View Full Version : Bell 407 cyclic cam question?
Weads
26th Apr 2022, 15:31
So in my training the cam “ The cam assembly automatically adds 12% of the total lateral movement cyclic input proportional to the fore and aft cyclic movement.” My first thought was for transverse flow and the right roll in the takeoff assuming the cam applied 12% left cyclic. But since it states that it applies 12% in proportion to forward and aft movements there has got to be something else more to it? Any info would be appreciated
6% left for forward cyclic and 6% right for aft cyclic perhaps? Just a thought.
Weads
26th Apr 2022, 18:42
6% left for forward cyclic and 6% right for aft cyclic perhaps? Just a thought.
Well I have seen the cam and it’s in one direction. If memory serves me correct then it was cam was for left cyclic
Bell_ringer
26th Apr 2022, 19:36
Why on earth is this relevant? Push left, it goes left, push forward, it goes forward.
Cyclic cams, the number 1 thought on every 407 driver’s mind :}
Tayters
26th Apr 2022, 19:50
Why on earth is this relevant? Push left, it goes left, push forward, it goes forward.
Cyclic cams, the number 1 thought on every 407 driver’s mind :}
Just because you're not interested in the subject doesn't mean he can't ask the question. I for one would be interested in the answer.
Weads
26th Apr 2022, 20:09
Why on earth is this relevant? Push left, it goes left, push forward, it goes forward.
Cyclic cams, the number 1 thought on every 407 driver’s mind :}
That was nice……
AKA I don’t know either
ApolloHeli
26th Apr 2022, 20:55
I don't fly the 407 so this is a guess, but might it be to aid in eliminating tail rotor drift when in the hover, and in more forward cyclic positions it reduces left cyclic input as there is less / no requirement to counteract tail rotor drift?
Fun_police
26th Apr 2022, 21:17
The training manual says it adds a lateral component proportional to the amount of fore/aft cyclic movement. It doesn’t say which way or why though but it might have something to do with the mast being canted to the left…
So in my training the cam “ The cam assembly automatically adds 12% of the total lateral movement cyclic input proportional to the fore and aft cyclic movement.” My first thought was for transverse flow and the right roll in the takeoff assuming the cam applied 12% left cyclic. But since it states that it applies 12% in proportion to forward and aft movements there has got to be something else more to it? Any info would be appreciated
During initial Bell 430 flight testing it was discovered that in high speed forward flight excessive left cyclic was needed to maintain forward flight. On the 430 the fix was to re index the upper swashplate a few splines to couple lateral to longitudinal inputs. If you look at the 430 pitch links, you will notice they are angled like a helix.
During 407 flight testing, a similar characteristic was observed, however the affect was non linear compared to the 430. With increasing speed, increasing lateral cyclic input was required, however before reaching full forward cyclic the need for lateral cyclic became reduced. So just re indexing the swashplate was not a viable solution.
To add non linear lateral coupling to forward longitudinal input, Bell on the 407 incorporated a cam mechanism under the pilot seat. The cam profile adds minimal lateral input for aft cyclic, and maximum lateral input at approximately 65 to 75% max forward longitudinal input. By 100% almost all the lateral input is removed. If you remove the pilot seat, and examine the cam profile, you can see the magnitude of lateral input with longitudinal cyclic.
FYI if you box the 407 cyclic without hydraulic boost, you may hear a popping noise under the pilot seat. This noise is generated by a anti jam spring cartridge in the cam linkage.
JimEli
26th Apr 2022, 22:32
Why on earth is this relevant? Push left, it goes left, push forward, it goes forward.
Cyclic cams, the number 1 thought on every 407 driver’s mind :}
Never denegrate another pilot for wanting to know more about his craft.
twinstar_ca
26th Apr 2022, 23:04
During initial Bell 430 flight testing it was discovered that in high speed forward flight excessive left cyclic was needed to maintain forward flight. On the 430 the fix was to re index the upper swashplate a few splines to couple lateral to longitudinal inputs. If you look at the 430 pitch links, you will notice they are angled like a helix.
During 407 flight testing, a similar characteristic was observed, however the affect was non linear compared to the 430. With increasing speed, increasing lateral cyclic input was required, however before reaching full forward cyclic the need for lateral cyclic became reduced. So just re indexing the swashplate was not a viable solution.
To add non linear lateral coupling to forward longitudinal input, Bell on the 407 incorporated a cam mechanism under the pilot seat. The cam profile adds minimal lateral input for aft cyclic, and maximum lateral input at approximately 65 to 75% max forward longitudinal input. By 100% almost all the lateral input is removed. If you remove the pilot seat, and examine the cam profile, you can see the magnitude of lateral input with longitudinal cyclic.
FYI if you box the 407 cyclic without hydraulic boost, you may hear a popping noise under the pilot seat. This noise is generated by a anti jam spring cartridge in the cam linkage.
Great explanation, CTR,,, :ok:
Phase lag? (https://en.wikipedia.org/wiki/Phase_lag_(rotorcraft))
Sometimes they don't quite get it right. I have had one machine where the swashplate was moved on the fixed part as the fore and aft was not acceptable after certification.
Arm out the window
27th Apr 2022, 02:56
Never denegrate another pilot for wanting to know more about his craft.
Hear, hear! Learning about the systems is beneficial in all sorts of ways, as is helping out in the hangar and seeing the bits and pieces normally covered by panels. Explanations like the one by CTR above are great too - it's all well and good to know what something does, but why it does it gives you another level of understanding altogether.
Ask more questions, I reckon ... the answers are often interesting and helpful for others too.
Bell_ringer
27th Apr 2022, 07:06
Hear, hear! Learning about the systems is beneficial in all sorts of ways, as is helping out in the hangar and seeing the bits and pieces normally covered by panels. Explanations like the one by CTR above are great too - it's all well and good to know what something does, but why it does it gives you another level of understanding altogether.
Ask more questions, I reckon ... the answers are often interesting and helpful for others too.
Steady on, untwist the knickers.
It was an observation, and one punctuated with an emoji so those prone to ease-of-offence may notice that some levity was intended.
It is marvellous to want to know all that you can about the collection of bolts and rivets that we frequently find ourselves strapped to.
Is training not to make better, safer pilots?
If that is the goal, how does drawing focus to things that the pilot has no control over, can't preflight nor is in the FM, achieve that?
Are there not other areas which get neglected - fadec to manual reversion get's glossed over, which is probably why Misters Rolls and Royce (presume that is their pronoun) get to service so many after someone got curious and pushed the button.
I sleep better at night knowing why the mast is offset at the angle it is, but will knowing that make anyone better or safer?
There is no such thing as too much knowledge, just some times the really valuable stuff gets lost beneath the trivia.
As you were. Keep calm and carry on.
Why on earth is this relevant? Push left, it goes left, push forward, it goes forward.
Granted on most types but not always - Go take a lap in a BO105 or the heavier versions with basically the same rotor system with all the augmentation off - lessons in phase lag and cross couple!
Would be fun in the dark or IMC!
No biggy asking which sh!t does what.
ericferret
27th Apr 2022, 10:22
Understanding how things work helps when discussing maintenance issues.
Some pilots haven't a clue. Others know more about the aircraft than the engineers.
Pilot of a 365C on an offshore platform.
"There is a lot of oil under the rear fuselage and the tail."
O.K check the oil levels.
Half an hour later
"Where are the engine oil tanks?".
CTR - thanks for that excellent explanation - there have been a multitude of designs for modifying the control runs over the years - mixing units, interlinks, mast tilt etc - all to overcome various aerodynamic issues, but that cam in the 407 is a new one on me:ok:
JimEli
27th Apr 2022, 14:59
FWIW,
…
Is training not to make better, safer pilots?
If that is the goal, how does drawing focus to things that the pilot has no control over, can't preflight nor is in the FM, achieve that?
Last I checked, I have no control over aerodynamics, can’t preflight it, and I can’t find a thing about it in the FM.
Are there not other areas which get neglected - fadec to manual reversion get's glossed over, which is probably why Misters Rolls and Royce (presume that is their pronoun) get to service so many after someone got curious and pushed the button.
I believe the 407 FM has a couple of emer procs that reference the cam…
I sleep better at night knowing why the mast is offset at the angle it is, but will knowing that make anyone better or safer?
I would suggest that’s up to the owner of the knowledge, not us.
There is no such thing as too much knowledge, just some times the really valuable stuff gets lost beneath the trivia.
...
We don’t get to define what’s trivia, but the next unforeseen accident will define what's not trivial.
Arm out the window
28th Apr 2022, 06:08
Steady on, untwist the knickers.
They were never twisted!
Just putting in a word in support of learning stuff about the types you fly which will probably come in handy some time down the track. The proneness to 'ease-of-offence' isn't at this end.
Reely340
28th Apr 2022, 08:28
6% left for forward cyclic and 6% right for aft cyclic perhaps? Just a thought.Why would one want that at all? This sounds like someone messed up cyclic rigging during design, and after discovering their mistake they added some corrective cam.
A properly designed cyclic mechanism would - for example for pure forward cyclic input - tilt the swashplate in a way that pure fwd action results,
thereby considering all 90° action delays, possible hiller-type stabilizer action shifts, leading or trailing edge pitch link attachments etc.etc.
=> I recall that Bell has some experience building helos, so why do they use that "crossmixing cam" at all?
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, 17:23
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.
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 (https://en.wikipedia.org/wiki/Phase_lag_(rotorcraft))
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, 22:08
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/253767-westland-lynx-merged-threads-3.html
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.