Axiom propellers

[luctor et emergo]

 

Well as Daslandia pointed out, improved stopping is at the expense of reduced forward efficiency.

 

Is that ok with you?

 

 

MtB

Dalslandia also pointed out that on a canal the forward efficiency at low speed is not very good anyway. I would say that loosing some forward efficiency, for a gain of stopping/reversing improvement, is worth it.

 

Unless you go boating against a significant flow on a river (which most nb don't), you are unlikely to get to use the full capacity of your engine or efficiency of your prop.

 

 

Edit for clarity.

Edited August 16, 2014 by luctor et emergo

[Neil Smith]

If anyone wants to donate an 18x12 prop I could do back to back tests.

 

Neil

[bottle]

I have one but it is on my boat and after discussions with Crowthers will not be changing it.

 

A thread I started after the discussion http://www.canalworld.net/forums/index.php?showtopic=63382&hl=crowthers#entry1218684

 

Boat is 60' powered by a Beta 43

[Farey]

I've had one for 3 years, and am happy with it. My (subjective) impressions after fitting were that it improved handling in forward and reverse, improved stopping and reduced wash. I also noticed that it seemed to reduce maximum speed; the RPM peaked at a lower throttle setting than before, and wouldn't go any higher. I was on the Trent with a hire boat, and had trouble keeping up, which hadn't been a problem with the previous prop. However, you'd never notice this on the canals, just on a large river at a high throttle setting.

[PaulG]

I also noticed that it seemed to reduce maximum speed; the RPM peaked at a lower throttle setting than before, and wouldn't go any higher.

So it's the wrong prop then!

[ditchcrawler]

I know one boater who changed there prop for an Axiom and a couple of moths later had it taken off again. There was also a boater on the Middle Level when I was there who had one and then took that off as well.

[Farey]

So it's the wrong prop then!

 

Are you able to explain why this is? I don't mind if it gets technical.

[nicknorman]

Are you able to explain why this is? I don't mind if it gets technical.

It means the prop is over-pitched. The prop "bites" too much water with each revolution. You could liken it to having a very tall top gear in a car. The engine can never reach max rpm because it doesn't produce enough power to get the car to that speed. Changing down to a slightly lower gear makes the car go faster.

 

A correctly sized (pitched) prop will allow the engine to just about reach max rpm at full throttle in deep water. However, some folk prefer to have their prop slightly over-pitched because it means that the engine can be run more slowly to achieve normal canal speeds, with concomitant reduced noise and fuel consumption, at the expense of top speed in deep water.

Edited August 18, 2014 by nicknorman

[David Mack]

So it's the wrong prop then!

Not necessarily the wrong prop, but certainly more overpropped than you had previously. So the difference in performance could be due to the change in prop size, or due to the change to an Axiom or some combination of both. So you can't tell whether or not a different sized conventional prop would have had the same effect.

[Sabcat]

I'm amazed anyone bought these/believed the hype. As has been pointed out in this thread already, propellers have been around for over a century and the technology has been perfected long ago. Another vote from me for Crowthers or in fact any correctly sized conventional prop.

[nb Innisfree]

I'm not sure if enough is known about how to correctly size an Axiom.

[MtB]

not even Axiom themselves...

 

 

 

MtB

[Dalslandia]

If we look at the S or is it ~ curved blade (airfoil) the leading edge meet the incoming water, especially with a lot of AoA like when accelerating or stopping. in a good way, but when up to speed the underside of the leading edge probably ad drag,

 

The reduced pitch at the trailing edge have two effects it force the leading edge or whole blade to have a higher pitch angle then otherwise, that is good for the dropped leading edge, but not good for thrust. it reduce pitch

 

Many high speed props have a cusp or cup at the trailing edge, that goes the other way, that accelerate the water at the trailing edge, but keep the leading edge at a smaller angle (pitch) so the propeller act like the pitch was higher then it is, 1-2" or so.

 

If we think about it as an airplane wing with a flap, set positive flap (down) and the whole wing will need less angle (nose down) to produce the same lift, set a negative flap (up) the wing need to be tilted nose up to make the same lift, and will stall easier.

 

An airliner coming in for landing often have leading edge flap or slats down, and trailing edge flaps down, to make a lot of lift at slow speed, without risk for stalling.

 

the only feature of an s airfoil in a propeller is to make it equally good at stopping as going forward, and that was most all user of them say here.

[nicknorman]

If we look at the S or is it ~ curved blade (airfoil) the leading edge meet the incoming water, especially with a lot of AoA like when accelerating or stopping. in a good way, but when up to speed the underside of the leading edge probably ad drag,

 

The reduced pitch at the trailing edge have two effects it force the leading edge or whole blade to have a higher pitch angle then otherwise, that is good for the dropped leading edge, but not good for thrust. it reduce pitch

 

Many high speed props have a cusp or cup at the trailing edge, that goes the other way, that accelerate the water at the trailing edge, but keep the leading edge at a smaller angle (pitch) so the propeller act like the pitch was higher then it is, 1-2" or so.

 

If we think about it as an airplane wing with a flap, set positive flap (down) and the whole wing will need less angle (nose down) to produce the same lift, set a negative flap (up) the wing need to be tilted nose up to make the same lift, and will stall easier.

 

An airliner coming in for landing often have leading edge flap or slats down, and trailing edge flaps down, to make a lot of lift at slow speed, without risk for stalling.

 

the only feature of an s airfoil in a propeller is to make it equally good at stopping as going forward, and that was most all user of them say here.

Sorry, can't let that aerodynamic lesson go unchallenged! Putting down flap in an aircraft only affects stalling speed if they are fowler-type flaps that also increase the wing area when the are down and out. Flaps that don't change the wing area, such as fitted to gliders and our Robin DR400, don't change the stall speed despite the "tilting" effect you mention. Well OK there is a very slight change in stalling speed due to slightly less down-force required by the tailplane, which in turn needs slightly less counter action by lift from the main wings - ie a slight reduction in overall main plane lift required - but it is only a knot or two. So it is the increase in wing area caused by fowler flaps that significantly affects the stalling speed of a jetliner.

 

Anyway, I am still slightly puzzled by the behaviour of our boat. It is really good at stopping. Hitting reverse big-time has folk swaying with the deceleration. But the same is not true of hitting FWD whilst going backwards. Initially there is significant deceleration but after a couple of seconds, this noticeably stops and the boat continue to drift backwards for a sometimes embarrassingly long time.

 

I put it down to "vortex ring" as suffers by helicopters and particularly noticeable with a little indoors model helicopter I have, but why the boat only exhibits this very clear behaviour in one direction, escapes me.

[Dalslandia]

Well I didn't want to make it to complicated with aerodynamics in a boat forum, but we see if I can, when home from todays boat drive.

 

Maybe your boat prop get cleaner water working in reverse then forward?

[Farey]

Putting down flap in an aircraft only affects stalling speed if they are fowler-type flaps that also increase the wing area when the are down and out. Flaps that don't change the wing area, such as fitted to gliders and our Robin DR400, don't change the stall speed despite the "tilting" effect you mention. Well OK there is a very slight change in stalling speed due to slightly less down-force required by the tailplane, which in turn needs slightly less counter action by lift from the main wings - ie a slight reduction in overall main plane lift required - but it is only a knot or two. So it is the increase in wing area caused by fowler flaps that significantly affects the stalling speed of a jetliner.

 

 

 

Hmm - I don't think that statement is correct. On our TB20 the stalling speeds are 70 kts (clean), 65 kts (1 stage) 59 kts (full).

 

I presume that the principles for an aeroplane and a boat propellor are the same, but the differences in design are because of the difference in air versus water density and the speeds at which they operate?

 

I asked Axiom a while back about the lower maximum RPM. Here's the reply they gave:

 

A cause of the Axiom not allowing the engine to over rev is that, the Axiom has no slip (skidding of the propeller) The Axiom propeller will only move the water in the boats swim in a direct cylindrical column and not pulling water from all side's (up and underneath the prop). This is what gives the Axiom good slump / hull characteristics to keep the boats hull flatter.

For the extra rpm used the boats reynolds numbers will only allow for slight increase's in speed for the extra revs, due to drag and displacement of the hull. In co junction with the boat theoretical hull speed and the a propellers advanced coefficients will fall off drastically, inline with the engine power curve falling off in efficiency and increasing the fuel consumption. For this extra revs, the boats swim will change its feed in and over 60ft the boats hull angle will increase by 2 degrees in slump or stern squat, drastically altering the rake and wash of the boat.

[PaulG]

Sorry, can't let that aerodynamic lesson go unchallenged! Putting down flap in an aircraft only affects stalling speed if they are fowler-type flaps that also increase the wing area when the are down and out. Flaps that don't change the wing area, such as fitted to gliders and our Robin DR400, don't change the stall speed despite the "tilting" effect you mention. Well OK there is a very slight change in stalling speed due to slightly less down-force required by the tailplane, which in turn needs slightly less counter action by lift from the main wings - ie a slight reduction in overall main plane lift required - but it is only a knot or two. So it is the increase in wing area caused by fowler flaps that significantly affects the stalling speed of a jetliner.

 

Anyway, I am still slightly puzzled by the behaviour of our boat. It is really good at stopping. Hitting reverse big-time has folk swaying with the deceleration. But the same is not true of hitting FWD whilst going backwards. Initially there is significant deceleration but after a couple of seconds, this noticeably stops and the boat continue to drift backwards for a sometimes embarrassingly long time.

 

I put it down to "vortex ring" as suffers by helicopters and particularly noticeable with a little indoors model helicopter I have, but why the boat only exhibits this very clear behaviour in one direction, escapes me.

I disagree.

The major function of most landing flaps as fitted to airliners is to change the curvature, or camber of the wing, which increases lift at any given airspeed. This reduces stalling speed.

Many designs also incorporate slots that help to keep the airflow attached to the wing at high attack angles, which further reduces stalling speed (the stall being cause by the laminar flow breaking down and becoming turbulent with subsequent sudden reduction in lift).

[nicknorman]

Hmm - I don't think that statement is correct. On our TB20 the stalling speeds are 70 kts (clean), 65 kts (1 stage) 59 kts (full).

 

I presume that the principles for an aeroplane and a boat propellor are the same, but the differences in design are because of the difference in air versus water density and the speeds at which they operate?

 

I asked Axiom a while back about the lower maximum RPM. Here's the reply they gave:

 

 

A cause of the Axiom not allowing the engine to over rev is that, the Axiom has no slip (skidding of the propeller) The Axiom propeller will only move the water in the boats swim in a direct cylindrical column and not pulling water from all side's (up and underneath the prop). This is what gives the Axiom good slump / hull characteristics to keep the boats hull flatter.

For the extra rpm used the boats reynolds numbers will only allow for slight increase's in speed for the extra revs, due to drag and displacement of the hull. In co junction with the boat theoretical hull speed and the a propellers advanced coefficients will fall off drastically, inline with the engine power curve falling off in efficiency and increasing the fuel consumption. For this extra revs, the boats swim will change its feed in and over 60ft the boats hull angle will increase by 2 degrees in slump or stern squat, drastically altering the rake and wash of the boat.

On your TB20, it has fowler flaps that increase the wing area when the flaps are extended (note "extended", not "lowered"). I have never flown a TB20, only the Rallye which definitely does have flower flaps, but a photo I found of the TB20 trailing edge clearly shows the pivot point for the flaps being well below the trailing edge, ie they move out and down, not just down with a hinge at the trailing edge as per our Robin.

 

On Axiom's response, let me translate that for you: "Yes your boat is over-propped but never mind, the extra rpm of a correctly pitched prop would only give you a slight increase in speed and a big increase in wash."

 

Which is true, but it depends on how important that slight loss of top speed is to you.

[nicknorman]

I disagree.

The major function of most landing flaps as fitted to airliners is to change the curvature, or camber of the wing, which increases lift at any given airspeed. This reduces stalling speed.

Many designs also incorporate slots that help to keep the airflow attached to the wing at high attack angles, which further reduces stalling speed (the stall being cause by the laminar flow breaking down and becoming turbulent with subsequent sudden reduction in lift).

You may well disagree, it is your right, but you would be wrong in your second sentence. The increased curvature does increase lift for a given angle at a given airspeed, but it does not increase the ultimate lift attainable just before stalling. That is increased only due to the increased wing area from the "fowler action" and also due to the slats which both increase the wing area but also help the airflow to remain attached to the top surface at higher angles of attack. But my original point was about flaps so can we stick to them?

 

Landing (fowler) flaps have 3 main advantages: allows slower flying speed due to fowler action increasing wing area. Effectively rotates the wing on the fuselage meaning the fuselage attitude is more nose down for a given speed, thus visibility of the runway is much better for the pilot. Increases the drag significantly. This makes it easier to maintain the desired approach speed and increases the deceleration when the throttles are closed in the flare, and in fact during the whole landing manoeuvre, thus requiring less runway. In the case of big jets, it also means the engines are operating well above idle which means they can much more quickly reach full power in the event that a go-around is necessary. Jet engines take a very long time (relatively) to accelerate up from idle to full power, with the longest time being at the lower end of the rpm range.

 

Edited to say I fly aircraft both with fowler flaps, and with trailing edge (non-fowler) flaps. The former gives a big difference in stalling speed. The latter, virtually none and what there is is caused by two factors

 

1/ less downforce on the tailplane required to maintain slow flight, so less compensating up force required by the wing. But since the proportion of lift produced by the tailplane is small compared to that of the main wings, this effect is small. In any case it has not increased the max lift the wings can produce, merely reduced the demand for lift slightly

 

2/ there is often a pitot/static factor produced by the airspeed system. ie with the flaps down, the airspeed indicator tends to read slightly less due to the attitude of probe and the different disturbance of air around the static port. If you look at the GPS groundspeed at low speed between flaps up and down, you can see this effect.

Edited August 19, 2014 by nicknorman

[Farey]

On your TB20, it has fowler flaps that increase the wing area when the flaps are extended (note "extended", not "lowered"). I have never flown a TB20, only the Rallye which definitely does have flower flaps, but a photo I found of the TB20 trailing edge clearly shows the pivot point for the flaps being well below the trailing edge, ie they move out and down, not just down with a hinge at the trailing edge as per our Robin.

 

On Axiom's response, let me translate that for you: "Yes your boat is over-propped but never mind, the extra rpm of a correctly pitched prop would only give you a slight increase in speed and a big increase in wash."

 

Which is true, but it depends on how important that slight loss of top speed is to you.

 

Apologies for digressing into aerodynamics, but you don't need Fowler flaps to get reduced stall speed when flaps are deployed. There's a very good description of the aerodynamic principles here. Section 5.5 deals with the effect of flaps. Does anyone know of a similar article descriping the principles of a boat propellor?

 

I'm happy to accept a slight loss of top speed for improved performance in reverse.

[nicknorman]

Apologies for digressing into aerodynamics, but you don't need Fowler flaps to get reduced stall speed when flaps are deployed. There's a very good description of the aerodynamic principles here. Section 5.5 deals with the effect of flaps.

Unfortunately, that description of flaps, if it is meant to refer to non-fowler flaps (not specified), is wrong. Not only can you not believe everything you read on the internet, but it is also a common myth held by aviators who haven't really thought about it properly and of course were taught this myth at flight school by instructors who were taught the myth by their instructors etc. The fact that the article doesn't differentiate between fowler and non-fowler flaps is a big give-away.

 

Regarding your prop, you could have a correctly sized Axiom which would probably still give the same or maybe better stopping power (finer pitched) but allow the engine to reach max rpm and thus deliver max power. However I suspect that Axioms are optimised for low speed operations on "normal" canals and even with the correct pitch, you may well find top speed in deep water is down a bit compared to say a Crowther.

Edited August 19, 2014 by nicknorman

[PaulG]

You may well disagree, it is your right, but you would be wrong in your second sentence. The increased curvature does increase lift for a given angle at a given airspeed, but it does not increase the ultimate lift attainable just before stalling. That is increased only due to the increased wing area from the "fowler action" and also due to the slats which both increase the wing area but also help the airflow to remain attached to the top surface at higher angles of attack. But my original point was about flaps so can we stick to them?

 

Landing (fowler) flaps have 3 main advantages: allows slower flying speed due to fowler action increasing wing area. Effectively rotates the wing on the fuselage meaning the fuselage attitude is more nose down for a given speed, thus visibility of the runway is much better for the pilot. Increases the drag significantly. This makes it easier to maintain the desired approach speed and increases the deceleration when the throttles are closed in the flare, and in fact during the whole landing manoeuvre, thus requiring less runway. In the case of big jets, it also means the engines are operating well above idle which means they can much more quickly reach full power in the event that a go-around is necessary. Jet engines take a very long time (relatively) to accelerate up from idle to full power, with the longest time being at the lower end of the rpm range.

 

Edited to say I fly aircraft both with fowler flaps, and with trailing edge (non-fowler) flaps. The former gives a big difference in stalling speed. The latter, virtually none and what there is is caused by two factors

 

1/ less downforce on the tailplane required to maintain slow flight, so less compensating up force required by the wing. But since the proportion of lift produced by the tailplane is small compared to that of the main wings, this effect is small. In any case it has not increased the max lift the wings can produce, merely reduced the demand for lift slightly

 

2/ there is often a pitot/static factor produced by the airspeed system. ie with the flaps down, the airspeed indicator tends to read slightly less due to the attitude of probe and the different disturbance of air around the static port. If you look at the GPS groundspeed at low speed between flaps up and down, you can see this effect.

I do not wish to be overly argumentative, but you are plainly wrong. Trailing edge flaps have a considerable effect on stalling speed:

 

From Wikipedia:

" Flaps are devices used to improve the lift characteristics of a wing and are mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed at which the aircraft can be safely flown and to increase the angle of descent for landing..They shorten takeoff and landing distances. Flaps do this by lowering the stall speed and increasing the drag."

 

From the Civil Aviation Authority (NZ) Flight Instructor website:

"Flap increases lift and therefore the stalling speed is reduced. However, flap also changes the shape of the wing, and this results in a lower nose attitude at the stall.
"

 

You may also like to read the paper "Stall Speed", written by Professor Cavcar at the Anadolu University School of Civil Aviation, where the characteristics of the 747-400 are analysed mathematically.

 

http://home.anadolu.edu.tr/~mcavcar/common/Stall.pdf

 

In this he clearly shows that "When flaps are deflected, maximum lift coefficient increases and stall speed decreases" and "since higher flap angles are used during landing, stall speeds in the landing configuration are lower."

 

Of course, there is always the possibility that we are all wrong.

Edited August 19, 2014 by PaulG

[bizzard]

There is also ''The Hanley Page slot'' Invented by Mr Hanley Page in 1919. He had an old bi-plane fitted with them and flew it himself. He was so complacent and sure about the almost gravity defying powers of his ''slot'' invention, ''so he thought'' that he stalled the plane almost dead at about 100ft off the ground. The plane didn't defy gravity of course and plummeted earthwards breaking both his legs. Although the slots performance didn't quite perform as well as he expected he was convinced that they reduced the stall speed considerably and the plane plummeted more slowly to earth than it would have done without them.

And of course he was right. The Hanley Page slot on the leading edge of certain aircraft, mainly STOL or slower aircraft these days is still used although I think they call em ''adjustable slats'' now and not fixed. However the idea is that the slat pops out and droops over the leading wing edge leaving a slot of nothingness between slat and leading wing edge.The airstream passes under the drooping slat, then up sucked through the slot and is blown over the wings upper surface at a higher velocity than it would without them and so causes extra vacuum, ''suck'' on the upper wings surfaces and so giving that extra lift at low speeds.

Deya like thaat.

[Dalslandia]

You guys have all this fun when I am working. not fair.

 

A flap, any flap when deflected change the airfoil camber, and the amount of lift coefficient (CL) it can take, a fowler flap and also some slotted flaps increase wing area.

A high cambered airfoil (al'a Flaped) stall at a lower angle but still carry more CL.

with a slat, slot or slut? the angle can also be increased before stall

 

some airplanes have poor flap designs, and mostly ad drag.

I have also flown the Rallye, 100,150 & 180 hp fun and safe plane, despite it being a Frog plane.

 

when testing new airplanes the pitot is feathering into the wind to reduce angle error.

but the data in the hand book is with the fixed pitot, and often have lot of error, especially at high angles.

The wing don't stall because of low speed, but because of high angle, and can be stalled at any speed. until the wing come off the fuselage.

 

we say the flap change the stall speed by only 2 kts. at ? 45/43 kts

Cl flap up is 1.35

10 sq. meter wing area.

so weight is 452 kg, right?

at Vso 43kts the CL is 1,48

an increase of only 0.13 but maybe half or less of the span is flapped, so double that, 0.26

but the flapped part of the wing stall earlier, (angle) so full lift of the remaining wing can not be used, but it make it safe, not going over a wing tip.

 

A slotted flapped wing can max at CL 2.0

a wing with slats and slotted or fowler flaps along the whole wing maybe 3.0

 

What does it have to do with propellers? not much, but the airfoil used on the propeller don't know that, if the trailing edge is bent down (or back) it increase lift - thrust

and the opposite if the trailing edge is bent up-forward it reduce lift-thrust, and have to be compensated with other means.

[nicknorman]

I do not wish to be overly argumentative, but you are plainly wrong. Trailing edge flaps have a considerable effect on stalling speed:

 

From Wikipedia:

" Flaps are devices used to improve the lift characteristics of a wing and are mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed at which the aircraft can be safely flown and to increase the angle of descent for landing..They shorten takeoff and landing distances. Flaps do this by lowering the stall speed and increasing the drag."

 

From the Civil Aviation Authority (NZ) Flight Instructor website:

"Flap increases lift and therefore the stalling speed is reduced. However, flap also changes the shape of the wing, and this results in a lower nose attitude at the stall.
"

 

You may also like to read the paper "Stall Speed", written by Professor Cavcar at the Anadolu University School of Civil Aviation, where the characteristics of the 747-400 are analysed mathematically.

 

http://home.anadolu.edu.tr/~mcavcar/common/Stall.pdf

 

In this he clearly shows that "When flaps are deflected, maximum lift coefficient increases and stall speed decreases" and "since higher flap angles are used during landing, stall speeds in the landing configuration are lower."

 

Of course, there is always the possibility that we are all wrong.

Certainly the paper referring to the 747, and probably all the other references, refer to fowler flaps which increase the wing area when lowered. But it is the increased wing area, not the flap deflection, that is the primary reducer of stalling speed. Most light aircraft have fowler flaps, only a few are like the Robin having just flaps hinged at the TE. This I suspect is why the general teaching is that lowering flap reduces stalling speed. If you can find me a reference that compares and contrasts fowler vs plain flaps I might be convinced, but then again my experience of flying TE flapped aircraft tells me that I am right. Fowler flapped aircraft of course exhibit a significant reduction in stalling speed when deployed. Edited August 19, 2014 by nicknorman