No Flybar



First flown in summer 2001
Using two Futaba GY-240 gyros for pitch and roll, it does aerobatics and fast forward flight just fine.
The only non-stock part is the machined aluminum block attached to the bottom of the rotor head.
It is held in place with M2.5x25 bolts into the guide pin holes in the head.
The former mixing levers reduce the load on the swash plate and double the servo torque going to the main rotor blades.
If you want to make your own, here is the drawing of the hub.

What happens with no flybar ?

For the record, when you give up elevator, the advancing blade increases pitch and the retreating blade decreases pitch. The blades in their fore & aft positions have neutral pitch and do not change. This is standard setup for all helis, pod & boom, scale and full size.

The rotor disk will pitch up when the advancing rotor blade gets more lift than the retreating blade (as in forward flight or up elevator) and the blades in their for and aft position will have neutral lift and pitch.

Imagine the blade(s) flying around the rotor disk circle. With more lift or up pitch, the blade will fly up. With less lift or down pitch the blade will fly down and with neutral pitch, the blade is already at it's lowest or highest point in the rotor disk (fore and aft in the case of forward flight). When the blade is pitched up or down (when the blade is off to one side or the other), it is in transition from high side to low side.

When the rotor disk tilts like this, it takes the main shaft and the rest of the heli with it.

The mechanics of gyroscopic precession are also exactly described by this process.

When in forward flight, the flybar is responsible for decreasing the pitch of the advancing blade and increasing the pitch of the retreating blade to keep the heli in straight and level flight. You can see this by noticing the angle between the rotor disk and the flybar disk in forward flight.

Some conclusions

If all you do is hover around, there is really no problem. If, however, you want to get into forward flight, you will find that the heli wants to balloon up in the direction of flight (forward, sideways or backwards). The faster you go, the more it wants to balloon up. This requires stick input to keep this from happening. The advancing blade creating more lift causes this problem and it is quite noticeable the faster you go.

Gyros on the pitch and roll axis do a very good job of helping with this problem but even when in HH mode, are still not quite as good as a real flybar.

As far as phasing goes, I have only one answer, keep your setup square. That means with a rotor blade over the tail, it should not change pitch when you give forward or aft cyclic. Only the blade(s) off to the side should show a change in pitch.

Rotor tip speed is where it's at.

For "nimble" flight characteristics, you want a relatively high rotor tip speed. Mach 0.5 is a good hi-ish number. For 710 blades and a 6.5" rotor hub, the rotor disk diameter is 5.2 ft. At sea level and 59F the head speed comes out to 1116.4 * .5 * 60 / 3.1416 / 5.2 = 2050 RPM.

Now the centrifugal force for 710 SAB blades at 2050 RPM -- that's 850 lbs trying to pull the blades off the rotor hub. You just need to verify that everything will hold together at that speed. At 1800 RPM it's only 660 lbs.

Before you say that's too high, keep in mind that the typical full size is 20% higher yet on the tip speed.

Helitip speed
Mach
rotor
diameter
RPMmax wt
lbs
disk loading
lbs/ft
Raptor 50 0.37 4 ft - 5 in 1800 7  0.457
MA Fury 0.43 5 ft - 1.6 in1800 10.5  0.507
Bell 47G 0.59 37 ft - 2 in340 2950  2.72
Robinson R22 0.60 25 ft - 2 in510 1370  2.75
V-22 Osprey - forward flight0.59 38 ft 333 6050026.67
V-22 Osprey - hover 0.73 38 ft 412 6050026.67
CH-47D Chinook 0.63 60 ft 225 5000018.84
CH-53D Sea Stalion 0.62 72 ft - 2.7 in185 4200010.25
CH-53E Super Stalion 0.66 79 ft 179 7350015.00
BK-117 Eurocopter 0.65 36 ft - 1 in383 7900  7.73
H-60 Black/Pave/Sea Hawk0.65 53 ft - 8 in258 2350010.39
Bell 222A (AirWolf) 0.69 42 ft 348 7850  6.25
Bell 412 0.70 46 ft 324 11900  7.16
Bell 204 (UH-1C Hog) 0.66 44 ft 324 9000  5.92
Bell 205 (UH-1E Huey) 0.73 48 ft 324 9500  5.25
EC-135 Eurocopter 0.77 33 ft - 6 in492 6400  7.26

Will more head speed reduce the pitching up in forward flight ? If so, how much ?

A numeric analysis of head speed, forward speed and nose-up-pitching.

A typical scale heli is about 1250 rpm with 810mm blades giving a rotor diameter of about 72".
That gives a tip speed of 393fps, 268mph or Mach 0.35

It is convenient, for the following numerics, to use a flight speed of 1/10 of the tip speed.

A simplified formula for lift adequate for this analysis is L = V - - - - Lift = Velocity squared

I will take the lift from the rotor disk and split it to left and right sides and include forward speed.
V = Blade velocity, F = Forward speed
( V + F ) + ( V - F ) = total lift

multiplying this out gives
V + 2 V F + F + V - 2 V F + F = total lift

re-arranging and simplifying gives
2 V + 2 F = total lift

An interesting observation is that the pieces of the equation that cause the pitching cancelled out in the total lift equation. What does remain is that an increase in forward speed will increase the total lift (this is transitional lift). This increase works out to be ( F / V ) which is usually compensated for by lowering the collective slightly.

The disappearing pieces are + 2 V F and - 2 V F. These pieces are a rolling force which is gyroscopically precessed to a pitch-up motion in forward flight.

To get an idea how much this force is, let's put some numbers in the basic equations. I'll use a forward speed of 1 and a blade velocity of 10 to keep it simple. (multiply by 27 to get real world MPH numbers)

( V + F ) + ( V - F )
( 10 + 1 ) + ( 10 - 1 )
11 + 9 = 121 + 81 = 202 = total lift value

Using the expanded equation
V + 2 V F + F + V - 2 V F + F
100 + 20 + 1 + 100 - 20 + 1 = 202 = total lift value

This shows that total lift is 202 and the rolling force is 40 (+ 20 on one side and - 20 on the other side).
The rolling force is 20% of the total lift which is gyroscopically precessed to a pitch-up motion in forward flight.

That's a bunch ! At only 27 MPH ! And this is increasing proportionately with air speed.

Using the expanded equation, but this time with 50% higher blade velocity and the same forward speed
V + 2 V F + F + V - 2 V F + F
225 + 30 + 1 + 225 - 30 + 1 = 452 = total lift value

The rolling force is reduced to 13% of the total lift

To put it another way, by increasing your head speed 50%, you reduce the nose up pitching by 33%.

My analysis is VERY simplified. In reality these numbers are somewhat less because I'm using tip speeds and leaving out the mitigating effects of blade flapping but you get the idea.

The ability of multibladed heads to handle greater stresses at higher head speeds is still in question. The new (Jan-06) solid heads from Century with flexible blades to allow flapping show some promise.