TeeDee 049 real power output?

John Goddard Tue May 08, 2012 7:00 am

Ok
In my never ending quest to get a 55 inch wingspan model to rise off ground I've taken to doing several
calculations to equate BHP to Watts which (imo) is a more reliable guide to performance.
However I've stumbled across YET ANOTHER of those inacuracies/errors that abound the tinterweb.
To witt.
Most of the site's we get our info from quote the TD049 at 78 watts power output.
Like an idiot I've been basing my calcs on this 'truism'.

But looking at Wilfs 1961 test http://coxengineforum.forumakers.com/t1453-next-test-report-td-049-dec1961?highlight=teedee+test
the output appears to be 108 Watts ie .145bhpx746=108 Watts @21,000 rpm
This throws several spanners in the works of my calculations but does explain why a plane with
only 70/75 Watts (which I'd based on 'it must be nearish to a TD') seems almost as powerful as when it was electrified and actually measured at 130 Watts.
Obviously my KB'stard is much more powerful than I previously thought and probably exceeding 100 watts given that it's spinning at 22,00 rpm.
Very Happy

Kim Tue May 08, 2012 7:11 am

Say What ?? !!!!

Surfer_kris Tue May 08, 2012 7:48 am

An electric setup of 130W, usually means that you are drawing 130W from the battery, and not that you are getting 130W at the prop. The efficiency of all components combined is probably around 60% (ruffly speaking). This then gives you 78W at the prop and that is what the TD produces too...

John Goddard Tue May 08, 2012 8:07 am

Naw naw naw Kris
10/12 % loss max these are lipos not nimh.
Very high c ratings as well. Not much lost as heat.
Not that I dabble in lekkys obviously cough.

ZACATTACK Tue May 08, 2012 9:45 am

Surfer_kris wrote:An electric setup of 130W, usually means that you are drawing 130W from the battery, and not that you are getting 130W at the prop. The efficiency of all components combined is probably around 60% (ruffly speaking). This then gives you 78W at the prop and that is what the TD produces too...

There are lots of factors effecting what your actual wattage is at the prop...you have IR (internal resistance on your batteries), heat, weight of the plane, type of prop being used, condition of the LiPos, all are going to effect the actual wattage being used. That being said..you have to do some math130 watts at what voltage? How many Amps are being used? What is the motor KV rating ? The pitch of the prop? Eflux rating of the prop? Eagletree V4 is the most convienient way to monitor these parameters in your aircraft during flight. Prior to using your system it should be monitored with a watt meter to establish and recognize all perameters at different throttle levels. Hope this helps.

Surfer_kris Tue May 08, 2012 9:46 am

10-12% possibly in the batteries, but you have to include the speed control and motor also for the total efficiency...

It is also very easy to measure this, just take your typical TD prop and see how much power an electrical version would need to spin the same prop at the same rpm.

ahrma_581 Tue May 08, 2012 12:18 pm

Condition/type/efficiency of battery doesn't enter into consideration of power output: juice could be provided by a chimp turning a hand crank generator. Calculating power consumption of an e-motor is pretty simple: supplied voltage x current = wattage.

However as mentioned this idn't the motor output, just what is going in. Kind of like trying to figure out how much power a TD makes by measuring how much fuel it burns in a minute.

It may be that model e-motors all fall into fairly narrow bands of efficiency: brushless are all pretty much 85%, brushed all around 80%. Don't know (don't care Smile

). IF that is the case, you could probably get a good rough estimate of output just knowing voltage and current. If not, you need some kind of a dyno, just like IC motors. Of course, all dynos are not equal, either....

Surfer_kris Tue May 08, 2012 1:39 pm

The type of battery does enter if you only measure the current and not the voltage supplied to the engine, but with out a specific example this is all speculation.

A dyno or calibrated props would easily show why electric power is weaker than glow power...

John Goddard Tue May 08, 2012 1:58 pm

Can't agree Kris.
Don't get me wrong I love glow and always will
But the serious boys are all going lekky these
Days simply because it's more reliably powerful.
As for calculations that's what the watt meter is for
And certainly for my purpose ie getting a larger
Ostensibly electric powered model off the ground
With a tiny glow motor it's the only worthwhile
Yardstick.
To give an example my hacker a30 14 3 cell set up
Which is a 30 sized replacement makes my
Super Tigre G34 look daft, stick 4 cells on and it's
In another galaxy And it's 4 years old so positively
Ancient as far as tech is concerned.
Very Happy

Anyway back to the original point which was on most of the sites
Cox fans tend to get their (mis?)information from
The misquoted power out put is wrong by a considerable margin.
Flying

Surfer_kris Tue May 08, 2012 2:53 pm

My point is that if you take the power drain from the battery as a measure of the power delivered to the prop you will always get an over estimate, hence the comment that electric power is weaker than glow power. If the electric power is calculated properly then of coarse it is the same as glow power, the prop cannot tell where the power comes from.

75W from a TD sounds about right to me, you can use the power_prop_calculator (from MVVS) to get an estimate for the power needed to spin a certain prop at a certain rpm. One problem is that all props are not created equal, but to spin a generic 5x3 prop at 20000rpm only takes about 60W.

Here are a few power curves from the aerotools website;

TeeDee 049 real power output? Cox_performance_2

TeeDee 049 real power output? Cox_performance_2

microflitedude Tue May 08, 2012 3:18 pm

Surfer_kris wrote:My point is that if you take the power drain from the battery as a measure of the power delivered to the prop you will always get an over estimate, hence the comment that electric power is weaker than glow power. If the electric power is calculated properly then of coarse it is the same as glow power, the prop cannot tell where the power comes from.

75W from a TD sounds about right to me, you can use the power_prop_calculator (from MVVS) to get an estimate for the power needed to spin a certain prop at a certain rpm. One problem is that all props are not created equal, but to spin a generic 5x3 prop at 20000rpm only takes about 60W.

Here are a few power curves from the aerotools website;

I think we deemed that chart inacurate, as different nitro percentages were used. Also, the testing was spread out over 13 years.

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