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-   -   1/2A THRUST-O-METER (https://www.rcuniverse.com/forum/1-2-1-8-airplanes-70/1562254-1-2a-thrust-o-meter.html)

combatpigg 02-25-2004 12:30 AM

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I just got done mentioning to AJC that it would be nice to have a way to see real world output of our tiny powerplants. The whole package needs to be considered, for example ROSSI can boast about their 1.2HP .15, but let's throw away the HP numbers and see what the little sucker can lift. You can compare props, fuel, lots of ways to get use out of this thing. The semi- friction free rails are 3/16" steel rod, and i'm using kite rod to slide over the steel rod. The cross piece is 3/4 plywood, and I rounded the leading edge to make it look more scientific;). The fuel tank is on a seperate stand to isolate it from vibration, and to keep the "gondola" as balanced as possible. The tank could be mounted on center beneath the gondola maybe. I got a baseline weight, which was 5ozs, and added enough steel nuts to equal 7.4ozs on the first "pull". It hauled it easily, so I added more weight, 9ozs. It could lift it barely, but that was pretty much all it could maintain in a hover. Now I will switch props later to see what the difference is.BTW, I was running a 5-3 on 15%, don't have an rpm reading yet.

Remby 02-25-2004 12:41 AM

Neat idea!

propjobbill 02-25-2004 12:56 AM

I really like your idea. That's what really counts is how much Thrust an engine can produce. I could use something like this to find out how much thrust an electric motors produces with various gears, props, batteries, etc.
I see the fuel line going to the tank.
I don't understand how hooking a line up to the fuel nipple will allow it to feed the engine. Can you please explain?
Your Idea is Too Cool

BMatthews 02-25-2004 01:10 AM

I've got a rig I use for testing electric motors and prop combos. It's an arm about 18 inches long with 12 for the motor arm and 6 for the spring side with ball bearings for the pivot. I made up a couple of springs that I use for resisting and measureing the thrust. One is quite a heavy spring and let's me measure up to 40 oz of thrust and the other is good up to about 14 for small FF motors and props.

I'll get a pic of it in the next few days for you to check out.

propjobbill 02-25-2004 01:17 AM

Thanks Bruce,
I would love to see that. I've been looking for a way to know how much thrust my electric motors produce.

combatpigg 02-25-2004 01:54 AM

HI PROPJOBBILL! So I could get a longer motor run, I drilled a hole in the side of the tank, and connected small silicone tubing directly to the nipple inside the tank. 2 ozs is enough to run for 12 to 15 minutes on 15% fuel.

combatpigg 02-25-2004 02:07 AM

HI BRUCE! I like the idea of the ballbearing pivot! Do you also look at current draw? Maybe nowadays they have wattmeters that monitor current and applied voltage simultaneously? With electric you have the advantage of precision instruments [multimeters] that are easy to afford. Are the peak thrust readings smooth and easy to read?

propjobbill 02-25-2004 05:48 AM

Hi combatpigg,
I had thought about drilling a hole in the side of the tank on some of my baby bee engines to hookup an external tank so I could get a longer run. I think I may try building something similar to what you have there. I was thinking if the engine could be mounted on a swinging arm that pivoted some what like Bruce described, only instead of a spring, I could use a string go backward to a pully. Let the string hang down then attach the string to a small container which could be filled with weights as needed to determine how much the engine could lift.
I would first have to determine how much pull was required to lift the empty container and then add that figure to the weight as needed. I already have lots of 1/2, 1, and 2 oz. fishing sinkers. In my own mind my idea will work, but I will know for sure after I build one and try it out.
I have a universal type of mount that I use on 1/2a engines. I mount them on a 1/4" back plate made of plywood which has 4 lobes that stick out at the four corners. I mount one on the plane and one on the back of the engine. It's not for scale looks, but you can change an engine out in about 10 seconds. they are held on by wrapping rubber bands around the lobes. This mount also works real well when you teach a new flier to fly control line, on a hard crash the engine will pop lose saving the engine from damage.
By using the thrust meter I could experiment with different engines and determine what gives the most thrust.
I'm new at posting things on the web but if my idea works I will try to post some pictures.

combatpigg 02-25-2004 02:14 PM

Hi PROPJOB! Your ideas sound real good, to give you comparative data, but I still think the only way to know exactly how much my engine can "bench" is to have it do a dead lift with free weights[like at the gym]. The fancy machines at the gym with the cables, pulleys and springs don't truly duplicate the effort, [but they sure are alot safer]! My gizmo is more of a hassle to get a reading with because you have to fumble with the little weights. To make it easier to use, I'm going to glue a dowel to the gondola that the weights can be slid onto quickly. Now I have to go out and buy a white lab coat, and get a clip board, and maybe a real frizzy hair-do!

propjobbill 02-25-2004 03:01 PM

Hi Combat I know your idea is more accurate. Im just trying to find a way so that if I modify the back plate, or want to change from engine to engine I can with little problem. I guess I could add a tube to thefill spout and turn it upward to keep fuel from running out. Thanks for the imput. good luck with the frizzy hair-do!

MyWay 02-25-2004 03:52 PM

The best no fiction actual thrust meter that i have used is a plane set up ready to fly. Start the motor, hold the plane straight up by the fuselage and slightly release your hold. Assuming you know the weight of the plane ,if the plane will raise you know that your thrust is more than the weight of the plane. Add known weight to the plane untill the plane will not raise anymore. Crude but works.

combatpigg 02-25-2004 04:51 PM

HI MYWAY! Do that with a .40 NELSON on a mini pipe!? I do that myself with the little guys, and you're right about [air]being the most friction free, my 1/2A PBF could be trimmed to stand in a free flight hover for a few seconds, with my nervous, shaking hands ready to snatch it!

combatpigg 02-25-2004 09:15 PM

I picked up a stock TD from an RCU auction, and thought it would be interesting to see how much it could "bench press" with different props. I used 30% fuel, and a NELSON head. This engine didn't need a breakin by me, the original user did a nice job. For the test it was run on bladder. It was too dark to get tach readings, but I would say it sounded like a 21-22K engine with the 5-3 prop.


COX 5-3 14.4 wide needle

COX 6-3 16.6 that equals the weight of the gondola & nineteen 1/2" nuts.

COX 6-4 16.6 might as well run a 6-3, this one runs hot

FG 4.5-2.5 10.6 Great prop [fiberglass] for CL combat

APC 5.7-3 17.4 WOW!

APC 5.5-2 16.2 had to open the needle for this one, easy to needle, cool run

GRISH 5-3 13.8 good prop for weak engines

GRISH 5.5-4 15.0 never had any use for this one, needs to be cut down

GRISH 4.5-4 14.4 OOPS! see above comment

WOOD 7-2 14.4 had to close needle way down, ran ok

I think it would be interesting now to slap on a diesel head and go through this again. I don't have an APC 7-3 or I would do this test ASAP.

phuffstatler 02-25-2004 09:33 PM

My thrusto-meter had to do with how many "Whoa's!!!!" I got from my flying buddies when I come tooling by..... :) That's how I judged it... :D

phil in austin

Digger-RCU 02-25-2004 09:44 PM

Combat, I absolutely love it!!! That is totally cool! I really like the listing of the different prop combos! Keep it up dude.

Ps, I always knew that 5.7x3 apc was a great prop!

Randy (Digger) Birt

jessiej 02-26-2004 12:52 AM

Wather than the weights would it be easier to use a trigger pull gage? RCBS makes one that is quite inexpensive.


combatpigg 02-26-2004 01:02 AM

HI JESSIE J! I don't know. How does a trigger pull gage work?

Remby 02-26-2004 01:07 AM

I would add a meter for weight, or thrust. Suggest you look for a Fishing scale, can be had for a few dollars and would provive a easy basic mesure of the pull. The ones I've seen and used are small, with hooks on each end. Sould be just the ticket for this research !

combatpigg 02-26-2004 03:24 PM

WELCOME TO TODAYS' DYNO REPORT! The engine tested was the WASP .061, on 30%, compression adjusted for best rpm and needle. Ran with a stock head, and through the muffler.


5-3 COX 20000 16.2 can't go wrong, plus it's variable pitch

4.75-4 COX 20800 14.4 oink

6-3 MAS 16300 16.8 mistaken for COX in previous tests

6-4 COX 16500 16.2 outdone by MASTER AIRSCREW

5.7-3 19200 16.8 hard to beat

5.5-2 22500 16.8 better for 3D, and throttle response

7-2 WOOD 15000 12.0 good for low idle, airbrake effect

The mysterious confusion about the earlier test results was my fault. Yesterday it looked like the COX 6-3, and 6-4 props were giving identical results. The fact is I don't have a COX 6-3, it's a MASTER AIRSCREW instead[:@]. Bear with me, we'll get the story straight. I started to do a .061 diesel comparison with an old AME last night, but the THRUSTOMETER claimed its' first victim. The rod snapped in half cleanly in the middle! It was going to be a honkin' run too! Stay tuned, I'll slap a diesel head on the WASP next and get some #s.

jessiej 02-26-2004 10:44 PM

A trigger pull gage is basically a spring scale with a "L" shaped hook to be hooked around a trigger to test the weight and consistency of pull of a firearm. It records the highest pull reached. I think the one I have has a range of from a few ounces to twelve pounds.


codyhenning 02-27-2004 12:35 AM

Got any thrust measurements for a .061 norvel with a 6x4????

combatpigg 02-27-2004 01:02 AM

HI CODY! I think I'll "dyno" that one next. I'll take a wild guess and say it will be more than 18ozs. If it's much more than that, I'll have to go out and buy some more "precision" weights.

BMatthews 02-27-2004 02:14 AM

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As promised here is a couple of pics of my own thrust meter. So far I've used it only for electric motor testing.

In the first picture you see a close up of the test stand with the built in spring scale. I wound the springs from music wire myself as I could not find commercial ones that would work for my arrangement.... well, I actually built myself into a corner and HAD to make my own springs... but that's another story. The arm is some oak I had handy but I would have used a hardwood in any case as I wanted to push fit the ball bearing into the arm and it takes hardwood to hold the bearings without slippling around or enlargning the hole. My hand is pulling on the arm slightly so the tune of about 4 oz of simulated motor thrust. There's actually 3 springs in total. One for small thrust readings that goes from 0 to 16 oz and a second one that starts at 7 and goes up to 40 oz. The third spring is the calibrator so I can set the wingnut for each spring to a known setting so the calibrated scale on the board is accurate. The scale calibration was originally set using a very free pulley and a bucket of weight that I would measure and then hang on the line and mark the spot. This was checked a couple of times to ensure decent accuracy and repeatability. It's easily good and consistent to within 1/4 of an oz and that's close enough for what I'm doing.

The second picture shows the whole setup with a motor in place for testing. The big black box to the left is my new toy, a 25 amp 13.8 volt power supply. $59 US. If anyone is interested I can post the details. Before this I just used a tired but still decent car battery that I'd charge up for a day before the tests. The second box is my self made speed control that has a current meter in it and will handle up to 30 amps or so for long enough at a time to take readings. Note the test rig is clamped to the bench so the arm is over the side for prop clearance.

MR Flyer57 02-27-2004 05:43 AM

Hey CP,
Just love the KISS method you have used!
do you weigh the engine before you do the tests? Or do you figure it has to pull it's own weight and then see what else it can hoist up?
I think you are on to something, even if there are some problems matching different weight engines, you can make a case for different fuel and props. Have you tried any shims under the heads yet?
MR Flyer57

combatpigg 02-27-2004 09:18 AM

HI BRUCE,and MARSHALL! Bruce, your set up looks great! Placing the load spring next to the pivot point give you a nice, steady, jounce and bounce free indication, I'll bet.

Marshall, What I do is get a baseline measurement of the whole package, the engine mounted to the gondola, and enough weights tied on to bring the weight of the entire package up to 3/4 of what I figure it will do. My bob weights are 1/2" steel nuts, that have been weighed in advance. They weigh .6ozs each. I first tune each engine for max rpm, and needleability with head shims, the jump from 15% to 30% fuel usually takes one or two shims. The engine has to be able to hold it's position in a hover for 5 seconds before it gets credit for the effort. This test is of value to sport and 3D flyers, but isn't totally relevant to what the speed freaks need to know. They like the" in the air" testing because they unload those engines, and onboard speed recorders tell the whole story for them. whereas I am testing at max load the whole time. I have seen the thrust of electric motor/prop combos published before and thought it was high time to get some numbers on our powerplants. You can evaluate many different things this way quickly. I only burn an ounce of fuel or less to do these tests, and I spend more time searching for prop adapters than anything else[:@]!

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