Rules Change proposals and cycle

[ceandra]

The AMA has announced that the current rules change cycle will be moved to 2021. The revised deadline to submit rules change proposals is now March 15, 2021, instead of March 15, 2020. If something is urgent, there is still a possibility of submitting as urgent, which requires a board vote to agree with urgency before a vote on the proposal.

There are currently 6 proposals filed (filed before March 15, 2020, I believe),as well as one urgent proposal. The proposals can be seen at https://www.modelaircraft.org/competiti ... -proposals. Any AMA member can submit a proposal. You should be willing to support, socialize, and shepherd your proposal, rather than throwing it over the fence. It is important, after it clears the initial vote, that the board "perfects" the wording but abides by the intent, as we all have to live with successful rules. You likely WILL be contacted concerning your proposals so that your intent is clear and not violated with any adjustments.

The following proposals are currently on the table:

• Urgent RCP20-01U, concerns allowing composite wings in 424
• RCP21-01, makes EF1 an official event
• RCP21-02, clarifies wheels, allowing a tail skid to replace a tail wheel that was on the prototype in 422
• RCP21-03, Allows modified or replacement backplates in 422 to support RPM sensors
• RCP21-04, Allows foregoing grommets on aileron servo installations when two servos are utilized in 422
• RCP21-05, Allows limited wear on 422 wheels
• RCP21-07, Clarifies zeros resulting from loss of parts, including allowing loss of a single wheel and axle (all classes)

As a racer, it is YOUR RESPONSIBILITY to read these, and provide input to the Pylon Racing Board (Contest Board, not NMPRA Board). It is also an extended opportunity to put in additional proposals, as the deadline may have passed while you had more weighty things on your mind.

Chuck Andraka
NMPRA 11H

[Dave Yost]

So, for those of us up here in the cheap seats, please explain from a form, fit or function stand point, why you would not want to shock mount a servo in a Q40 airframe? I get the two servo redundancy piece, but are you not reducing the redundancy margin by leaving the grommets off? The explanation in the proposal does not explain the reasoning for removing the grommets, I.E. if there is only an increased safety margin by leaving them in place, why would you want to remove them?

Thanks, Dave

[GaryS]

Over the years, several people (to include myself) have used methods of mounting outboard servos that don’t allow for grommets. I personally used “servo frame” mounts intended for gliders in my Stregas for tons of flights. I’ve also seen servos literally glued in without issue.

With the margin on having an extra servo, the proposed rule is actually better than many installations where people either crush the crap out of the grommets or use the grommets properly, but have the servo case rubbing on the servo tray.

[Dave Yost]

From:
g. All screws holding the servos to the servo rails or trays and holding any trays to the airframe
shall be in place and secure. Rubber grommets shall be used on all servos designed to accept
them. If the heads of the servo mounting screws are small enough to pull through the grommets,
washers shall be used to prevent this.

To:
g. All screws holding the servos to the servo rails or trays and holding any trays to the airframe
shall be in place and secure. Rubber grommets shall be used on all servos designed to accept
them. If separate servos are used for each aileron, the servos may be mounted without grommets.
If the heads of the servo mounting screws are small enough to pull through the grommets,
washers shall be used to prevent this.

Why not just say:

"Rubber grommets shall be used on all servos designed to accept them, with following exemption: flight controls equipped with two or more servos; an alternate means of mounting maybe approved by the contest committee during the initial airframe design approval process"

I suspect servos mounted further out on the wings are subjected to less vibration, as the core material used in the wing is dampening the vibration frequency over the distance traveled from the source. Two servos mounted next to each other in the center of the wing, closer to the wing saddle may have a different outcome based on the quality of the servo, the installation process and proximity to the vibration source.

As written, it implies that we are applying a different safety standard from one safety critical primary flight control to another for convenience.

Example: If we would approve redundant servos with no grommets on the ailerons, why would we not on the elevator/rudder servos on a v tail configured airplane?

Thanks for the explanation,

Cheers, Dave

[fizzwater2]

I wouldn't bet on there being less vibration out on a wing panel. Lots of wings have no cores, other than spars, with molded skins. Early Miss Daras with servos in the wings went through a LOT of servos, before the design was changed to move the servo into the fuse and use torque rods.

Resonance can play havoc with things, making vibration MUCH worse than it should be.

Had a project at work once that required testing to an airborne helicopter vibration standard. There was a push on cover on a RF circuit in the receiver front end, that would blow off every time at a specific frequency of vibration. The vib table was set to 1G. I dialed the frequency off just a small amount (a few Hz at most) and had the vib table running at 10G, the cover stayed in place. Back down to one G, shift the frequency back to the resonance, and the cover would come off every time.

[Dave Yost]

Yep.....I was on a UAS project that relied on a FCU that was shock isolated on a payload tray adjacent to a heavy fuel converted DA50. The vibration and the ignition played havoc with the FCU. They hired an outside consultant and payed big bucks to identify the proper density materials (for the shock mounts) for the frequency and still had major issues.

For that reason, I suspect the grommets on our servos probably don't do much. BUT.....the OEM recommends them. So how do you write a "one size fits all" rule?

Dave

[GaryS]

“If we would approve redundant servos with no grommets on the ailerons, why would we not on the elevator/rudder servos on a v tail configured airplane?”

With a v-tail, if you lose one you’re also losing Half your control along another axis (either yaw or pitch). Safely landing a plane with one aileron is much easier than landing with only one V-tail servo working.

[Dave Yost]

Servos typically have three failure modes; hard over (short,wire chafing) last position (signal or power loss) in trail(gear train). Two of those are most likely going to lead to loss of control if it’s on the ailerons or elevator regardless of how many servos are installed.

As written, You must have grommets on the rudder and fuel cut off, but not the ailerons? Fails the logic test in my line of thinking.

We are changing a rule based on data from one servo type in one airframe.

Seems to me that requesting a deviation for the airframe type based on empirical data makes more sense then changing the rule book.

I guess we will see what they come up with.

Dave

[GaryS]

I’ve watched multiple outboard servos fail with grommets, and I’ve personally had and seen others use them without grommets successfully.

I’ve also seen servos fail using grommets on single aileron servo setups, elevator, rudder, and throttle.

Grommets are completely useless if the servo case comes in contact with the structure of the airplane, yet there isn’t a rule saying the only contact the servo can have is through the grommets. Back when people mounted aileron servos on their side, it wasn’t unusual for people to have the servo laying against the wing, yet they didn’t seem to have any additional failures. I’d be willing to bet if you looked at every airplane at a race, you’d find at least one with a servo case contacting the servo tray or other structure.

[Dave Yost]

Concur.

Cheers, Dave

[vonderhey]

A good quality silicone wing saddle would help.
Dow Corning 999