Using RC airplane props in water?


#1

RC airplane props and ducted fans are cheap and purpose built to move fluid at exactly the scale we need for small to medium sized ROV's. Its only logical that we should try using them but the question is always raised " its designed for air, will it be effective in water?" fortunately the answer is YES and I just got bored enough to spend the 15minutes to look up the numbers to prove it.

Reynolds number is a dimensionless number that is the ratio of viscous forces to inertial forces in a fluid.

Re= VL/v V= velocity L= "characteristic length" v="kinematic viscosity"

one application is that the same shape will have the same flow around it regardless of fluid if the Reynolds number is the same. So if we take our airplane prop and put it in water, the velocity is proportional to rpm, the characteristic length doesn't change, and the kinematic viscosity goes from 15.68m^2/s *10^-6 in air to 1.004m^2/s *10^-6 in water. (yes air has a larger kinematic viscosity but remember its on the bottom of the equation)

All you have to do is spin the prop 1/15th the rpm in water than in air and it will be at the same Reynolds number and be as efficient as it was in air.


#2

Ah ha! Thanks for this post Ace!

We had the ROV in the pool last weekend and were having some issues with it being under-powered. There were a number of potential tweaks and fixes to try, but slowing down the RPM was one of the big ones. This is VERY helpful.

David


#3

Here's the prop we're planning to use. It looks like this is normally meant to spin between 20,000 and 30,000 RPM, so it spinning the props somewhere around 1500 RPM in water would probably be ideal. Thanks Ace!


#4

That’s exactly what we have been doing see videos


#5

Have you tried using the props that people use for RC boats?


#6

No but we should! Actually, we could use help finding some that are meant for one or two thousand RPM, and are roughly 70mm in diameter. Also, the would ideally be low cost and easily available so they would fit with our "openhardwarenesstisity". Any thoughts?


#7

These guys have a lot - I have to pass my Nitrox/Enriched Air course this morning so I don't have time to look at this moment. Will dig around more later, but I believe many of these are meant to be used on COTS brushless motors.

http://www.hobbyking.com/hobbyking/store/__255__254__Boats_Parts-Props.html


#8

Misc pointers: Propeller design program: JavaProp http://www.mh-aerotools.de/airfoils/jp_applet.htm

and note that static thrust is very different than thrust in motion, so keep that in mind. might be worth building a "trough" (long enough so you can get nice laminar water flow) or doughnut (say a kiddie pool with a round center blockage to make a channel) so you can test various props with moving water. super cheap way of doing things like this is to get plastic sheeting and place it inside of some sort of form (like a trench dug into the ground). remember water is heavy, so cardboard boxes won't work; too weak. tupperware tubs are somewhat strong, but will not hold their shape.


#9

Thrust produced by this propellers should be compared and the right combination of

> RPM

> ROTOR diameter should be found

You can use LARGER a larger propeller with less RPM

Or

Smaller propeller with more RPM

IMPORTANT

EFFICIENCY of PROPELLER at THE OPERATING SPEEDS

QUESTION: When spinning the PROP at 1/15th of the RPM in WATER.. is it OPERATING near its MAX efficiency?

ITS important to MATCH operating RPM and propeller efficiency - This will MINIMISE POWER requirements and increase the RANGE of the ROV

Usually WATER props have 2-3 blades..2

Important parameters PITCH/DIAMETER/Material used etc

http://www.modelgasboats.com/Knowing_Your_Prop/

http://www.olds.com.au/marine/maximizing_propulsion_efficiency/

etc


#10

QUESTION: When spinning the PROP at 1/15th of the RPM in WATER.. is it OPERATING near its MAX efficiency?

A: Yes, Assuming that you also have 1/15th the forward speed as in air and that the rpm/foward speed in air was at max efficiency

In reality the assumption sated above is rarely true but thats ok because...

max efficiency can only be accomplished at only one chosen forward speed. Deviating form this forward speed will affect your efficiency at least as much as your rpm diameter and pitch.

as it turns out larger diameters are more efficient at low speed(and in less dense fluids). since most ROV's operate at low speed in order to operate efficiently they need bigger props than the equivalently sized rc boat. rc airplane propellers and ducked fans are available in larger sizes(because air is a less dense fluid) hence the interest.


#11

I was thinking about boat props as well, and I found an interesting place:

http://www.harbormodels.com/site08/main_pages/propellers_propshop.htm

In particular, the PS3022 is a 3.0"x2.2" and is claimed to provide 5 lbs of thrust at 3500RPM. They also have a PS3033 which has a steeper pitch and so runs better at a slower RPM. These aren't cheap ($50 each), but they seem much more appropriate than a fan blade designed for air.


#12

that website is a good find there are also cheaper props of decent size there too 3.75" for $14

http://www.harbormodels.com/site08/main_pages/propellers_robbe.htm

However in defense of airplane props(and ducted fans): boat propellers are designed with strong disadvantages on high tip speed and are unlikely to be as efficient as an airplane prop when turning at low speed. This is because boat propellers (even small ones) are designed to use far more power at a far higher forward speed than most ROVs

In different words ROVs generally use boat propellers outside their design rage both in terms of forward speed(which is important to "max efficiency" and Reynolds number(which while related to forward speed/rpm is important to foil design and blade shape)

Interesting stuff: while cavitation is unique to liquids it has two causes that are common to all fluids

1) Tip vortex cavitation: when the low pressure caused by the tip vortex gets below the vapor pressure of the liquid. while this doesn't happen in air which is already a gas tip vortices are large source of drag and the long narrow profile of air propellers is much more efficient in this respect.

2) Suction side surface cavitation: again pressure falls below vapor pressure this causes detached flow better known as a stall (it must be noted that stalling in gas controlled by only foil shape and Reynolds number; while vapor pressure is an additional concern of liquids). Suction side surface cavitation is unlikely to happen to airplane propeller in water because they have a lower pitch vs diameter(much lower) and rpm(for the same Reynolds number in water)

The reason that boat propellers don't look like airplanes goes back to the post about maximizing efficiency:

In order to get high efficiency at the desired foward speed(usually 10-40knots) requires a very high pitch low diameter prop and the blades are made wide as a design compromise between efficiency(narrower is better) and thrust (wider is better) (the additional thrust from a wide blade allows a smaller diameter giving less drag and (potentially)better overall efficiency).


#13

I agree that the ROV design point is different to the high speed boat design point. My guess is that, for the ROV, the key aspect is power consumption (really efficiency) for low thrust.

However, thinking some more, it isn't clear to me whether the majority of the power is consumed in the coarse maneuvering trying to get the ROV from the launch point to the exploration zone and back.

Do you have any guesses on the thrust required to do 1 m/s and 1 cm/s? I'm thinking that a mission might involve 2 minutes (each way) of high speed travel, and then (say) 40 minutes of slow speed. Based on this (unrealistic) assumption, it seems entirely possible that the high speed section consumes the vast majority of the energy -- and hence we need to optimize on the high speed portion of the journey.

p.s. I have no experience in aquatic design -- but I did work on an odd airplane design. I'll have to dig out my books.....


#14

Ducted fans are very un-efficient under water. The blades are too big and hard to spin ( low RPM = low thrust), that causes excessive amp-draw and heat. RC 3-bladed props are best choice for under water use, they are efficient and give lots of thrust on smaller amp-draw = longer battery life

One more tip: Props with 2-4 blades are good under water (best are 3-bladed), but when you will add more blades it will start losing efficiency.


#15

I think that some actual performance tests would be useful here. In particular, I think it is all about power consumption.

I.e. what is power versus thrust curve for a particular prop?

Given that the drag is proportional to the square of the velocity, you can then discover the optimum speed to minimize power consumption to move any distance. You have to factor in the power used by the controller, leds, camera etc to make it a fair assessment.


#16

what motor/voltage/fan are you using?

the ducted fan I've been messing with seems pretty efficient producing about 5lbs of thrust(static) and moving a very substantial amount of water while consuming just over 100watts

if your using an off the shelf ducted fan with preinstalled motor then bad results are to be expected. The preinstalled motor is designed to spin very fast and doesn't have much torque at all. If its put in a dense medium like water it will suck ridiculous amounts of current and still not have enough torque to spin very fast.


#17

They use this motor: Link Esc: Link

And like I have understand they use 8 C-batterys to get 12v.

I have messing with underwater ducted fans to. I have found out that you need to use low Kv torquey outrunner, get a 6-blade impeller (Something like this) and remove 3 of the blades so you will get Y 3-blade impeller - that will give best thrust and will use less current.

I dont know why less blades under water are better, but have you seen boat outboard with more than 3-blades (some big cruise ships have 4-blades)


#18

ahh... they are using the the impellers backward for mounting convenience and don't have a thrust ring (the lip on the front of a ducted fan produced a significant amount of lift(in some cases up to 60%) and improves flow to the impeller further increasing efficiency)

those impellers were designed for ease of assembly and would preform much better if the ROV drove backward and had thrust rings

less blades on a propeller is more efficient for the same reason that mono wing aircraft are more efficient than bi-planes or tri-planes, the blades/wings are exposed to air that is affected by another blade/wing. theoretically one blade is the most efficient but has imbalanced forces, hence two blades is generally considered the most efficient practical design. Three is more balanced with respect to gyroscopic and cyclic forces while still maintaining good blade spacing. Higher blade count gets more thrust per disk area this in some cases can save drag that would have been caused by a larger prop and can result in increased overall efficiency(this becomes more likely to be the case as forward speed increases).


#19

Can this 3-blade EDF be good underwater with right motor. We can get 3-blade prop from it :D.


#20

looks good to me though it does not have a thrust ring/ intake lip; it will operate much more efficiently with one. it's supposed be mounted in a nacelle or body of an aircraft with intake geometry built in so i would use it the same way under water. so as long as you do that, remember the 1/15th rule and don't use it backward it should work pretty well.

if your thinking of using it as an open prop it will probably produce ok thrust, but as with all fan impellers it lose will lose efficiency from tip vortexes that the duct prevents.