30465 Tail rotor blowing in the wrong direction?

[Sadap]

While watching Sariel's review of the mini helicopter (https://www.youtube.com/watch?v=3NlCf0H2Nwk) I have checked the rotation direction of the rotors.

Here you can see the main rotor turning CCW (from above). The tail rotor is turning CCW too (seen from the left side). That means the tail rotor is blowing the air from the left to the right (blue arrows) which generates a CW torque on the helicopter, but the main rotor generates a CW torque too.

Can someone please verify my train of thoughts? Did I make a mistake somewhere?

CW = ?
CCW = ?

[Sariel]

It's my mistake, the instruction is clearly showing that the main rotor blades should be installed in opposite direction. I was playing with the rotor, replacing the 3L axle with a 3L bar and I must have put the blades back in wrong direction. Sorry!

[Sadap]

If you reverse all directions, you have the same result. You would have to replace the tail rotor with a mirrored one or place the gear on the main rotor axle above the other gear to fix it.

[Sariel]

Well, considering that this is a $4.99 set with 70 pieces, it's possible that the designer just put the gear where it was convenient to him, not where it made sense aerodynamically.

[Sadap]

Yes, that's probably how it happened. But it still bothers me a bit, because lego also serves a bit as a educational toy. Imagine explaining why helicopters need a tail rotor to a kid using this set and then it blows in the wrong direction...

[anyUser]

@Sadap: your scheme appears to match the drawing that i found on a wikipedia site:

However, I am lacking knowledge on relation between rotational direction, propeller pitch and the resulting direction of air flow. It may be possible that the tail rotor technically has the wrong pitch.
(But that's not the reason I used a plain piece as propeller in my version).

The issue that you brought up should be fixed easily: swap the 12tooth gear in the rear with the half bushing - it may not be the most stable setup but technically more correct.

[Jurss]

I think, we need to boicot now this set, write some petition to lego.

[Gray Gear]

Why dont you just put the Tail Rotor on the other Side? As far as I can see that would fix the problem...

[anyUser]

49 minutes ago, Gray Gear said:

Why dont you just put the Tail Rotor on the other Side? As far as I can see that would fix the problem...

Wouldn't that require the use of panel #21? ;-)

In a quick search i found -only- three technic helicopters where the pitch of main and tail rotor appears to be defined:

• the small one from Ocean Explorer (42064)
• 9396 Helicopter
• 8812 (8429) - recognizeable

Any other one typically does not show (any) sign of pitch on either main or tail rotor. On some models there is no transmission to the tail rotor (8005, 8253).

 

[Sadap]

1 hour ago, Gray Gear said:

Why dont you just put the Tail Rotor on the other Side? As far as I can see that would fix the problem...

Yes it would. Just put the rotor + tan 12T gear + black 3L axle pin in the hole from the other side. You also need to change the red light + stud pin so it doesn't collide with the gear.

Quote

However, I am lacking knowledge on relation between rotational direction, propeller pitch and the resulting direction of air flow

I always put some imaginary ping pong balls in the area where the propeller blades are and rotate the propeller in my mind. then i see in which direction the imaginary ball would be hit. This is also the direction the air will flow. I know it's a bit stupid but it works for me

Quote

In a quick search i found -only- three technic helicopters where the pitch of main and tail rotor appears to be defined:

42064 is wrong

9396 according to Sariel's video (https://youtu.be/N6_O1y1MVmM?t=537) the main rotor rotates CCW and the tail rotor too, but I think the tail rotor has 0 pitch

8812 wrong too

 

Edited January 5, 2021 by Sadap
sent too early

[steph77]

Hello

A tail rotor has, by definition, NO specific orientation.

As the blades have both positive OR negative pitch, there are no Reason to put it in one or another side of the aircraft.

You could object that the tail of the aircraft is an obstacle at the air blow, but it is also an obstacle to the air intake to the rotor.

Dynamic of air flow is quiet complex, but you can figure out of it that the airflow is hooverd all around the rotor in a vertical plane and swooshed perpendiculary by the blades effect.

So, no care of it for this little set.

That looks cool by the way...

@SarielThanks for reviewed it

[anyUser]

On 1/5/2021 at 5:37 PM, Gray Gear said:

Why dont you just put the Tail Rotor on the other Side? As far as I can see that would fix the problem...

Turning over only the tail rotor to the other side (= rotating by 180°) does not change (the sign of) the pitch angle. Only by moving the 12tooth gear to the right side of the tail the rotational direction will change and therefore, as we are talking about propeller with  fixed pitch angle, the direction of the airflow.

I did a check on some parts: 2952 and 4617 have same direction:

• If the propeller rotates clockwise, the airflow is towards the observer
• If the propeller rotates counter-clockwise, the airflow is away from the observer

Of course steph77 is right as a fixed angle pitch tail rotor would do no good any (real) helicopter. The variable pitch angle is used to change the yaw angle of the aircraft during flight.

To compensate just for the torque of the (counter-clockwise rotation) main rotor the thrust of the tail rotor in 30465 should be OK (according to the drawings above).

[Gray Gear]

1 hour ago, anyUser said:

Turning over only the tail rotor to the other side (= rotating by 180°) does not change (the sign of) the pitch angle. Only by moving the 12tooth gear to the right side of the tail the rotational direction will change and therefore, as we are talking about propeller with  fixed pitch angle, the direction of the airflow.

Oh really? No way. Of course I meant that you could swap the Rotor mount and gear to the other side. The other poeple before you got exactly what I meant. But it does seem like they thought a bit what would make sense. Noone is that dumb to think that flipping over the rotor piece itself would make any difference