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determining grams of thrust

Old 09-23-2011, 02:48 PM
  #1  
chambo88
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Default determining grams of thrust

hi guys,

just wanting to know if there is some kind of formula for determining the grams of force an eletric motor puts out. (or if it has to be tested manually)

i have been flying for a while now and have started to design my own aircraft. only problem is im not sure how to find this figure if its not listed in the specs list b4 i buy the motor.
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Old 09-23-2011, 05:28 PM
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JetPlaneFlyer
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No there is no simple way to calculate thrust from the power of the motor. There are many variables such as motor kv, prop diameter, prop pitch etc. all of which can make a huge difference to how much thrust you get per Watt of power. Also consider that static thrust is not much use on it's own anyway because it's actual thrust at the plane's flying speed that matters.

Motocalc is a good tool for working this stuff out.

Steve
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Old 10-18-2011, 07:58 PM
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Theoretical as in Motocalc is fine but at end of day ... it's practical that matters.

In this end I start with the rubbish figures that motor sellers state ... reckoning that they are probably overstating by about 25% ... I see what prop they recc'd and what rpm / watts / thrust subject to the 25% ... This then gives me a starting point.

So I now have my motor / prop combo ...

It's then a matter of fine-tuning the amps draw via wattmeter and the thrust req'd / made .. measured by the "Luther Vertical Hanging Scale" ...

I use a digital fishing scale bought of ebay for about $5 ... I hang that from ceiling beam ... have a strap from hook down, round tail of model so it hangs vertically well clear of floor and other objects. I switch on scale and zero the reading so model weight is accounted for.
Now advance throttle and watch the scale reading showing the thrust of the motor / prop combo. With care you can put to full throttle while just stopping the mdel from torque turning ...

The thrust figure obtained is static thrust i agree ... but gives a good indication of whether suitable for the model. You can play with props etc. to see the changes as well as running a wattmeter for other readings. The combiantion is a powerful tool and in my opinion gives more worthwhile results than any theoretical calculation.

My opinion anyway ...
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Old 10-18-2011, 09:46 PM
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kyleservicetech
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Originally Posted by JetPlaneFlyer View Post
No there is no simple way to calculate thrust from the power of the motor. There are many variables such as motor kv, prop diameter, prop pitch etc. all of which can make a huge difference to how much thrust you get per Watt of power. Also consider that static thrust is not much use on it's own anyway because it's actual thrust at the plane's flying speed that matters.

Motocalc is a good tool for working this stuff out.

Steve
Agreed:
Only problem is motocalc seems to be about 20% high in their estimate of static thrust with a given prop/esc/motor. This 20% high value was found in four or five of my KW sized models.

Only guess is motocalc may not allow for the prop blast hitting the wings and fuselage. Don't know for certain.

At any rate, as others have indicated, performance in the air is what counts. You could have a large diameter prop with very low pitch that could put out a LOT of thrust. But because of that low pitch, that prop/motor configuration might not get the model off of the ground with enough forward velocity to fly.
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Old 10-19-2011, 12:28 AM
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Yeah, I've found Motocalc overestimates ststic thrust too. As you say perhaps the installed losses due to airframe drag are neglected. Possibly a bench test rather than in installed thrust test would give figures closer to the Motocalc estimate?

Steve
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Old 10-19-2011, 01:09 AM
  #6  
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It's perhaps also worth making it very clear that MOTORS don't produce any thrust at all. It's only the propeller that produces thrust.

If you can work out what rpm a motor with a particular battery/voltage can turn a particular prop at then you can relatively easily calculate the static thrust for that combination. But change the voltage or the propeller and the same motor will behave completely differently and produce a very different thrust figure.

Steve
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Old 10-19-2011, 03:49 AM
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Originally Posted by slipstick View Post
It's perhaps also worth making it very clear that MOTORS don't produce any thrust at all. It's only the propeller that produces thrust.

If you can work out what rpm a motor with a particular battery/voltage can turn a particular prop at then you can relatively easily calculate the static thrust for that combination. But change the voltage or the propeller and the same motor will behave completely differently and produce a very different thrust figure.

Steve
That is precisely why I use my practical method of actual measurement ...
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Old 10-20-2011, 02:22 AM
  #8  
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If you don't trust calculators such as DriveCalc, Badcock, Motocalc etc, here's one way to do it: http://www.rcgroups.com/forums/showt...t=thrust+stand
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Old 10-20-2011, 03:16 AM
  #9  
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One way I calculate thrust is with a tach measuring RPM 's of the prop. I then use this formula to estimate thrust...

http://personal.osi.hu/fuzesisz/strc_eng/index.htm

I just started doing this and am a newby at this calculation...my question is...should the static thrust estimated from this method be greater than the weight of the plane in order for the plane to fly ( ie right motor and prop)? In other words...how do you interpret this information?
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Old 10-20-2011, 03:53 AM
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Originally Posted by Nieuport Fly Guy View Post
One way I calculate thrust is with a tach measuring RPM 's of the prop. I then use this formula to estimate thrust...

http://personal.osi.hu/fuzesisz/strc_eng/index.htm

I just started doing this and am a newby at this calculation...my question is...should the static thrust estimated from this method be greater than the weight of the plane in order for the plane to fly ( ie right motor and prop)? In other words...how do you interpret this information?
Ok ... this is where all the theory falls apart in fact ... sorry gents !!

Thrust is fine and the more you have - the better you are. But thrust is only a part of the equation. The pitch of the prop is also an important factor in whether you fly or not.

Pitch determines the distance a prop will move in one rotation.

So a combination of the two determines the flyability of the prop combo.

You can incredible high thrust but if pitch is very low - the model will never fly fast enough to have wing generate lift. The prop reaches it's pitch speed forward too early. If you increase pitch too high ... then motor is overloaded and again not enough speed. So you have to aim for an ideal speed (rpm) to obtain a good balance of thrust and pitch distnace forward to achieve flying speed ...

Think on this .... A harbout tug boat has incredible power and thrust ... but it's speed through water is slow ... it reaches prop speed at very low speed and then thrust is ineffective other than to maintain that slow speed ... is why it can move ships far more huge than itself.
Get a powerboat of similar size ... it is far faster but with a lot less thrust ... because it has a high pitch prop ... we'll ignore the much higher RPM as really it's the thrust / pitch we're looking at.

In answer to your question .... it is good to have thrust as near to model weight as possible ... you can always throttle back. As you reduce thrust vs model weight ... you get harder to fly, assuming that prop is suitable for model. Personally I aim for about 80% of model weight in thrust and that seems to give me ample power and flight .. 100% if I can !! many actually fly on a lot less ...

Another factor that affects the answer ... a Clark Y or other flat / inverse cambered wing will fly at lot lower speed, lower thrust requirement than a symetrical or semi-symet wing by virtue of it developing lift much more than the symet. So you can get away with low powered trainers ... but need to up the anti when you get to aerobatics etc.

OK ?
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Old 10-20-2011, 04:23 AM
  #11  
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Originally Posted by Nieuport Fly Guy View Post
One way I calculate thrust is with a tach measuring RPM 's of the prop. I then use this formula to estimate thrust...

http://personal.osi.hu/fuzesisz/strc_eng/index.htm

Just tried this with several of my model setups. One is a Hacker A60-16M motor that turns an APC-E 19X12 Wide blade prop at about 7000 RPM. Results on this web site are very close to the measured results. Same for a Hacker A50-16S motor and a 16X12 APC-E prop/ that turns at about 6700 RPM. Quite close to measured results.

Good info!
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Old 10-20-2011, 02:07 PM
  #12  
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Data for a lot of the props in Badcock, and some of the props in DriveCalc come from measurements from my thrust stand... since the equations they use for estimating thrust are derived from those data... it follows that their estimates match what one would get from a similar thrust stand quite well!

Circular reasoning... but I believe that my measurements have been reasonably "accurate" and certainly "consistent"........and I'm fairly sure that data from others who have contributed to those calculators can be relied on too.....any inconsistencies/errors are usually quickly discovered by those who use the programs.
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Old 10-20-2011, 02:26 PM
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Originally Posted by Nieuport Fly Guy View Post
I just started doing this and am a newby at this calculation...my question is...should the static thrust estimated from this method be greater than the weight of the plane in order for the plane to fly ( ie right motor and prop)? In other words...how do you interpret this information?
Static thrust is not really the most useful measure and it's certainly not the only one as solentlife has said. Pitch speed is also important.

But anyway, there is definitely no need for the static thrust to be even equal to the weight of the plane unless you want it to hover or do other "3D" stuff. So it depends on the type of flying you intend doing. I have slow fliers and motor gliders flying well with static thrust less than 30% of the weight and even aerobatic planes will fly beautifully with measured static thrust around 70-80% weight.

Steve
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Old 10-20-2011, 04:48 PM
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Originally Posted by slipstick View Post
Static thrust is not really the most useful measure and it's certainly not the only one as solentlife has said. Pitch speed is also important.

But anyway, there is definitely no need for the static thrust to be even equal to the weight of the plane unless you want it to hover or do other "3D" stuff. So it depends on the type of flying you intend doing. I have slow fliers and motor gliders flying well with static thrust less than 30% of the weight and even aerobatic planes will fly beautifully with measured static thrust around 70-80% weight.

Steve
Thanks....i have been building a number of kits...then taking the thrust measurements. And afraid to fly them as the thrust was less than the weight! Knowing this I'll take them for a test flight!
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Old 10-20-2011, 06:23 PM
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Originally Posted by slipstick View Post
Static thrust is not really the most useful measure and it's certainly not the only one as solentlife has said. Pitch speed is also important.

But anyway, there is definitely no need for the static thrust to be even equal to the weight of the plane unless you want it to hover or do other "3D" stuff. So it depends on the type of flying you intend doing. I have slow fliers and motor gliders flying well with static thrust less than 30% of the weight and even aerobatic planes will fly beautifully with measured static thrust around 70-80% weight.

Steve

Out of curiosity, anyone know the approximate static thrust ratio of a full sized plane like a Piper Cub?
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Old 10-20-2011, 07:49 PM
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Originally Posted by kyleservicetech View Post
Out of curiosity, anyone know the approximate static thrust ratio of a full sized plane like a Piper Cub?
I believe a full scale J3 Cub had 390lb of static thrust, and a normal flying weight in the 1000 to 1200lb range.. So that's 0.33 - 0.40 thrust/weight.

Steve
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Old 10-21-2011, 01:31 AM
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Originally Posted by JetPlaneFlyer View Post
I believe a full scale J3 Cub had 390lb of static thrust, and a normal flying weight in the 1000 to 1200lb range.. So that's 0.33 - 0.40 thrust/weight.

Steve
Thanks
That sure explains why those full size planes have a 1000 foot take off, and a model of that plane can take off in 20 feet!
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Old 10-21-2011, 04:16 AM
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Originally Posted by kyleservicetech View Post
Thanks
That sure explains why those full size planes have a 1000 foot take off, and a model of that plane can take off in 20 feet!
Lets be honest - most models can take of in less than 20' ... and are grossly overpowered compared to their full-size cousins.

Put another way ....... most fullsize need a shallow dive before considering a loop or other manouevre ... but most models will pull straight into it. take it further ... there are many full size that cannot loop ... but the model can.

It is generally accepted that the model will outperform it's full size counterpart in nearly all areas except weight carrying and size.

Model extremes :

In 1980's my display machines were a Biplane that was aerobatic but about 70% thrust to weight ... and the other was a WOT4 with 61 engine and tuned pipe putting out 19K rpm ... with a thrust /weight ratio that exceeded 1:1 by a large margin giving me vertical out of hand take-offs ...
Other extreme was my Sopwith WW1 job scale with about a 35% thrust to weight that relied on airspeed to even think of flying ... similarly my Competition Nieuport 28 in the avatar ...

In the 80's in the UK the mags published a plan of an F-15 built entirely of flat-sheet 3/16 balsa. No aerodynamic aerofoil at all, wings / tail were absolutely flat sheet ... no curve or form ... it relied on +incidence and airspeed to fly ... and it flew very very well ........ In fact at the club where a guy built one that I flew at ... it was not far behind a Club 20 pylon machine when WOT ...
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Old 10-21-2011, 06:39 AM
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Originally Posted by solentlife View Post
Lets be honest - most models can take of in less than 20' ... and are grossly overpowered compared to their full-size cousins.

Put another way ....... most fullsize need a shallow dive before considering a loop or other manouevre ... but most models will pull straight into it. take it further ... there are many full size that cannot loop ... but the model can.

It is generally accepted that the model will outperform it's full size counterpart in nearly all areas except weight carrying and size.

Model extremes :

In 1980's my display machines were a Biplane that was aerobatic but about 70% thrust to weight ... and the other was a WOT4 with 61 engine and tuned pipe putting out 19K rpm ... with a thrust /weight ratio that exceeded 1:1 by a large margin giving me vertical out of hand take-offs ...
Other extreme was my Sopwith WW1 job scale with about a 35% thrust to weight that relied on airspeed to even think of flying ... similarly my Competition Nieuport 28 in the avatar ...

In the 80's in the UK the mags published a plan of an F-15 built entirely of flat-sheet 3/16 balsa. No aerodynamic aerofoil at all, wings / tail were absolutely flat sheet ... no curve or form ... it relied on +incidence and airspeed to fly ... and it flew very very well ........ In fact at the club where a guy built one that I flew at ... it was not far behind a Club 20 pylon machine when WOT ...
Yeah
The power to fly a model increases something like the cube of the ratio of the wingspan. Something found out very quickly when you go from a 45 inch electric to a 90 inch electric model. The $$$$ cash register really increases fast on those larger electric models.
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