Designing best 11" Prop for thrust at 0-5 mph

[boomergeek]

I've got a special application for the best prop design to get the most thrust out of an 10-11" prop turning 8000-12000rpm on a specific brushed electric motor that can reach about 150W of output power but only needing the thrust when the prop is traveling 0 to 5 mph.

I tried APC 11x3 prop and Thrusthp indicates I should be able to get 2.6 lbs of thrust out of it at 7800 RPM: I only measure about 1.5 lbs of thrust at 0 mph.
When I used an APC11x6 prop at the same output power, I also get about 1.5 lbs of thrust at 0 mph. I was expecting to see the 11x3 prop produce significantly more thrust at 0 mph.

The props are only used to help taxi a very heavy non-flying object at less than 5 mph so the stall speed can be less than 8 mph.

When I set Thrusthp up for 11x1 (APC profile) it indicates I should be able to spin my motor at 11000 RPM at that wattage and produce over 5 lbs of thrust.

I can't find a 11x1 prop to test. I was thinking of trying to fabricate a prop optimized to the application if it can give me substantial improvement over the standard APC 11x3.

I'm a newbie and thought that some experts here may be willing to suggest something. My local RC dealer could not sugest anything.

Would a few days work to fabricate a prop produce a doubling of the thrust for the same wattage? How would I figure out the optimal shape?

Is there any real data for extremely fine pitch props? How far off is Thrusthp for pitchs below 6%?

Thanks in advance for any ideas.

Boomergeek

[lincoln]

Well, you could find Marten Hepperle's Javaprop and mess with that. You might figure out something. It disagrees with your results somewhat. Shows 4 inch pitch.

[boomergeek]

Thanks for the suggestion. Earlier in weekend I had started experimenting with Javaprop but when I reduced velocity I was expecting the optimum pitch to fall below 11x4. Tonight, I tried changing the geometry to lower the angle of attack from 3 degrees at all radii to be only 1 degree. As soon as I did that the beta goes to about 5 degrees for my very low target velocity. And the thrust versus velocity chart matches the needs of my application. If I can trust the results, then it indicates that an optimum prop with produce close to twice the thrust that the APC11x3 is producing for me.

The thrust profile is more consistent for what I'm looking for.

http://www.mvvs.nl/prop-power-calculator.xls seemed to indicate that I should not see a thrust improvement moving from an APC 11x6 to an APC 11x3. My actual test results seem to match the XLS program better than they match ThrustHp.

[slipstick]

ThrustHP is no help in this application since the formula it uses for thrust takes no account of pitch at all. It assumes "normal" flying with a sensible pitch speed required where diameter has by far the greatest effect on thrust.

You may need to do some real testing rather than relying on calculators which are intended for a completely different application of a propeller.

Steve

[vintage1]

For a pitch speed this slow, all you need is a helicopter style blade. i,e no twist is needed.

In fact the propellor wont really have any true (helical) pitch at all. For static thrust alone, you want a pair of blades set at a reasonably constant angle of attack.

You could make this by carving/planing a profile into some hardwood sheet.

[boomergeek]

[For a pitch speed this slow, all you need is a helicopter style blade. i,e no twist is needed.

In fact the propellor wont really have any true (helical) pitch at all. For static thrust alone, you want a pair of blades set at a reasonably constant angle of attack.

You could make this by carving/planing a profile into some hardwood sheet.

Thanks for the advice. I spent some time with javaprop- (what an awesome tool!). I am a neophyte at airfoils and this problem may get me hooked on rc. Is there any tool that predicts the thrust of heliprops? I found http://www.s4a.ch/eflight/helicalc_e.htm but I am sort of stuck with 11 inch or less diameter in the application I'm using. I also need to use a shroud. I was thinking of simply attaching a pair of eflite blades from their 10.25 adjustable pitch props... (but I'm sure they are narrower than the optimum.

The javaprop embodiment really looks like two clark foil airplane wings stuck on a big spinner.

Here is what I came up with in JavaProp...

[HX3D014]

For a pitch speed this slow, all you need is a helicopter style blade. i,e no twist is needed.

In fact the propellor wont really have any true (helical) pitch at all. For static thrust alone, you want a pair of blades set at a reasonably constant angle of attack.

You could make this by carving/planing a profile into some hardwood sheet.
what if he was to cut down a larger prop ?
would that be easier or ?

Bryce.

[Brandano]

Mainly "or" in this case. cutting down larger prop will leave you with the coarser pitch portion, that in this specific case is the most inefficient for the task. As Vintage correctly remarked this prop is mre akin to an helicopter rotor for the task it's supposed to accomplish. Mind you, some helical pitch should be built into it, partly because it's still moving through a relative wind, and partly because it's still creating its own moving column of air to spin through.

[vintage1]

I'll accept that on the day you show me a helically cut helicopter blade.. :D

[Brandano]

the PiccoZ and many other "toy" fixed pitch helicopters

[Jurgen Heilig]

I'll accept that on the day you show me a helically cut helicopter blade.. :D

Helicopter rotor blades optimized for maximum lift normally have less incidence towards the tips.

:) Jürgen

[boomergeek]

[

Thanks for the advice. I spent some time with javaprop- (what an awesome tool!). I am a neophyte at airfoils and this problem may get me hooked on rc. Is there any tool that predicts the thrust of heliprops? I found http://www.s4a.ch/eflight/helicalc_e.htm but I am sort of stuck with 11 inch or less diameter in the application I'm using. I also need to use a shroud. I was thinking of simply attaching a pair of eflite blades from their 10.25 adjustable pitch props... (but I'm sure they are narrower than the optimum.

The javaprop embodiment really looks like two clark foil airplane wings stuck on a big spinner.

Here is what I came up with in JavaProp...

Thanks for all the suggestions. The optimum airfoil from Javaprop for my application was definitely the Clark Y airfoil: all other canned airfoils (e.g., E193, ARA, MH 1XX) where the bottom curves inward along the chord produce 10 Newtons less of thrust (for the same hp). The RC helicopter blades I found in approximately the right radius all had highly curved blades along the underside of the airfoil. The beta, and airfoils along the radius for the 10x3.8 SF props seem to match the best I could do with javaprop. The big difference is that the optimum chord values for my application are about three times the length of 10x3.8 SF.

[Brandano]

I'd consider increasing the chord and using a thinner, more cambered profile. the sort of application you describe is pretty close to the design specs for a fan. Actually, since weight is not an issue, why not try and adapt the blades from an electric fan?

[boomergeek]

I'd consider increasing the chord and using a thinner, more cambered profile. the sort of application you describe is pretty close to the design specs for a fan. Actually, since weight is not an issue, why not try and adapt the blades from an electric fan?

I've simulated the thinner, more cambered profile airfoils using Javaprop: this particular application is very underpowered (130W) for the amount of thrust we are trying to get out of the motor/prop/shroud combination.

I think the lift (vertical thrust) to hp ratio I want is equivalent to the challenge of human powered flight. I think I am going to try to fabricate something close to what javaprop is recommending- three to four inch chord Clark Y airfoils twisted to give a relatively constant pitch of about one third the diameter of the blade.