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Old Apr 28, 2012, 12:39 PM
222 km/hr Parkjet flyer
solentlife's Avatar
Latvia / UK visits
Joined Jan 2010
7,370 Posts
Quote:
Originally Posted by GryphonRCU View Post
cjmartinez103,

Hans is correct about how much more power you need.
Here is how to do a bit of your own calculation in future:

Relationship between how much power you need to gain speed is pretty much a cubic rule.

100 MPH / 80 MPH = 1.25 times the speed is desired

1.25 X 1.25 X 1.25 = 1.95

Your desired(new) speed will need 1.95 times the power (95% more than before) while at max speed compared to your old power level at its max speed.
Static Wattmeter readings will help you a bit, but data loggers and ICE ESCs will help more.
Prop diameter to pitch ratios and also drag all play a role.

***The 1.95 times power level only applies if your new prop is not too small for your application at the new desired speed.....and the motor runs efficient enough!!!

________________________

Working that calculation backwards:

If you have 4 times the power level, what is the best you can hope for speed wise?
Cubic root of 4 = 1.587

best you can get is 58.7% more speed compared to before.

_____________

What about if you have just half the power?

Cubic root of 0.5 = 0.794

If your power level becomes half, you can still retain 79% of original speed.



Have fun,

Gryphon
So far all speeds registered via Eagle Tree Speed sensor show a 60% of theoretical calculated speed ......... that is prop rpm x pitch .... assuming that prop diameter is correct for the model.
The % figure actually improves as you reduce speed and rpms ... it's the high speeds where drag of airframe saps power at a fantastic rate ... it's same with boats .......... slow to medium speed is easy and moderate power ... but push for extra and the demands escalate dramatically.

I have a 3800KV light 250 Heli motor and that is lucky to hit 180kph on the model, fit the heavy 450 motor 3700KV back on and speed jumps up over 200 ...
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Old Apr 28, 2012, 01:16 PM
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GryphonRCU's Avatar
United States, CA, SF
Joined Mar 2008
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60%... that is horrible.

We are getting much better % values than that for our applications(different airframe than yours), ....AND we know enough and have been doing it way long enough to know that you cannot just go by a % of pitch speed for actual flight speed.

That 60% can only apply for a particular model with a particular setup/prop. There is just too many factors involved.
If you are talking about very similar airframe and setup as him then great!!!


Not trying to make it a full blown tutorial here....just demonstrating where "Hans's" 1.95 figure came from.


Have a great weekend,

Gryphon
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Old Apr 28, 2012, 10:14 PM
Flying!!! Got to love it
raido56's Avatar
USA, PA, Philadelphia
Joined Jun 2006
589 Posts
[QUOTE=solentlife;21451075]You need a far stronger bigger motor that can provide the probable power levels of well above 700W ..... but watts are not the answer as they can be absorbed into slow speed as well as fast. So we need a big powerful motor that can swing your prop or higher pitch at a much faster rpm. That will also mean a serious level ESC and a high C rated LiPo .......

You will trade 2 things for speed :

1. Duration of flight will be markedly reduced as you compromise on higher cell count and higher C rating but keeping weight down.
2. Torque effects will be much greater and you'll find that launching / speeding up will have some strange effects until model is flying fast enough to stabilise via it's wing .....
I would suggest looking in the EDF motor ranges - but make sure that the shaft is locked BOTH sides of brushless can. Many EDF motors do not have a circlip as they are designed for one way mounting only unlike ordinary motors.

To give an example of the extreme power levels we go to for speed :

My Parkjet designed for about 2400kv, 2S - 3S lipo, 10A ESC and up to 5x5 prop .... is running a 3700kv 450 Heli motor with 50A ESC, 3S - 4S Lipo and 5x5 prop. That is producing over 550W ..... against the stock set-up of about 150W. I should actually runa higher rated ESC - but I'm running out of space to fit !

My replacement Parkjet ordered will have a 4400kv Heli Motor, 4S, 50A ESC, 5x5 or 5 x 6 prop. The intention to approach 300kph.[/QUOTE]

Can you say smoke em if you got em . Cause that's all your gonna see is smoke with that set up with a 50 amp esc and 5x5 or 5x6 prop .

Keep em flying
raido56/ Herb
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Old Apr 29, 2012, 12:20 AM
222 km/hr Parkjet flyer
solentlife's Avatar
Latvia / UK visits
Joined Jan 2010
7,370 Posts
Quote:
.......................My replacement Parkjet ordered will have a 4400kv Heli Motor, 4S, 50A ESC, 5x5 or 5 x 6 prop. The intention to approach 300kph.[/QUOTE]

Can you say smoke em if you got em . Cause that's all your gonna see is smoke with that set up with a 50 amp esc and 5x5 or 5x6 prop .

Keep em flying
raido56/ Herb
That's not whole story anyway ....as there already has been similar set-up run on my PKJ .... the ESC is one part that is subject to change .... I already know it's on its limits. Tests show AMPs run at just over 55 on the 5x5. Second the speed runs are short and majority of flight is at much reduced throttle. Aim is to restrict first runs at 650 - 700W ... giving amps draw of 43 to 47.5A

My previous 222kph with similar set-up - some said would not work .... but it did.

Until I can find a suitable ESC that is light enough, small enough, economically priced ... I have to make do. I don't have a bottomless cash fund on this and I use what I already have for much.

The other matter as I posted for OP .... is that in these levels - the capacity of LiPo that can be carried becomes a problem. On my PKJ - I cannot carry a large LiPo - so my duration is extremely short.
On the 2200 small standard motor I can run a 1000mAh 4S for approx. 6mins. Put that with the above motor and even with careful throttle management between speed runs I will be lucky to see 1.5mins

With greatest respect - I am not new at this game and am fully aware of the limitations / risks I take with my gear.

FYI : The previous set-up I pushed one time to over 60A on the 50A ESC ... on the test bench ... ESC got hot but did not go up in smoke as expected. But it was purely a test to see what it would take and to make sure my limiting throttle to just under ESC rating would work.
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