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-   -   Bell 407 cyclic cam question? (https://www.pprune.org/rotorheads/646383-bell-407-cyclic-cam-question.html)

CTR 28th Apr 2022 14:10


Originally Posted by Reely340 (Post 11222066)
A properly designed cyclic mechanism would - for example for pure forward cyclic input - tilt the swashplate in a way that pure fwd action results……..=> I recall that Bell has some experience building helos, so why do they use that "crossmixing cam" at all?

The easy part is designing the mechanical controls to provide pure lateral and longitudinal input to the swash plate.

The hard part, is the nonlinearities of the rotor performance with changes in forward speed, center of gravity, downwash over the fuselage, deflection in the transmission mounts, etc.

Prior to flight testing of a new helicopter design, manufactures often produce prototype bell cranks of slightly different ratios to allow for adjustment of control sensitivity. As mentioned before, swashplates can be adjusted to couple lateral with longitudinal.

For larger helicopters equipped with stability augmentation systems like the 429, many of these nonlinearities or taken out by the SCAS actuators.

Engineers that don’t like the inability to exactly predict helicopter performance and handling qualities, quit their jobs and design fixed wing airplanes. :-D


wrench1 28th Apr 2022 18:23


Originally Posted by Reely340 (Post 11222066)
I recall that Bell has some experience building helos, so why do they use that "crossmixing cam" at all?

Just to add, there are various M/R systems out there that balance out certain aerodynamic forces with its design similar to the 407 cyclic cam. For example, the R22 delta 3 M/R and swashplate design was partially influenced to reduce the lateral inputs required when the cyclic is moved forward. However, you need to be at the Ray Prouty level of understanding to see why the cyclic cam or delta 3 designs are needed. But from my experience there is a lot more going on upstairs when you push that cyclic forward.

RVDT 28th Apr 2022 21:27

There is a thing in the design specs that sez when you push forward and pull aft that is actually what should happen?

Sometimes it doesn't and the certification authorities require it to.

Posted again for those who didn't bother - Phase lag

[email protected] 29th Apr 2022 13:49

Nice to see Wiki have pointed out phase lag is not precession - at last some sense :ok:

The Lynx - with an effective hinge offset of 17% - produces a marked right roll with a lot of aft cyclic application ie high g pull up or high AoB turn

212man 29th Apr 2022 23:08


Originally Posted by [email protected] (Post 11222823)
Nice to see Wiki have pointed out phase lag is not precession - at last some sense :ok:

The Lynx - with an effective hinge offset of 17% - produces a marked right roll with a lot of aft cyclic application ie high g pull up or high AoB turn

for some nostalgia and well missed names: https://www.pprune.org/rotorheads/25...threads-3.html

[email protected] 30th Apr 2022 09:47


for some nostalgia and well missed names
That was a trip down memory lane :ok: Further down there are posts about the Oasis video featuring 671 Sqn Lynx which I was in for day 2 flying with the then CFI.


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