Weird design decisions

[FalcoGer]

I am noticing some really confusing and nonsense design decisions in this aircraft.

For example there is an audio knob on the CMWS but the audio is actually being controlled by the ADF knob. Why? That just sounds ridiculous. Just don't install the knob, save yourself 2 bucks of hardware and tape over the hole/don't drill it in the first place.

Or there is an A/A switch or two, but A/A has been phased out for years. I understand that they reuse the hardware and manufacturing processes, but couldn't they at least scratch the labels off/not print them on?

No bullseye support? I mean there is all this amazing navigation hardware and software, but they couldn't program that in?

I'm sure there are plenty of other whack things that are just plain counter intuitive or make no sense. Why is that?

[Flagrum]

The CMWS panel is a later addition to the aircraft, probably using a standard component. They probably had to tie in the audio into an existing channel - but then having two volume knobs for the same audio channel would make no sense. So they did not use the one on the panel.

But if you find this already weired, then you must have never flown the FA-18 or even the Harrier ...

 

Edited March 13, 2022 by Flagrum

[Raptor9]

1 hour ago, FalcoGer said:

For example there is an audio knob on the CMWS but the audio is actually being controlled by the ADF knob. Why? That just sounds ridiculous. Just don't install the knob, save yourself 2 bucks of hardware and tape over the hole/don't drill it in the first place.

It was a retrofitted configuration using existing hardware and standardized components like Flagram said. Not everything in aviation is built to order for a specific aircraft.

1 hour ago, FalcoGer said:

Or there is an A/A switch or two, but A/A has been phased out for years. I understand that they reuse the hardware and manufacturing processes, but couldn't they at least scratch the labels off/not print them on?

There was never any Air-to-Air equipment fielded to US Army AH-64D's, however it was originally built into the aircraft for future growth. Having the possibilities for future growth, even if they aren't ultimately used, saves money over the long term, and allows for foreign export configurations to use the aircraft for plug and play. Defense contractors typically have multiple customers with varying needs or requirements.

The presence of labels on buttons or switches that have no function has zero impact on the operation of the airframe.

1 hour ago, FalcoGer said:

No bullseye support? I mean there is all this amazing navigation hardware and software, but they couldn't program that in?

I'm sure there are plenty of other whack things that are just plain counter intuitive or make no sense. Why is that?

Bullseye was primarily a joint air forces thing, it was never used in Army Aviation operations, and therefore was never needed by AH-64D's in the avionics.

The reason a lot of these things seem counter-intuitive is because you are applying fixed-wing fighter logic to an Army attack helicopter, which operates on a completely different set of doctrine and tactics.  It's sort of why the rocket steering cursor seems counter-intuitive to people used to the CCIP's in the jets. AH-64's were designed to employ unguided rockets in a completely different way compared to attack jets.

[TacticalOni]

3 hours ago, Raptor9 said:

There was never any Air-to-Air equipment fielded to US Army AH-64D's, however it was originally built into the aircraft for future growth.

I will add to this that the lugs are there for Aim-9s apparently, just never used

[Dragon1-1]

They're probably not even being wired for Sidewinders anymore, but AH-64A definitely did have that capability, even if it was never utilized. Also, on foreign Apaches, ATAS probably uses these controls. Sometimes, it's cheaper to leave some controls, just in case, in than to make a custom panel for each version of the helo. Counterintuitive, but that's how economics of scale work.

It's like the MAX POWER switch in the Viper. It doesn't do anything, and it haven't since the A model, but they kept it for some reason.

[FalcoGer]

Okay what's with the engine indications then?

101% rotor/engine RPM is normal? what is 100% a reference to? Why 101%, shouldn't 100% be normal? What's the design limit (aka rapid unplanned disassembly point)? why isn't that 100%? Anything else would make more sense.

[doedkoett]

Just speculating here, but the nominal value was probably 100% at some point, but maybe an upgrade of the gearbox, new blades or something led to 101% being the best RPM. 
In aeronautics 100%, in my experience at least, 100% is usually max allowable in normal use. When it comes to engine RPM for example, values exceeding 100% RPM can be allowed for some time without any ill effects - The Harrier is an example of this. 
So in short, it´s just a number. You should probably not overthink things. Just memorize 101% as the nominal rotor RPM. A simpler figure to remember than the 424 RPM that´s the optimal rotor RPM of a BO 105. 
 

[Raptor9]

7 hours ago, Dragon1-1 said:

They're probably not even being wired for Sidewinders anymore, but AH-64A definitely did have that capability, even if it was never utilized.

The Alpha models didn't have that capability either. It was only fired in a few tests, and from the underwing outboard stations. There wasn't even an ATA weapon select for it in A models. IIRC, the WAS switch aft position was used to dispense chaff.

2 hours ago, FalcoGer said:

Okay what's with the engine indications then?

101% rotor/engine RPM is normal? what is 100% a reference to? Why 101%, shouldn't 100% be normal? What's the design limit (aka rapid unplanned disassembly point)? why isn't that 100%? Anything else would make more sense.

The nominal rotor RPM operating range is actually rated for 95-105% without issues, it's just the engines are automatically referenced to 101% so that's what they are programmed to throttle up or down to maintain.

Most likely, as doedkoett said, it's a value that moved over time. Same thing with the other values of Np and Ng. There are times they will be above 100% with no adverse effects.

But aside from bleeding the rotor RPMs due to too much collective application, you don't need to worry about these things anyway. They are all automatically managed by the avionics. Only information that is presented to the pilot is important for flight, such as the torque indication in the helmet display. If something goes wrong or malfunctions, the avionics will notify the pilot appropriately.

It's management by exception.

Edited March 14, 2022 by Raptor9

[AKarhu]

2 hours ago, doedkoett said:

Just speculating here, but the nominal value was probably 100% at some point, but maybe an upgrade of the gearbox, new blades or something led to 101% being the best RPM. 
In aeronautics 100%, in my experience at least, 100% is usually max allowable in normal use. When it comes to engine RPM for example, values exceeding 100% RPM can be allowed for some time without any ill effects - The Harrier is an example of this. 
So in short, it´s just a number. You should probably not overthink things. Just memorize 101% as the nominal rotor RPM. A simpler figure to remember than the 424 RPM that´s the optimal rotor RPM of a BO 105. 
 

It is, in fact, very common to see values quoted in percentage to exceed 100 % - or sometimes to come short. Usually the 100 % reading refers into some design reference value shared by different variants and designs, only recently it has become a norm to use readings that are in format of percent available.

For instance, some very common CFM56 turbofans (snip from the TCDS):

In case of some turboshafts / turboprops, where the power section is effectively driven to a constant speed, my understanding is that slight variations from the design speed are used to fine tune the torque and vibration characteristics of the entire powertrain for a specified power output. You know, as the same power can be produced with higher speed and lower torque or vice versa, and all that good stuff. While it is, in principle, a choice of just calibrating the indication, it also makes much sense to keep every mention of, say, 100 % to refer into same actual rotational speeds everywhere. Including in technical documentation and all that.

Edited March 14, 2022 by AKarhu