Measuring Thrust - RC Groups
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Mar 11, 2002, 06:36 PM
drbird's Avatar

Measuring Thrust

Are there any devices that are moderately inexpensive and readily available to measure thrust?
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Mar 11, 2002, 07:26 PM
Registered User
Daren's Avatar
Any moderately accurate spring scale will work. You can either hang it nose up from the tow hook (if bungee launched) or nose gear (if retractable) and check the weight without thrust, then run it up to full throttle and see how much lighter it gets and figure the difference to get your thrust. Or, you can hang it nose down and see how much heavier it gets to figure thrust.

As a last resort, you could place it nose down on a postal scale, run it up and check the weight difference. All are accurate enough for our purposes. Remember, these are all static thrust which is not what the jet will get once the fan unloads in the air.

Tailpipe velocity is a different story. Dan and I are lucky enough to have access to an airspeed indicator which has been adapted for this purpose.

Mar 11, 2002, 07:30 PM
drbird's Avatar
How do i get a "how to" to make that device?
Mar 11, 2002, 07:36 PM
Registered User
dave morris's Avatar
make something like this out of foam, should pivot easy and thrust line to pivot pivot to scale should be the same distance. I also calibrate it by pushing the motor forward with a postal scale, and reading the digital scale at the same time.
Mar 11, 2002, 08:00 PM
Lithium Member
Herb's Avatar
For thrust measurement it's enough to hang the plane from the ceiling of your garage with two wires. The deflection from the vertical will give you the thrust, you don't need a scale (In any case upside down placement on a kitchen scale will only work if the there is no front intake).

For efflux speed measurements we use a $2 electric motor hooked up to an ordinary voltmeter. It needs to be calibrated, otherwise it will give you only relative speed measurements:

It's accurate enough to measure small deviations from uniform speed throughout the fan diameter.
Last edited by Herb; Mar 12, 2002 at 01:56 PM.
Mar 11, 2002, 08:12 PM
drbird's Avatar
that is a good idea but what is the votage compared to thrust - i mean how would you calibrate in order to measure thrust?
Mar 11, 2002, 10:41 PM
Tim H.'s Avatar
I just took a pic of how I did it. I'm using a digital scale I picked up from Walmart. Put a piece of foam on the scale, place airplane's nose on the scale, zero out the scale (gets rid of the aircraft wieght), throttle up and read scale. My F-18 is putting out 1lb. 9oz of thrust (25oz.) Not too shabby. All up wieght of airplane is 36oz. .7 thrust to wieght. 1 to 1 would be great!

Tim H.
Mar 12, 2002, 04:52 AM
drbird's Avatar
Originally posted by russ6421
I have been thinking about the samething, but what I wondered is if you took a good scale made some type of open platform made up of some type large screen. Mounted the EDF to blow down would you get a correct reading when you adjust scale to 0.???????

Do you mean like a "wind catcher" that would measure the thrust coming out?
Mar 12, 2002, 11:05 AM
Registered User
russ6421's Avatar
Some type of open mesh that wouldn't restrict the airflow. I think what I should have said is would blow up and put a downward thrust on the scale.
Mar 12, 2002, 12:20 PM
Dude, where's My Plane?
JasonJ's Avatar
Dave Ms rig looks the best. You dont want to measure the force of the air as there will be losses from divergence?? What you want is how much thrust can the fan put into the airframe.
With Dave's rig like he said the moment arms must be = so the distance from the axle of the fan to the piviot and then from the pivot to the scale contact point must be the same, calibrating it with two scales like he said it the way to go, then you can change the moment arms to account for any loss from the mechanics.
Its also as cheap and simple as you could want, his is made from foam.
I too use the in jet, nose on scale but there are fluxuations as you try to hold the jet up plus you are to far from the fan moment arm at the nose, the ideal way to do that would be to make a foam cradle that grabs the wings and sits on the scale.
Measurent of the fan thrust inside and outside of the plane is usefull to see your ducting losses but ohter than that, its the thrust in the plane that is the most relivant I think, again this is only staic thrust, but thats the best we have,,,,, .
Mar 12, 2002, 12:50 PM
drbird's Avatar
I saw a device a while a go that was just a small box that had a very thin ducted fan like setup and them a meter under it - it was basically used to measure wind velocity - I wonder if that device would work or if it would even be accurate?
Mar 12, 2002, 02:24 PM
Lithium Member
Herb's Avatar
Originally posted by drbird
i mean how would you calibrate in order to measure thrust?
Thrust=weight * tan (angle). So 45 degrees deflection -> thrust = weight etc.
Last edited by Herb; Mar 12, 2002 at 02:32 PM.
Mar 12, 2002, 02:31 PM
drbird's Avatar
The motors with the fans on the end - the thrust of the EDF blows the fans and rotates them creating a current which can then be measured - is that right?
Mar 12, 2002, 02:37 PM
Lithium Member
Herb's Avatar
Originally posted by drbird
is that right?
(rpm of sensor probe microfan) proportional to (efflux speed) ;
also (voltage out) proportional to (rpm of sensor probe).

Thus measured voltage is lclose to linear in efflux speed. Static thrust can also be measured locally, it is proportional to the rpm^2 or measured voltage^2 .
Mar 12, 2002, 03:59 PM
Registered User

I think your math is a little off. Thrust would equal weight at 90 degree (the motor would be hovering). At 45 degree, the thrust would be 71% of the weight. I think you forgot to account for the tension in the string that is supporting some of the weight.

It has been a while since I did static analysis, but I think it goes something like this (see attached image):

horizontal components

t * cos a = s * sin a

s = t * cos a / sin a

vertical components

w = s * cos a + t * sin a

combining to replace s

w = t * cos a * cos a / sin a + t * sin a

w = t * ( cos a * cos a/sin a + sin a )

w = t * ( ( cos a * cos a + sin a * sin a ) / sin a)

t/w = sin a / ( cos a * cos a + sin a * sin a)

Using this equation, at 0 degress t/w = 0, at 90 degrees t/w = 1, at 45 degrees t/w = .7071.


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