why do prop manufacturers recommend against extensions, particularly with aerobatic props? I see you did so here (thank you!), but also GT recommended the same sort of "keep it as short as possible". Is there a particular reason?
So to start with, and I know that this isn't exactly your question but it's worth clarifying for everyone reading, there are two possible interpretations here: 1) prop extensions/spacers, meaning something that's bolted between the propeller and the engine, or 2) Propellers with integral hub extensions that accomplish the same thing without the extra bolted joint, which is what we make and recommend. We (Hartzell) advise against bolted in spacer/extensions for two reasons: 1) the added complexity of the bolted joint, which may not be defined or controlled to the same specification, and 2) propeller vibratory loads are affected by system stiffness and a bolted in extension, of unknown design, affects those loads in unknown ways. As a result, when I talk extension I mean integral extension that's part of the propeller hub.
We know that moving the prop forward with respect to the engine makes loads higher (worse). Still acceptable? It depends, a lot of information is needed in order to make that determination. We don't have that information for an experimental installation, so the guidance is to keep the extension short. And when someone say they want to do aerobatics in their RV, do they mean they're going to do a loop and a roll once a month on the way to breakfast, or practice for and compete in IAC contests?
Why is the guidance on extensions something vague (keep it short), as opposed to a specific limit (no more than X G's)? We don't publish limits on extended hubs, in particular a limit on maximum G, for a variety of reasons. Firstly, a G limit is shorthand that only works when you know other things (which blades, which extension, which aircraft). Secondly, there's not one defined limit that we can publish because it's not really a singular limit, the limits often depend on the installation, and propellers in particular get complicated because their limits are often related to fatigue rather than peak strength. So it depends on not just how much, but also on how many times. Fatigue is insidious and cumulative: everything seems fine until it suddenly isn't.
Because the prop is effectively a gyroscope, it's not just the G, it's also the rate at which the G is applied (pitch and/or yaw rate) that can drive the loads. The specific propeller (total extension length and stiffness, blade material, blade design) matters. It's also likely going to be a fatigue issue, rather than an overload issue, both in the sense of repeated maneuvers and also in the sense that the prop is rotating so the loads will be alternating; the cumulative loads depend on the vibratory load pattern associated with a given maneuver, how often a given maneuver is performed, and the mix of maneuvers. For a certificated aerobatic installation, this is something that would be evaluated for that specific installation as appropriate based the specific components used and their capabilities, the performance envelope of the aircraft, and measured loads. This doesn't just apply to the propeller either, those gyroscopic and weight loads are moments, meaning load times distance (arm), applied to the crankshaft flange, the engine main bearing, the engine mount, the firewall, the fuselage. That doesn't necessarily mean that a longer prop extension isn't possible, but it's definitely a step towards higher loads. When we've got a nice, neat, way to minimize these loads then that's the preferred engineering solution in the absence of some other requirement, and in particular when the installation loads are unknown or uncertain.
Another piece is that, again, the prop is a gyroscope so the further you move that gyroscope from your CG the more effort (stick/pedal force, tail load) it's going to take to make it change direction and therefore the greater the affect on your handling qualities. Consequential? It depends, and for an experimental, amateur built aircraft the decision is up the builder/integrator. The top aerobatic performers love our new Talon aerobatic prop compared to the older Claw (which they really liked), in part because the new prop has a lower polar moment of inertia (less of a gyroscope) and therefore makes it easier to start and stop maneuvers precisely. So, given that handling is a higher priority for aerobatic aircraft, and increasing the extension is a step in the opposite direction of nice handling, again the advice is to avoid extended hubs.
In a nutshell, the more frequent and more aggressive the aerobatics, the more we would advise to use a shorter or the shortest hub extension and the more likely a certificated installation would require the same (and/or life limits) in order to keep the system safe. A Cirrus SR20 uses the longest extension we make while an Extra 300/L uses the shortest; RVs can live in a spectrum between those two. The lowest risk choice is the shortest hub and using a longer one incurs more risk; how much more risk depends on the particulars, and whether that risk is acceptable depends on the individual experimental operator. The advice on hub extensions is to reduce/limit risk. Sometimes this advice has been communicated as "if you're doing aerobatics, use the shortest hub" or "don't do aerobatics with extended hubs or prop extensions" or similar for the sake of clarity and brevity. This post is anything but brief, though hopefully educational.
