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Help! Prop Sizing Questions (PLEASE HELP!!)

#1
For my university project I am calculating the performance of a UAV however I am struggling to calculate the dynamic and static thrust produced by a prop. Which leads me to a couple of questions...

1) When calculating the thrust of a motor with a prop, how do I determine the RPM of the motor with a prop (adding load to a prop)?
2) Are there any calculations I can do to determine the ideal prop size without having to do any physical tests?
3) What is the most accurate dynamic thrust and static thrust equation I can use to calculate the thrust of my motor with a prop?


Motor specs:
kV = 1070; V = 11.1; unloaded RPM; 11877
Battery Specs:
3 Cell LiPo battery of 2200mAh of capacity.

If someone could help me out that would be awesome!
 

Merv

Well-known member
#2
1) When calculating the thrust of a motor with a prop, how do I determine the RPM of the motor with a prop (adding load to a prop)?
You will need to measure it with an RPM meter. The old school optical ones need to be used outdoors. You will get a false reading if used indoors, the flicker of the artificial light will mess up the reading. In fact that is how you can know your meter is accurate. Hold it to an artificial light, it should measure the hertz of the electricity in your country. In the USA it should measure 1,800 (60Hz times 60 seconds)/2, on the 2 blade setting.
2) Are there any calculations I can do to determine the ideal prop size without having to do any physical tests?
None that I know of. The best way to determine prop size is with a watt meter, that is, with a physical test. Your first step should be to look at the vendors recommendations for prop size.
3) What is the most accurate dynamic thrust and static thrust equation I can use to calculate the thrust of my motor with a prop?
That's the wrong question.
First ask, what do I want the UAV to do? Is the goal speed, heavy lifting, endurance or something else. These goals are pretty much mutually exclusive.

In short, I don't know anyone who answers these questions with an equation. Unless calculating the answer is part of the assignment, physical testing is the practical way to go. If you let us know your goals, budget and other limitations we can get your pretty close to the answer. That is, you will only need to test 3-4 props, not tens or hundreds, to pick the best.
 
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#3
Thank you so much for the information!
It has cleared my confusion on whether determining propellers sizes can be done mathematically rather than just practically.
My assignment is very vague, which means that its worth me exploring and discussing physical tests to determine values rather than assuming everything can be done mathematically.
 

Headbang

Well-known member
#4
I know you can calculate the load on a prop, but to do that without computer modeling is a bit insane considering all the factors of a prop. With the load figured one can go from there, but you need a lot more data on the motor, and manufactures never provide the data required. So I am saying it is possible, just not practical.
 

Jackson T

Well-known member
#6
I can't provide any information, but I'm very interested in your assignment! Any chance you could share with us your findings when you're done? The dynamic vs static thrust test would be particularly interesting.
 
#10
I can't provide any information, but I'm very interested in your assignment! Any chance you could share with us your findings when you're done? The dynamic vs static thrust test would be particularly interesting.
Yea sure. My Assignment is part of a group project to design, develop and test a UAV for a heavy lift challenge as part of the BMFA Competition. I'm incharge of the aircraft Performance so I am responsible for calculating all the performance values of our aircraft eg. take off, cruise, thrust etc. I've created a MATLAB tool where you input all the dimensions and specs of your aircraft and the velocity range and as an output you should get all the estimated performance values.
 
#13
If anybody has any tips or advice on how to determine the performance of an RC aircraft before any physical tests, it would be very much appreciated and go a long way in helping me ace this uni final year project!
 

FDS

Well-known member
#14
The best known flight calculator is e-Calc. That only does power systems, the rest comes down to test flight, that’s why plane companies build prototypes still. There’s obviously rules of thumb and general characteristics determined by airframe design but that still leaves everything together to a physical test.
https://www.ecalc.ch
 

Merv

Well-known member
#15
If anybody has any tips or advice on how to determine the performance of an RC aircraft before any physical tests,
So you are in a heavy lift contest with an a plane. The plane with the highest loaded weight to empty weight ratio will win.

You are going to want a large diameter, shallow pitch prop. Fine the largest diameter prop the motor can safely pull. You will need a watt meter to insure that you are not over taxing the motor. The spec sheet suggests a 12x6 prop on 3S, I suspect the motor will pull a 13x4 prop without exceeding the 33A rating.

Building a light airframe and wing loading will be the key. For heavy lifting I think I would start with 15 oz/sqft. I think I would start with a wing measuring 6' by 13" and work up from there. A larger wing and/or heaver wing loading may also work. You will want a flat bottom wing with a 15% thickness located about 30% back form the LE. This plane could easily be build under 30 oz (850g) AUW (battery, motor, everything) with US foamboard. Something resembling an oversized FT Bushwhacker.

4-Max PO-3541-1070 motor - https://www.4-max.co.uk/po-3541-1070.html
 
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FDS

Well-known member
#16
If you want the lightest weight foamboard in the UK, you will need FliteTest branded board, available from a Model Shop Leeds, Gliders UK and a Sussex Models, plus a few others. It’s about 40% lighter than EU foamboard in 5mm, with the option of removing the backing paper on the inside of folded sections like fuselages or wings, which won’t massively compromise strength but saves even more weight. Look out for added weight of a wing spar if you are lugging cargo with a long wingspan, braces may also be required to stop it lifting up under load.
 
#17
So you are in a heavy lift contest with an a plane. The plane with the highest loaded weight to empty weight ratio will win.

You are going to want a large diameter, shallow pitch prop. Fine the largest diameter prop the motor can safely pull. You will need a watt meter to insure that you are not over taxing the motor. The spec sheet suggests a 12x6 prop on 3S, I suspect the motor will pull a 13x4 prop without exceeding the 33A rating.

Building a light airframe and wing loading will be the key. For heavy lifting I think I would start with 15 oz/sqft. I think I would start with a wing measuring 6' by 13" and work up from there. A larger wing and/or heaver wing loading may also work. You will want a flat bottom wing with a 15% thickness located about 30% back form the LE. This plane could easily be build under 30 oz (850g) AUW (battery, motor, everything) with US foamboard. Something resembling an oversized FT Bushwhacker.

4-Max PO-3541-1070 motor - https://www.4-max.co.uk/po-3541-1070.html
Thats correct, we will be awarded points for having an aircraft that is able to lift 4kg of water (payload) but the aircraft must be the lowest it can be. Myself and my team are working to build a plane that is around 1.75kg in weight by using balsa for majority of the structural components as well as ply for reinforcements.
Currently based on our calculations and our FEA analysis, our wing span will be around 2 meters and so we can generate the required lift. Our groups aerodynamiast has found that eppler 422 aerofoil generates the most amount of lift for our low speed aircraft even though we are all aware of its design complexities.
Okay thats really useful, I can discuss tests of using a 13x4 prop in my report too.
 
#18
If you want the lightest weight foamboard in the UK, you will need FliteTest branded board, available from a Model Shop Leeds, Gliders UK and a Sussex Models, plus a few others. It’s about 40% lighter than EU foamboard in 5mm, with the option of removing the backing paper on the inside of folded sections like fuselages or wings, which won’t massively compromise strength but saves even more weight. Look out for added weight of a wing spar if you are lugging cargo with a long wingspan, braces may also be required to stop it lifting up under load.
We found that using balsa and ply conservatively allows us to be lighter than using foam boards but it is something we have deeply considered. Although we might consider using foamboards for trailing edges or control surfaces (Elevator and Rudder only since we won't be adding ailerons due to the weight factor)
 

Merv

Well-known member
#19
build a plane that is around 1.75kg in weight by using balsa
That is a very heavy plane. I have build many foamboard planes of similar size all under 850g. I grant that my planes would not lift 4kg, they would easily lift 2kg. My plane would likely carry more than 2kg.

The way the contest is structured (Payload/Empty weight) x 80, my plane carrying 2kg would beat your plane carrying 4kg. The idea is not who can carry the most weight BUT who can build the lightest plane. US foamboard is much lighter than EU foamboard. A 20"x30" sheet is about 4 oz.

It will not do any good to build a plane to carry 5kg, the judges will not weigh anything over 4kg.
 
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Jackson T

Well-known member
#20
If you want the lightest weight foamboard in the UK, you will need FliteTest branded board, available from a Model Shop Leeds, Gliders UK and a Sussex Models, plus a few others. It’s about 40% lighter than EU foamboard in 5mm, with the option of removing the backing paper on the inside of folded sections like fuselages or wings, which won’t massively compromise strength but saves even more weight. Look out for added weight of a wing spar if you are lugging cargo with a long wingspan, braces may also be required to stop it lifting up under load.
How heavy is a FT foamboard sheet? Are they 20x30in?