1/16 Tiger I - Truely Scale Performance
02-20-2012 | 12:36 PM
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From: , MD
1/16 Tiger I - Truely Scale Performance
First things first - Specs of the actual Tiger I tank (full scale) from this site http://www.alanhamby.com/technical.shtml
Combat Weight = 57,250 kg
Engine = Maybach HL 230 12 cyclinder = 700 horsepower (522.2 kW)
Power to Weight Ratio = 9.12 Watts/kg
Transmission = 8 speeds forward (4 reverse) = max speed 1st gear 2.8 kph (1.8 mph); max speed 8th gear 45.4 kph (28 mph)
now we can do the calculations to determine the 1/16 scale values
Scale Top Speed (covered elsewhere) - simply 45.4 kph / 16 = 2.84 kph (1.75 mph) = 0.788 m/s (2.58 ft/s)
Scale Weight - its not just divide by 16, or the scale weight would be 3578.5 kg! The weight is roughly proportional to the volume of the tank, or scale cubed. So 1/16 cubed is 1/4096, so our scale weight should be 13.98 kg (which is a little heavier than most of our tanks)
Scale Power to Weight Ratio - same as full scale 9.12 Watts/kg
We can calculate how fast the max rpm of our motors should be if whe know the ratio of the gearbox. I measured my Mato 3:1 and the motor turns 70 times for every single turn of the final drive axle. The outer diameter of the drive wheel/track is 55 mm, so each turn of the axle moves the tank (pi x diameter) 172.8 mm (6.8 inches). So the motor speed should be (788/172.8)x70 = 319.2 turns per sec, or 19,152 rpm (Mato 3:1), or 6,384 (standard HL gearbox). My previously reported test of the standard HL motor http://www.youtube.com/watch?v=LIjS_rLGX5g showed a low load rpm of 12,880 at 8V. This explains why the HL Tiger I is too fast with the stock gearbox, and too slow with the 3:1 gearbox.
Now this is where it gets tricky. Comparing power to weight ratios is difficult because the real tank had an 8 forward speed transmission, and our 1/16 tanks only have one speed (unless you are a lucky Tamiya 2-speed owner.) So the full scale tank could put all of its 700 horsepower into 1.8 mph in 1st gear, giving it tremendous power for going up hills etc. If our electric motors had a totally flat power band (same shaft power output at 10 rpm and 10,000 rpm) then we would be in good shape. But they don't. So if we want the same hill climbing ability as the real Tiger I in first gear, we need something much greater than 9.12 Watts/kg. What that number should be - I have no idea - at least 2-3 times that I would guess (arbitrarily.) So lets say 25 Watts/kg just for example.
So the stock plastic with metal tracks HL Tiger I is about 6 kg I think, so in our example you would need 150 Watts of power. Thats 75 Watts per channel (each motor). The RX-18 esc's have a max output of about 58 Watts[per side according to Phil at RCTankWars.com. So with the most powerful motors the RX-18 can handle (like maybe ProMaxx 480's), your in the ballpark. If your tank has a "to scale" weight of anything close to14 kg then the RX-18 is not going to cut it no matter what motors you are using.
My rationale for Brushless
First, I suppose there is little point in thinking about it if you know you want an aftermarket battle system/proportional controller. I think there is one system that has standard servo outputs for speed, so that you could use brushless speed controllers (I can't remember the name.) Most of the aftermarket systems that I know of have brushed motor esc's incorporated into the board. So if you have to have one of those, there is no point in thinking about brushless.
I have an all metal Mato Tiger I that will weigh over 10 kg with batteries, maybe more. But lets say you are at 14kg - the correct scale weight. The target power (from our example) should be about 350 Watts total, 175 Watts per motor. (That's shaft output power, so the power drain of the motor is actually a bit higher to account for heat production etc.) The target max rpm of the motors should be 19,152 with the 3:1 gearbox to acheive scale top speed. I chose 22 turn Hobby King motors that are 1800 Kv. With 3S batteries that is 19,980 max rpm, and the max power output is 18 amps x 11.1 volts = 200 Watts which are close to the target values.
Sorry if I rambled on.
Combat Weight = 57,250 kg
Engine = Maybach HL 230 12 cyclinder = 700 horsepower (522.2 kW)
Power to Weight Ratio = 9.12 Watts/kg
Transmission = 8 speeds forward (4 reverse) = max speed 1st gear 2.8 kph (1.8 mph); max speed 8th gear 45.4 kph (28 mph)
now we can do the calculations to determine the 1/16 scale values
Scale Top Speed (covered elsewhere) - simply 45.4 kph / 16 = 2.84 kph (1.75 mph) = 0.788 m/s (2.58 ft/s)
Scale Weight - its not just divide by 16, or the scale weight would be 3578.5 kg! The weight is roughly proportional to the volume of the tank, or scale cubed. So 1/16 cubed is 1/4096, so our scale weight should be 13.98 kg (which is a little heavier than most of our tanks)
Scale Power to Weight Ratio - same as full scale 9.12 Watts/kg
We can calculate how fast the max rpm of our motors should be if whe know the ratio of the gearbox. I measured my Mato 3:1 and the motor turns 70 times for every single turn of the final drive axle. The outer diameter of the drive wheel/track is 55 mm, so each turn of the axle moves the tank (pi x diameter) 172.8 mm (6.8 inches). So the motor speed should be (788/172.8)x70 = 319.2 turns per sec, or 19,152 rpm (Mato 3:1), or 6,384 (standard HL gearbox). My previously reported test of the standard HL motor http://www.youtube.com/watch?v=LIjS_rLGX5g showed a low load rpm of 12,880 at 8V. This explains why the HL Tiger I is too fast with the stock gearbox, and too slow with the 3:1 gearbox.
Now this is where it gets tricky. Comparing power to weight ratios is difficult because the real tank had an 8 forward speed transmission, and our 1/16 tanks only have one speed (unless you are a lucky Tamiya 2-speed owner.) So the full scale tank could put all of its 700 horsepower into 1.8 mph in 1st gear, giving it tremendous power for going up hills etc. If our electric motors had a totally flat power band (same shaft power output at 10 rpm and 10,000 rpm) then we would be in good shape. But they don't. So if we want the same hill climbing ability as the real Tiger I in first gear, we need something much greater than 9.12 Watts/kg. What that number should be - I have no idea - at least 2-3 times that I would guess (arbitrarily.) So lets say 25 Watts/kg just for example.
So the stock plastic with metal tracks HL Tiger I is about 6 kg I think, so in our example you would need 150 Watts of power. Thats 75 Watts per channel (each motor). The RX-18 esc's have a max output of about 58 Watts[per side according to Phil at RCTankWars.com. So with the most powerful motors the RX-18 can handle (like maybe ProMaxx 480's), your in the ballpark. If your tank has a "to scale" weight of anything close to14 kg then the RX-18 is not going to cut it no matter what motors you are using.
My rationale for Brushless
First, I suppose there is little point in thinking about it if you know you want an aftermarket battle system/proportional controller. I think there is one system that has standard servo outputs for speed, so that you could use brushless speed controllers (I can't remember the name.) Most of the aftermarket systems that I know of have brushed motor esc's incorporated into the board. So if you have to have one of those, there is no point in thinking about brushless.
I have an all metal Mato Tiger I that will weigh over 10 kg with batteries, maybe more. But lets say you are at 14kg - the correct scale weight. The target power (from our example) should be about 350 Watts total, 175 Watts per motor. (That's shaft output power, so the power drain of the motor is actually a bit higher to account for heat production etc.) The target max rpm of the motors should be 19,152 with the 3:1 gearbox to acheive scale top speed. I chose 22 turn Hobby King motors that are 1800 Kv. With 3S batteries that is 19,980 max rpm, and the max power output is 18 amps x 11.1 volts = 200 Watts which are close to the target values.
Sorry if I rambled on.
