General RPM question

[D.O.G.]

Does anyone know if your RPM increases in your heli when you change from straight main gear to slanted main gear even if the motor pinion gear and the throttle curves stay the same?

 

[heli-maniac]

same tooth count on the main gears

 

[D.O.G.]

same tooth count on the main gears

Yup, same everything.

 

[Smoggie]

If the tooth count is the same then the gear ratio is the same. So RPM should be the same.

 

[Tony]

Actually, helical gears use larger teeth than a straight cut gear, so there are less of them. However, your final drive ratio will be almost identical since the circumference is pretty much the same on the two main gears.

What helicopter is this? Do you happen to know the main gear tooth count (straight and helical) and the pinion tooth count (straight and helical)?

 

[Smoggie]

Tony,
Helical teeth aren't always larger than straight teeth. D.O.G. already said in post #3 that the tooth count was the same.

FWIW the tooth size is specified by the 'Modulus' (typically something like M0.6 , M0.8 or whatever) which is the gear diameter divided by the number of teeth. You must always make sure that both meshing gears have the same modulus.

EDIT.. well it seems from post #7 like the tooth count isn't actually the same, so all bets are off! By keeping the same tooth pinion and reducing the main gear tooth count the reduction will be much less, so your RPM will go up a lot. If you want to keep the same RPM then you need to fit a smaller pinion, the calc says 10.5 tooth, which is obviously immpossible so either 10 or 11 depending on if you want a small increase or decrease in RPM.

 

[D.O.G.]

Actually, helical gears use larger teeth than a straight cut gear, so there are less of them. However, your final drive ratio will be almost identical since the circumference is pretty much the same on the two main gears.

What helicopter is this? Do you happen to know the main gear tooth count (straight and helical) and the pinion tooth count (straight and helical)?

Hey Tony, this is for my 450DFC. You mention "Final drive ratio" should be the same since the circumference is pretty much the same. I thought so too. The slant tooth has 121T and the straight has 150T. The motor pinion has 13T on both slanted/straight. When I switched from straight gear to slanted gear "motor/main" the RPM increased and when I went back the other way "Slanted to straight" the RPM decreased. Not a big deal because all I did was change my settings. I was wondering if I could of just changed my motor pinion gear to make up the difference in the increase or decrease of RPM? BTW, the gears that I'm using on any of my helis aren't Helical. I haven't that try that brand yet.

- - - Updated - - -

Tony,
Helical teeth aren't always larger than straight teeth. D.O.G. already said in post #3 that the tooth count was the same.

FWIW the tooth size is specified by the 'Modulus' (typically something like M0.6 , M0.8 or whatever) which is the number of teeth on the gear divided by the gear diameter. You must always make sure that both meshing gears have the same modulus.

EDIT.. well it seems from post #7 like the tooth count isn't actually the same, so all bets are off! By keeping the same tooth pinion and reducing the main gear tooth count the reduction will be much less, so your RPM will go up a lot. If you want to keep the same RPM then you need to fit a smaller pinion, the calc says 10.5 tooth, which is obviously immpossible so either 10 or 11 depending on if you want a small increase or decrease in RPM.

Sorry smoggie, the teeth count aren't the same, Slanted =121T and the straight is 150T. I should of counted first.

- - - Updated - - -

Tony,
Helical teeth aren't always larger than straight teeth. D.O.G. already said in post #3 that the tooth count was the same.

FWIW the tooth size is specified by the 'Modulus' (typically something like M0.6 , M0.8 or whatever) which is the number of teeth on the gear divided by the gear diameter. You must always make sure that both meshing gears have the same modulus.

EDIT.. well it seems from post #7 like the tooth count isn't actually the same, so all bets are off! By keeping the same tooth pinion and reducing the main gear tooth count the reduction will be much less, so your RPM will go up a lot. If you want to keep the same RPM then you need to fit a smaller pinion, the calc says 10.5 tooth, which is obviously immpossible so either 10 or 11 depending on if you want a small increase or decrease in RPM.

Thank you for that information smoggie.

 

[Tony]

Helical just means slanted. Not a brand. With the straight cut gears you are at a 11.538 and on the helical you are at 9.307. You need to run an 11 tooth pinion to get the head speed back down. With an 11 tooth pinion you will still have a slightly higher head speed since the motor will be turning a half turn less per revolution of the main head. If you want more torque then go with a 10 tooth pinion.

 

[D.O.G.]

Helical just means slanted. Not a brand. With the straight cut gears you are at a 11.538 and on the helical you are at 9.307. You need to run an 11 tooth pinion to get the head speed back down. With an 11 tooth pinion you will still have a slightly higher head speed since the motor will be turning a half turn less per revolution of the main head. If you want more torque then go with a 10 tooth pinion.

Thanks for that information also Tony. Now I have a better understanding, especially Helical means slanted not brand

 

[murankar]

From what I have seen slanted gears have less teeth. This mean head speed will change. Just my 2 cents.

 

[Tony]

You are only thinking numbers, and not circumference. Yes, there are less teeth, but they are larger teeth. The pinion goes down as well to keep the same circumference. Thus leaving almost the same final drive ratio.