Axiom propellers

[Farey]

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.

 

The aviation thread drift seems to be continuing so ... I think you're wrong stating that most light aircraft have Fowler flaps. All the definitions I've seen state that a Fowler flap is one which moves both backwards and down, whereas most light aricraft have simple hinged flaps - but the manufacturer's pilot operating handbooks still quote lower stalling speeds with flaps extended. However, I accept that the reduction in stalling speed may be higher with a Fowler flap than a simple flap.

[nicknorman]

The aviation thread drift seems to be continuing so ... I think you're wrong stating that most light aircraft have Fowler flaps. All the definitions I've seen state that a Fowler flap is one which moves both backwards and down, whereas most light aricraft have simple hinged flaps - but the manufacturer's pilot operating handbooks still quote lower stalling speeds with flaps extended. However, I accept that the reduction in stalling speed may be higher with a Fowler flap than a simple flap.

Plain flaps are hinged right at the trailing edge, like the ailerons are. They just pivot up and down. Fowler type flaps are hinged below the trailing edge so that they move in an arc - out and down. When up, they are tucked under / into the trailing edge so for example with the flaps up if you put a pen mark at the point where the wing stopped and the flap started, then lower the flap, the pen mark would have moved aft as well as down. More flap surface is exposed and so the wing area is increased. Your TB20 has fowler flaps, even if you don't realise it! Here is a photo and if you zoom / look closely under the trailing edge you can see the flap support hors, with the hinges being a few inches below the lower wing surface. Since this is the pivot point and the flaps are located further up on the other horns, they must move in an arc ie down and out.

 

 

By contrast, on the Robin the flap hinges are actually on the lower trailing edge surface / bottom of the flap (they are piano hinges). These are plain flaps that do not move out at all, and therefore don't change the wing area. Struggled to find a photo showing the flaps, this is the best I could do:

 

 

Anyway, next time you visit your TB20 have a look and I'm sure you will see what I mean. You have fowler flaps, you just don't realise it!

Edited August 19, 2014 by nicknorman

[David Mack]

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).

Really? A prop with no slip? If it just pulls a cylindrical column of water where does that come from? It can only come from drawing in water from all sides, not least because there is a boat hull a short distance in front of the prop.

[nb Innisfree]

When I did my second solo circuit in a Cessna 150 I made the approach on full 40 deg of flaps, on rounding out an unexpected side gust threw me off balance and I landed on one wheel and one wing down. Through panic at not being able to correct me veering off the runway I opened the throttle and pulled back on the collumn and to my surprise I staggered into the air. After failing to get max power I realised I still had carburrettor heat on so after turning it off full power returned. Got to about 40' but couldn't go faster than 60mph, ah I then remembrred I still had full flaps so I did the one thing I'd been told not to do and that is to take all 40 deg off in one go.

 

I can now state with authority that a mk1 Cessna 150 can take off at 20mph with full flaps and can just avoid hitting the runway at full power at 70 mph if full flaps are then taken off suddenly at 40' (which results in a dramatic loss of lift)

Edited August 20, 2014 by nb Innisfree

[MtB]

Didn't the hire company show you how to fly it before letting you out on your own?

 

:D

 

 

MtB

[PaulG]

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.

OK. Let's look at the Fowler flap

 

If you look at the graph on page four of the learned Professor's paper,

 

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

 

you will see that the "basic configuration" (i.e. no flap) wing has a Lift Coefficient of 1.5.

 

With the (Fowler) flaps deployed, the Lift Coefficient increases to 3.0. I.e. it doubles. The same wing under the same conditions of air density, airspeed etc will produce double the lift.

 

The available lift due to an increase in wing area of 10 or 15% i.e produced by deployed flaps, will be 10 to 15%, as lift is directly proportional to wing area, please see link below:

 

http://www.grc.nasa.gov/WWW/k-12/airplane/size.html

 

So it is actually the change in wing shape causes the major effect, not the increase in area.

[Dalslandia]

I agree with PaulG.

 

Those Nr is 2D, a 3D wing as all is in reality the Nr's is smaller, depends on Re Nr, and % of wing that have flap.

 

The flap on the airplane isn't there to reduce stall speed, but to reduce wing area and drag, and keeping the stall speed the (original) designer set up as goal. Then many airplane designs evolve into something heavier and more powerful then was first thought of.

 

What can we learn in boating from airplanes?

[Tony Brooks]

Really? A prop with no slip? If it just pulls a cylindrical column of water where does that come from? It can only come from drawing in water from all sides, not least because there is a boat hull a short distance in front of the prop.

 

Sorry to intrude into you flyboys discussions.

 

Not only that but any prop must impart a rotary motion of some degree to the "cylindrical column of water" so centripetal (centrifugal) force must cause the column to go cone shaped.

 

Also as the Axiom neither runs in a duct nor has "tiplets" on its blades it must spin some water outwards.

 

Now as far as I know Axiom have never sought to explain how this is not so in language ordinary people can understand.

 

The we come to the claims about less wash and the boat riding with less squat. A boat has a fixed mass so to provide a forward force to drive or accelerate it demands an equal and opposite force from the prop. I can see a large diameter cylinder of water moving at a slower speed could achieve this compared with a small diameter cone moving at higher speed but I feel the volume of water being displaced by the prop in a given period of time would be the same as it has to produce the same thrust. How can this cause less wash - in any case most of the wash is caused by the hull movement, not the prop.

 

I repeat (in answer to another post about draw bar pull) that Axion have published a paper from a research organisation that shows an increase of draw bar pull but they seem incapable of explaining how this is achieved so non-technical boaters can assess and understand it.

 

Until we get those non-technical explanations I think one needs to assume there is a reason they are not forthcoming so maybe the cartoon much earlier in the thread has some truth.

 

All I know is that I can stop well enough with my Crowther prop.

[Dalslandia]

I am out of vote this post up for today, but one + goes to Tony

 

the amount of thrust needed to move the boat is the same at the same speed, and as Tony say we can move a large area of water with a smaller increase (Delta V (speed)) in prop wash or with a smaller diameter with a higher delta V, to get the same needed thrust. the bigger diameter prop will probably do it with less power input, - better efficiency then the smaller diameter.

 

If the boat make less wave and wash with the axiom, is it going just as fast as with propeller brand X?

 

Stub pulling test, is as useful as it sounds, good for the pub at evening.

 

static thrust have nothing to do with efficiency when moving or flying for that matter. and surprisingly little to do with take off distance (airplane)

stopping the boat is as everyone understand important, it come with a price, the same goes with a very efficient propeller, it will probably have less stopping ability, and being sensitive for debris, so propeller as well as boats and planes is always a compromise in one way or other.

[MtB]

 

Sorry to intrude into you flyboys discussions.

 

Not only that but any prop must impart a rotary motion of some degree to the "cylindrical column of water" so centripetal (centrifugal) force must cause the column to go cone shaped.

 

Also as the Axiom neither runs in a duct nor has "tiplets" on its blades it must spin some water outwards.

 

Now as far as I know Axiom have never sought to explain how this is not so in language ordinary people can understand.

 

The we come to the claims about less wash and the boat riding with less squat. A boat has a fixed mass so to provide a forward force to drive or accelerate it demands an equal and opposite force from the prop. I can see a large diameter cylinder of water moving at a slower speed could achieve this compared with a small diameter cone moving at higher speed but I feel the volume of water being displaced by the prop in a given period of time would be the same as it has to produce the same thrust. How can this cause less wash - in any case most of the wash is caused by the hull movement, not the prop.

 

I repeat (in answer to another post about draw bar pull) that Axion have published a paper from a research organisation that shows an increase of draw bar pull but they seem incapable of explaining how this is achieved so non-technical boaters can assess and understand it.

 

Until we get those non-technical explanations I think one needs to assume there is a reason they are not forthcoming so maybe the cartoon much earlier in the thread has some truth.

 

All I know is that I can stop well enough with my Crowther prop.

 

I agree with what you say here Tony.

 

Never mind the failure of Axiom to back up their claims in ordinary language so non-technical boaters can understand it, as a technical person myself I find their technical explanations the engineering equivalent of the 'psycho-babble' spouted by various amateur psychologists out to sell books tapes and CDs by making bollux stuff up as they go along.

 

Blimey that was a long sentence!

 

MtB

[Farey]

 

The we come to the claims about less wash and the boat riding with less squat. A boat has a fixed mass so to provide a forward force to drive or accelerate it demands an equal and opposite force from the prop. I can see a large diameter cylinder of water moving at a slower speed could achieve this compared with a small diameter cone moving at higher speed but I feel the volume of water being displaced by the prop in a given period of time would be the same as it has to produce the same thrust. How can this cause less wash - in any case most of the wash is caused by the hull movement, not the prop.

 

Until we get those non-technical explanations I think one needs to assume there is a reason they are not forthcoming so maybe the cartoon much earlier in the thread has some truth.

 

 

Its a long time since I studied fluid dynamics, but surely there's more to it than that? Things are never 100% efficient - is propellor efficiency a factor? And a big issue with fluid frow is turbulence; I would expect a propellor which produced less turbulence to be more efficient and to produce better handling. I (possibly naively) thought that was where the Axiom got its benefit.

 

As for non-technical explanations - the problem is that to really understand anything in physics you need to understand the underlying mathematics and physical principles. While you can try to produce non-technical explanations, they always lose something in translation. As an extreme example, consider something like quantum mechanics. So, while Axiom failing to produce a non-technical explantion may be a concern, I'd be much more interested in the opinion of someone who has a good understanding of propellor science. MtB says he is technical and that Axioms technical literature is 'psycho-babble'; if we think Axiom's claims are incorrect, I think a technical refutation would be very valuable.

[Timleech]

 

 

if we think Axiom's claims are incorrect, I think a technical refutation would be very valuable.

 

My impression is that some of their claims may have some validity, especially the greater effectiveness going astern, but they ruin their case (to me) by grossly overstating them and surrounding them with poorly explained technobabble.

It seems to me that much of the other claimed benefit can be explained by correct size matching or even over-propping.

 

Tim

[PaulG]

 

I agree with what you say here Tony.

 

Never mind the failure of Axiom to back up their claims in ordinary language so non-technical boaters can understand it, as a technical person myself I find their technical explanations the engineering equivalent of the 'psycho-babble' spouted by various amateur psychologists out to sell books tapes and CDs by making bollux stuff up as they go along.

 

Blimey that was a long sentence!

 

MtB

I agree.

 

http://en.wikipedia.org/wiki/Pseudoscience

 

However, it does get people talking about their product!

[IanD]

Maybe Axiom haven't done themselves any favours by over-caliming what they can do, but I suspect people claiming that it's all snake oil are equally off the mark.

 

As has been said, conventional propellors (like Crowther) haven't changed much over the years because they're pretty close to the optimum design for propelling a boat forwards -- though if you look at some of the designs used on large container ships where efficiency is the #1 design priority, they're even more complex in design with spiral-shaped backwards-curved blades.

 

http://www.wartsila.com/en/propulsors/fp-propellers/fpp

 

However conventional props aren't so good going astern because the blades are then far from optimum -- the container ship ones are presumably even worse, but then they hardly ever have to go astern.

 

Looking at the blade forms and the explanations by people who've actually tried to understand what they're doing instead of dismissing it as rubbish, the Axiom trades off a little bit of ahead performance for a big improvement in astern performance by using symmetrical blades -- which is partly a throwback to the early days of prop design before people had cottoned on how to shape the blades asymmetrically for better forward performance. There's also more blade near the tips so you'd expect more thrust to come from this region, this together with the bigger blade area should mean more thrust from a given diameter prop, though this is complicated by the fact that unlike a normal prop the pitch is not constant along the blade.

 

If Axiom had just said this -- "symmetrical blades giving much better astern performance by losing a litle bit of ahead performance" -- then most of the ordure beiong dropped on their heads could probably have been avoided...

Edited August 21, 2014 by IanD

[BruceinSanity]

Another thought; is the square ended blade more vulnerable to chips and dings from junk on the bottom than a conventional round ended blade?