General the Mathematics behind a Helicopters lift

[Blackwarrior]

Im on a learning curve now whilst i wait for parts and i would like to know how one knows that a specific head speed will create lift.

i know from members in a previous thread that:-

Head Speed = Motor KV * Battery voltage * Efficiency * (Pinion Teeth count / Main gear Teeth count) this is at 100% throttle

my Blade heli that destroyed itself was due to excessive head speed, this is my setup.

Motor is 4200KV, 14T pinion, 140T main gear. (3S battery was fully charged)

HS = 4200 * 12.6 * 0.9 * (14/140) = 4762 RPM at 100% throttle, mine was set to 70% MAX in normal mode, therefore HS = 4762 * 0.7 = 3333 RPM

OK, this is great, BUT how do we know what head speed will give us lift, there must obviously be other formulae that have parameters such as rotor blade lift characteristics, weight of Helicopter etc etc, the Mathematics behind this must be phenomenal..

thanks

 

[Tony]

I'm sure there is a formula for this, but I don't know it. I do know that a 4200kv motor is way too much though for a 450 size heli.

 

[murankar]

There is a lot to take into account.
1)pitch range (+-10 11 or 12)
2) all up weight
3) main blade length
4) main blade width


Now for me i have my Gaui X3 set at 11° pitch, i have a 12 T pinion, 131 main gear and a scorpion 2221-8 (3595 kv ) motor and a standard 3S pack. My main blades have been the rails 326mm and i have the Gaui 325mm which produce about the same result for hovering.

My pitch curve in normal is real close to whats been suggested for you, 45 47 50 75 100 and my throttle curve is at 0 40 60 60 60.

My lift occurs around 55% throttle. When i say lift i really mean the point when the skids leave the ground.

Sent from my LG-E980 using Forum Runner

 

[Blackwarrior]

I'm sure there is a formula for this, but I don't know it. I do know that a 4200kv motor is way too much though for a 450 size heli.

That's why it destroyed itself on both those crashes I posted Tony, I'm rebuilding now and put the stock motor on it.. It had a 14T pinion on too... Being a novice I had no idea of the consequences...

I'm now on a learning curve...lol

 

[Tony]

Bro, we are all on a learning curve lmao. That is what I love about this hobby!

 

[Slobberdog]

Here you go, if you want a bit of fun,

Firstly



Then



A bait that's for a wing, when lift overcomes mass you get lift off,

Disclaimer, this is a down and dirty method and I don't want to offend anyone who might do this on a day by day basis.

 

[Blackwarrior]

That's what it's all about Kev, a bit of fun. I would be interested to see these formuli in action though based on a specific helicopter, maybe a trex 450

 

[Lee]

Disc loading makes a big difference too.
A light DL gives a floaty feel.
A heavy DL gives more punch.

There is a formula for this too, to do with rotor diameter, blade cord width etc...
Justin on RCHN did a show about it once, but i fell asleep with all the sums LOL

 

[Blackwarrior]

So when you guys set your helis up with a head speed of XXX RPM for normal flight, and YYY RPM for 3D flight etc. is this from experience then? Or from manufactures specifications? Someone, somewhere must have done the original calculations when the heli was designed!!

 

[murankar]

Basically it is set by how you want it. Keep in mind the lower and upper limits of the frame. If you go to low the frame will wobble and to high, well you know what happens. The high end is not much over 3000 for a 450, you can get away with 3300 but not much past that.

With lower head speed the heli is not as stable or responsive and will provide longer flight times. Generally you can run higher collective to get back the punchy felling but at the risk of bogging the motor. I was able to run at about 2500 with 13° of collective without issue. I am now closer to 3000 and went back to 11° of collective.

More and more pilots are running lower head speeds for safety and for a better flight experience. You can still throw down just not as hard. Collective management is your friend.

More newbies will run higher head speeds for the stability it offers and allows for less collective management.

You will need to find your happy spot. Start around 2700 to 3000 at about 12v. This will give a little more at the beginning of the flight and a little less at the end.

Personally i like to be around the 2900 to 3100 at 12.6 volts. I like to keep the head speed lower than most.

Sent from my LG-E980 using Forum Runner

 

[Blackwarrior]

Basically it is set by how you want it. Keep in mind the lower and upper limits of the frame. If you go to low the frame will wobble and to high, well you know what happens. The high end is not much over 3000 for a 450, you can get away with 3300 but not much past that.

With lower head speed the heli is not as stable or responsive and will provide longer flight times. Generally you can run higher collective to get back the punchy felling but at the risk of bogging the motor. I was able to run at about 2500 with 13° of collective without issue. I am now closer to 3000 and went back to 11° of collective.

More and more pilots are running lower head speeds for safety and for a better flight experience. You can still throw down just not as hard. Collective management is your friend.

More newbies will run higher head speeds for the stability it offers and allows for less collective management.

You will need to find your happy spot. Start around 2700 to 3000 at about 12v. This will give a little more at the beginning of the flight and a little less at the end.

Personally i like to be around the 2900 to 3100 at 12.6 volts. I like to keep the head speed lower than most.

Sent from my LG-E980 using Forum Runner

Thanks for that Matthius

I have put the stock motor back on so my head speed would be:

Motor = 3800kv with 10T pinion and 140T main gear

At 100% throttle I should get 3078 RPM head speed on a fully charged 12.6v battery

In normal mode I would set the TH curve to 0,40,60,60,60

Which will give me 1846 RPM..... Is this fast enough ?

Would I need 100% throttle for idle up?

Thanks

 

[murankar]

To be honest its going to depend on your airframe and head. If you want to test it start your lower limit at around 2500. If you dont have any wobble then call that your lowest limit. 2500 head speed will feel lethargic to you. Will it fly sure but you may have to make other adjustments to make it poppy again. This will also teach you about your heli.

Since your still learning you really dont need to go below 2700.

Sent from my LG-E980 using Forum Runner