Primarily for general aviation discussion, but other aviation topics are also welcome.
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By Miscellaneous
#1698716
Anyone able to explain the detail of why a prop designed for climbing (tugging) may not be suitably adjusted to give a sensible cruise close to that achieved on the same aircraft with conventional props?

Is it simply a case of the amount of adjustment required being beyond what is possible given the original design?

TIA
User avatar
By davef77
#1698721
Miscellaneous wrote:Anyone able to explain the detail of why a prop designed for climbing (tugging) may not be suitably adjusted to give a sensible cruise close to that achieved on the same aircraft with conventional props?

Is it simply a case of the amount of adjustment required being beyond what is possible given the original design?

TIA


Well it is all about angle of attack, of the prop blades. The blade of a prop works like a wing, it creates lift based, mostly, by its AoA.

You can change the AoA two ways, by literally changing the angle, the pitch of the blade, or by changing the direction of the wind that strikes the blade.

As the aeroplane speeds up the AoA reduces and so the prop doesn’t create as much lift, and so doesn’t pull as hard.

So ideally you want a coarse pitch when you are going fast and a finer pitch when you are going slow to achieve an optimal AoA.

So no fixed-pitch prop is optimal at all speeds, they are all a compromise. You simply choose at which speed you want maximum efficiency.

A c/s prop works by changing the pitch to be optimal for whatever rpm you choose. The mechanism tries to keep the rpm constant whatever the speed.

The prop on the tug is optimised for low speed, climbing, and so is less efficient in the, faster, cruise, by definition.

Is that what you meant?
User avatar
By Ben Twings
#1698722
Propellers can be re-pitched.

Wooden props are modified by reshaping the leading edge to move the stagnation point up or down. This can give about =/-100-150 RPM static adjustment.

Metal prop are physically twisted, if approved by the manufacturer.

This may mean heating before and doing the heat treatment again afterwards to get the correct temper and remove most of the internal stress.

Neither of these processes are suitable for the unschooled amateur.
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By TheFarmer
#1698725
As above.

A metal prop can be re-pitched (I can send you contact details) but only by about 2 degrees I think. So, a McCauley 48 inch pitch can be re-pitched to 50 (coarser) or 46 (finer). There’s about 100 rpm static change each way assuming your engine is fit. I had one done and it worked well.

Wooden ones can also be changed, but it’s normally easier to just buy new from someone like Hercules (assuming it’s LAA) and you can get exactly what you want then. I can vouch for his props. They’re excellent, and he’s a very good bloke indeed. In fact one of the best chaps I’ve met who trade within GA.
Flyin'Dutch', Kittyhawk liked this
By Boxkite
#1698731
Miscellaneous wrote:Anyone able to explain the detail of why a prop designed for climbing (tugging) may not be suitably adjusted to give a sensible cruise close to that achieved on the same aircraft with conventional props?

Is it simply a case of the amount of adjustment required being beyond what is possible given the original design?

TIA

When you say 'adjustment', are you envisaging rotation at the hub clamp? Some of the above explanations relate to the reshaping of the blades.
As you know, the blades have a twist; the angle of attack is greater near the hub than the tips. On a climbing prop the angle variation between the hub and tip is smaller than on a coarse prop. If one was to rotate the climbing (fine) prop outside of the permitted range so that the more effective area nearer the root was at the coarser pitch of a cruise prop then the tips would be too coarse and put a strain on the blades.
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By Miscellaneous
#1698732
Thanks for the replies. :thumright:

davef77 wrote:Is that what you meant?

Not really. :D

Why can't a (composite) prop which has been designed for towing (IE compromising on cruise) not be ground adjusted to sacrifice towing ability and give satisfactory cruising speeds?

Boxite wrote:On a climbing prop the angle variation between the hub and tip is smaller than on a coarse prop. If one was to rotate the climbing (fine) prop outside of the permitted range so that the more effective area nearer the root was at the coarser pitch of a cruise prop then the tips would be too coarse and put a strain on the blades.

That would make sense. :thumleft:
User avatar
By TheFarmer
#1698736
Misc

Many LAA type props that fit the Rotax series of engines do have props that are ground adjustable.

Kiev
Ivoprop
Woodcomp
GSC
WarpDrive

To name a few...

However, the typical Sensenich/McCauley props on Conti/RR engines don’t have this as you have identified which is a shame, as performance can be changed with simple angle re-pitching in a matter of minutes on the list above, with remarkable performance changes.
User avatar
By Miscellaneous
#1698738
The prop can be ground adjusted, manufacturer says don't, fit new prop. :(

I don't believe they are saying so for the sake of a sale and wanted to understand why it can't be adjusted.
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By TheFarmer
#1698741
Misc

Please dont take thisthe wrong way, but you haven’t really explained it very well to be honest! We are all answering a question that you didn’t ask. :D
User avatar
By Miscellaneous
#1698747
TheFarmer wrote:...you haven’t really explained it very well to be honest!

Seems to be the case. :oops:

Simple really, I wanted to adjust my towing prop to give reasonable cruise and asked manufacturer for settings. Response was I need to change to a cruising prop. Request for reasoning didn't really produce one. @Boxkite's explanation that towing props and cruising props have different variations across blade length makes sense. :D
User avatar
By kanga
#1698768
TheFarmer wrote:..

Wooden ones can also be changed, but it’s normally easier to just buy new from someone like Hercules (assuming it’s LAA) and you can get exactly what you want then. I can vouch for his props. They’re excellent, and he’s a very good bloke indeed. In fact one of the best chaps I’ve met who trade within GA.


Gloucestershire technology, of course; like those made for the much bigger stuff (ironically, including [C130] Hercules) by another outfit in the County ... :)
By Maxthelion
#1698789
Misc, perhaps you could let us all know what the make/model of the prop is? Maybe then someone can give substance to what the manufacturer has said from their own experience with the same product.
TheFarmer, Rob P liked this