somegeek

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From searching the archives, I can't find one prop thrust calculator that folks appear to swear by... or I am not looking in the right place. Anyone able to suggest a thrust calculator that they've successfully proven accurate with real world results?

I am turning an APC Sport 10x3 prop at 14,300rpm on a GMS .32. I've looked out there and tried a few Excel calculators and sites and the results vary from 4.4lbs to 6.5lbs. ThrustHP was also said to be overly optimistic.

Appreciate any input!

somegeek

rainedave

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Howdy somegeek! This is the one everyone around here uses: [link=http://mvvs.nl/prop-power-calculator.xls]Prop-Power Calculator[/link]. It was created by Pé Reivers. It's by far the best one on the web.

David

somegeek

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Excellent - thanks, RD!

microsprint9

I find ThrustHP to be fairly close, on my friends 42cc gas/glow conversion we are spinning a 20x6 at 7000, thrustHP said 22.19lbs of thrust, engine mounted on the plane and attached to a fish scale gave us 19.2lbs of thrust so that is pretty close. On my TT PRO.46 i turn a 12.25x3.75 at 12000, thrustHP said 9.73lbs of thrust, while i haven't tried the fish scale, i will say my 5lb TWIST will rocket out of my hand vertical until i let off which means i have way more than a 1:1 power to weight ratio. So all in all i find ThrustHP to be accurate enough for my calculations before mounting an engine/prop combo on my planes.

pe reivers

thrustHP in general is 10% to 20% too optimistic. For practical purposes, where pilots go out of their way to save an ounce of weight, I think that a thrust estimate of 22lbs instead of the measured 19lbs is a bit too much off target. (~16% in this case). The calculator should rather err low so it will make a promise it can keep.

FrancisPerson

In my testing I subtract 15% from whatever Thrust HP's APC numbers are and you will be close. master airscrew numbers are not so optimistic.

Francis

microsprint9

Well i was 16% low, but those were Zinger wood #'s in ThrustHP and we were using a dynathrust prop, so maybe that prop is 16% lower than the Zinger.

FrancisPerson

No slight was intended as I use Thrust Hp all of the time to try and predict performance. It is very inaccurate for oddly shaped props. For example I cut a master airscrew 8x5 undercambered electric prop down to 4.1" for use in a Turbo bee (see RCU videos) and mounted it on a stand and measured 19 oz of thrust at 20,000 RPM. Thrust HP says this will put out 5 oz of thrust???


Francis

microsprint9

So a 4.1x5 prop at 20000 got 19oz of thrust, wow, that must be a heavy plane, you better market that one.

How did you measure the thrust?

DarZeelon

Francis,


What you have there is essentially a ducted propeller.

The duct around it increases the effectiveness of the prop dramatically.
This is especially so in a static situation, where an 'open' prop may just churn the air, not making much effective thrust...


Besides, what diameter, pitch and most importantly, factor did you dial in.
Surely not the factor for a 'standard' 4.1x5 MA, with those very wide (in proportion to the diameter) blades...

So there are two things; the factor must be ~3 times as large and the duct...

FrancisPerson

My Turbo Bee weighs 23 oz and has great performance. My test rig was suspended by string for minimum friction. I will post a pic later.

Francis


Dar,

What is this factor that you mention? How do I set it? FYI the prop only turns 18,600 RPM in the plane???

pe reivers

unrealistic juggling of prop parameters will get me above 17oz thrust, (i.e. 24oz@prop power factor 5) because the prop is stalled when operated in free air due to it's very wide tips, and other non-conformancies near the prop hub. Stall sets in at abobe pitch/dia ratio of 1.2. In such small props, the tip vortex covers a major part of the prop disk, so I think there is no calculator around that will predict anywhere near correct at these extremes.
Removing the tip vortex as in ducted fan use, like Dar said, may yield higher values. However, this is not possible in my calculator which is for props, not ducted fans.

edit,
BTW Francis, If I set my calculator at prop factor 3.5, I get your thrust, at ~155W motor shaft power (~200W motor input power)
Have you measured motor voltage and Amps?
Just curious and willing to absorb.

AJsToyz

Is that a wide blade prop ?? Just wondering ...

DarZeelon

Francis,


Please read what Pé Reivers wrote... That should give you a good reply.

Although you had used the ThrustHP (which I cannot trust); the Reivers PropPower is much better and employs such a factor that the user must dial-in to get the results. You can use it, or download it from [link=http://mvvs.nl/prop-power-calculator.xls]right here[/link].

It requires MS Office, or Google OpenOffice to use independently on your PC.

blw

My Feedback: (3)

You can also run it with iWorks that comes with Macs if you have one of those.

pe reivers

The blade width and other things all is about ratios. If you compare the blade surface to the prop disk surface, then it is a very wide blade compared to the original MA ratio. Think of it as the percentage prop disk that is covered by the blades, and think of thrust as the aerodynamic force that is acting in the very thin disk, which is the prop disk. In front of the disk is low pressure, behind the disk is high pressure. Air tends to flow from high to low pressure, so in a small prop a lot of the total amount just escapes back to the front of the prop disk. In a fan, this is prevented by the fan shroud.

Grammar edit only.

FrancisPerson

Pe,

I used a MEGA 16/15/4 motor that pulled 23Amps @ 11V.

Thanks
Francis

pe reivers

11x23=253 Watts as compared to 155 Watts estimated by the calculator. Trust efficiency went down the drain fast, probably due to the high jetstream speed.
I may have some more thinking to do for such small high speed sizes. It would be nice if I could get it down to decent deviations like 5% off or so. Hopefully not too many variables like tailpipe diameter reduction etc. which reduce thrust, but increase speed and power drain. From the pictures, I guess your plane had about 60% tailpipe outlet area as compared to the prop disk section, in which case it may be as simple as 100/60*155=258Watts. Close enough for all purposes.

Edit:
OTOH, reducing airflow through a fan by adding restrictions, makes the rev's go up and reduces motor load, so above reasoning cannot be used.
end edit

Anyway, thanks very much for the data.

DarZeelon

Francis,


That calculated to 253 Watts of power input.

Amperes X volts is equal to Watts.

The actual output from this motor is this number of Watts X the efficiency of the motor in this situation.
If it is 80%, the output is roughly 200 Watts.

FrancisPerson

I will measure the inside diameter of the pipe and take some closeup photos of any obstructions. The motor supports are aprox .29" thick, about 1" front to back sanded in a symetrical airfoil and made smooth by being covered in monocoat.

Francis

scratchonly

google " Aero design propeller selector"; safe download, I have compared it to some static readings with a scale,it seems very accurate.

pe reivers

That is the program I still use to do cross checks as needed. It is not bad at all, but you cannot use it to simulate a prop running in very dense fluid like water, or at elevated field locations with thinner air, or produce power curves, or produce speed graphs, nor can you compare two engine/prop combo's with each other.

FrancisPerson

Pe,

The tube is 16.5" long, has a inside diameter of 4 3/16" and the prop is 4.75" back from the front.

Francis

pe reivers

Is the tube straight? (inlet and outlet diameter the same?)

edit:
much on propeller design can be found in the [link=http://naca.central.cranfield.ac.uk]NACA files[/link] Cranfield-UK. check the years 1925, 26, 27

FrancisPerson

Yes the tube is straight except for a slight lip in the front.

Francis