force ( 9,81 for kerbin, and 16,5 on Eve).

 

For a two motor helicopter with 4 large helicopter type s blades per motor, this would mean that it could carry:

2800kN / 16,5 m/s^2 * 2 motor with 4 props = ~336 tonnes

I tested this and It seems to check out.

I also wondered if I could translate tests on kerbin to match the results on Eve.

It seems that the there is a correlation between air density, RPM and lift on Kerbin. However, the maximum amount of RPM is different on Eve, compared to kerbin, so I think I need to add torque and drag into the equation for this. I didn’t have time to work this out though.

 
Btw, this was a very flawed study and many additional tests need to be done before any hard conclusion can be made. Several things come to mind:

-          Does the lift vary with different vertical velocities (anything else than 0 m/s)?

-          Does the optimum angle of attack vary as the density changes (it does)

-          What is the effect of the angle of the ship on the optimal angle of attack of the blades

-          Etc..

 

Anywho… It was a nice experiment. I’m now able to calculate how many rotors I need to carry a certain payload.

 

 

Edited by xendelaar

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The fastest propeller plane on Steam uses R-25 Ducted Fan Blade. I started using them and they seem to be overpowered. Would be interesting to see their numbers. Good testing!

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6 hours ago, Wilds said:

The fastest propeller plane on Steam uses R-25 Ducted Fan Blade. I started using them and they seem to be overpowered. Would be interesting to see their numbers. Good testing!

yeah, Ive noticed this myself as well! In my table you'll find to experiments with R-25 fan blades which had less lift than the helicopter blades. however, the tests were performed static, so the crafts didn't move. I think that the drag of the helicopter blades is much larger than of the R25 which could result in different values when the craft is moving at high velocities. 

I found a nice mod (wind tunnel mod) that perhaps could speed up the research. I'll try to look into it when i've got the time. 

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I always enjoy seeing players do this kind of work and share it. 

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