Author Topic: Scale Speed  (Read 1758 times)

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freightliner009

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Scale Speed
« on: February 05, 2014, 15:50:47 »
I've been reading past posts on scale speed and the use of reduction gearboxes etc., now I don't know anything about this sort of thing but have a question that someone may be able to answer.
Rather than using a reduction gearbox to achieve scale speed could you not program your TX with a plotted speed curve to get the same or similar results?

Sorry if it's a stupid question but to a complete novice it seems as though it could be a feasible and or cheaper alternative ;)
« Last Edit: February 05, 2014, 15:53:56 by freightliner009 »
Proud  father of a new Hobby Engine 1/35 scale Southampton tug and 1/24 scale hand crafted Zeeland tug

Puffin

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Re: Scale Speed
« Reply #1 on: February 05, 2014, 17:03:03 »
THAT  WILL STILL NOT TAKE THE LOAD OFF THE MOTOR .   PUFFIN

des

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Re: Scale Speed
« Reply #2 on: February 06, 2014, 00:20:07 »
G'day Andy

A reduction gear is not usually used in order to obtain a scale speed.  Rather, the gearbox is necessary to match the prop speed to the (usually higher) motor speed.

As a general rule, the faster the prop spins, the more it will simply churn the water while producing less usual thrust into the water.

Again generally, most model tug props would be designed to spin at 2500 - 3000 rpm - the smaller the prop the faster the spin rate.  So if you are using a 70mm prop it would need to spin slower than a 50mm prop.  Racing props are somewhat different, but this won't affect most tugboat modellers.

There are also physical limitations.  When the tip speed of a propeller approaches the speed of sound in water, large amounts of cavitation occurs which will eventually erode the propeller blades - this is a well known limitation in the full-size marine drive system world, so drive systems are designed to avoid this effect.

Now, if your motor spins at 6000 rpm, and you need only 2500 rpm for your prop, then you will need a reduction gear on the motor.

Once you've built and installed the drive system into your boat, and done your sea trials, you may then find that the boat travels through the water at a speed that doesn't "look right".  You can then use the programmable features of your DX6i to limit maximum motor speed to whatever you think appropriate to achieve a "scale speed".  But this is different from matching motor speed to prop speed.

Des.

des

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Re: Scale Speed
« Reply #3 on: February 06, 2014, 00:30:32 »
I forgot to mention - Puffin had it right in his comment, but he didn't provide any explanation.

When you install a reduction gear to a drive system, the torque available at the output shaft is increased, by the same factor as the gear reduction.  So if you use a 2.5:1 reduction gear, your output shaft speed will be reduced by a factor of 0.4, but the output torque will be increased by 2.5.

Now, your motor probably doesn't provide much output torque, and may not be capable of spinning your prop at full motor speed.  This does two things - firstly the motor will run at some slower speed at which its available torque, power and speed are "in balance".  Secondly it will overload your motor.  This will result in motor overheating, and possible burnout;  and the trapped heat within the closed confines of your hull may do other damage to adjacent components.

By adding the reduction gearbox the output shaft conditions are brought to a more usable set of conditions of speed, torque and power output to drive your prop.

Des.

west coast tug

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Re: Scale Speed
« Reply #4 on: February 06, 2014, 09:54:38 »
Everybody will sometimes ask what is scale speed in a model versus there real tug . These tables should help with that .
You can also calculate horse power with it by knowing your prop RPM.
We have used this with Robert Allan for technical trials with model tugs .
Gary 

Puffin

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Re: Scale Speed
« Reply #5 on: February 10, 2014, 22:02:32 »
THANKS DES  FOR FULLY EXPLANING THAT .    PUFFIN

Puffin

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Re: Scale Speed
« Reply #6 on: February 10, 2014, 22:06:49 »
HI GARY  THANKS FOR THOSE HANDY TABLES , IT SIMPLIFIES THINGS .  PUFFIN

Gashmore

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Re: Scale Speed
« Reply #7 on: February 21, 2014, 11:30:10 »
Simple formula for that table is Vm= Sqrt(scale)* Vf
Where Vm= model speed
Vf = Full scale speed.
For Example: at 14Knots in 1:24 scale Square root of 1/24=.204. 14*.204=2.857Knots.

That will pretty closely approximate the wake pattern of a full size displacement hull.