Best DIY gadget alert

[Andy Hall]

Another one that I use quite a bit is the minicut set. This has some vey small blades which can be used to get into very awkward places.

[Geoff Berrow]

Message-ID: from John Rumm contained the following:

I must have read this book as a kid (I remember doing most of the experiments - I never could make a successful hot air balloon that didn't immediately catch fire) and despite making many model aircraft I was always confused by the omission of any mention of angle of attack.

[John Rumm]

About £120 ;-)

Its a slimmer, smaller, lighter version. It has a different tool clamping mechanism and its own range of blades (although an adaptor is available to use "ordinary" blades on it (and possibly its blades on an ordinary multimaster). They also make a bunch of very specialised blades for this one for things like car glazing removal etc.

[Andy Hall]

I thought that it was the other way round, John

Both tools weigh around 1.2kg

Multimaster spec . says 250W (input) Supercut spec. says 400W (input)

[John Rumm]

Not seen them side by side, but I was under the impression the supercut was slimmer and designed to be easier to hold etc.

[Andy Hall]

Might be.

I couldn't find any dimensions.

[Julian]

The same theory holds good. Note that aeroplanes designed to routinely fly upside down tend to have a symetrical aerofoil section so that the wing will produce lift when inverted just as efficiently. A symetrical section still relies on Bernoulli for lift.

It isn't far more significant at all. The Bernoulli effect creates about 2/3 of a wing's lift, ie the upper surface of the wing does about 2/3 of the work, and the lower about 1/3. This is irrespective of the 'wing angle.' (angle of attack is the term you need here) You can prove this easily in a wind tunnel by taking a sheet of thin ply and cutting a wing profile from it. You will find that presenting it at an angle to the airflow to 'vector air mass downwards' will produce only a small proportion of the lift that a similar sized wind profile with a proper aerofoil section is capable of.

Julian.

[Roger]

The message from "Julian" contains these words:

With a symmetrical section you should get the same conditions above and below the wing so how does it generate lift?

Within reason the more asymmetric the aerofoil section the greater the lift and the greater the angle of attack likewise so fixed ratios don't make sense.

Without resorting to a wind tunnel (a paper dart will glide) you can easily prove you don't need an asymmetric section to get lift. When I was a child and toys were much simpler you could get crude balsa wood gliders that had flat sheet wings and they could be trimmed to glide perfectly but would nose dive if thrown upside down.

[Julian]

You need a positive angle of attack, when you have this conditions 'above and below the wing' differ and lift is produced.

Haven't you just answered (all by yourself!) the question you asked above?

Julian.

[Roger]

The message from "Julian" contains these words:

So when you invert the plane you get a negative angle of attack and the plane is sucked down.

I suppose you could always argue that any change in pressure in a fluid involves the Bernoulli effect but there is a world of difference between the lift generated by a flat plane at an angle and that generated by the an aerofoil section designed to provide lift.

[Stuart Noble]

>>> The moment you pick one up you realise that it bears no resemblance

And if you buy one, you'll be obliged to say it's wonderful so you won't look an idiot. That's how you sell expensive gadgets.

[Brian Sharrock]

snip

Yep; despite the popular lyrics; it's not " ... the wind beneath my wings ..." but the suction above the wings that takes weight-off-wheels . The effect is quite marked on aircraft when ,given the correct humidity conditions, as the aircraft accelerates down th runway -suddenly a 'cloud' of fog forms above the wings and the airframe 'rotates'.

[John Rumm]

I am not suggesting the Bernoulli effect is in any way mythical, just that the explanation given in the book is nonsense and does not describe the correct mechanism. It also seems to fly in the face of the law of conservation of momentum.

As you state the path length actually makes no difference and a symmetric or flat aerofoil will still generate lift.

The book states:

"Now the air which passes over the curved upper side of the wing must travel a much greater distance in the same time than the air which passes below the wing. That means that the air going over the top of the wing travels faster than the air beneath."

Which is flawed in two respects: firstly there is no reason why the air split at the leading edge should remain in alignment at the trailing edge, and secondly, the actual path length difference does not get close to accounting for the speed increase of the airflow over the wing. Feed numbers into the Bernoulli equation based just on that, and you will get the wrong answer.

The relative contributions will depend on the attack angle and the aerofoil shape (and many other factors like air speed, & density etc). The Bernoulli effect enhances the vectoring or turning effect of the airflow (and as you say, can in the right circumstances generate a sizeable part of the turning effect). It also explains the influence of top surface of the aerofoil on the overall lift generated (which a simplistic model that only looked at the underside of the wing would miss)

Indeed. An aerofoil section will produce greater turning effect for a given attack angle.

[John Rumm]

Unless you trim it so that the whole attitude of the airframe is steeply enough raked when inverted to still offer a positive attack angle of the wing irrespective of its natural upward sloping design when the right way up.

There are some nice interactive simulators and explanations of this stuff here:

[Lurch]

On Sun, 29 Jul 2007 12:00:26 GMT, Stuart Noble mused:

>>>>> The moment you pick one up you realise that it bears no resemblance

But it is.

I buy things that can pay for themselves. The Multimaster is one of my cheaper tools, most of the tools I have cost couple of hundred quid up to around a grand or so.

I suppose to be fair, DIYers will be reading this and they won't be able to justify spending hundreds of pounds on a tool whereas I can put it to work and have it pay for itself.

[Andy Hall]

.. and consumables can be built into the customer price.

[Roger]

The message from John Rumm contains these words:

Me being a bit careless with the terminology. I was using 'angle of attack' as the angle between the centre line of the plane and the centre line of the wing. If that has a separate definition I have yet to find it.

[The Medway Handyman]

But that is why the shower curtain sucks itself inwards AFAIK.

[Van Helsing]

Angle of attack is the angle between the centre line (chord line) of the wing and the direction of airflow.

The angle between the centre line of the fuselage and the wing is the angle of incidence.

The angle of incidence is fixed at design time, the angle of attack varies depending on what the plane is doing and its this that allows a plane to fly upside down. Increasing the angle of attack increases drag. At some point, depending on the airfoil section, too high an angle of attack will cause the air flow to detach from the surface of the wing causing a stall.

Wikipedia has several good write ups related to this, and can normally be relied upon for straight factual stuff.

VH.

[Geoff Berrow]

Message-ID: from The Medway Handyman contained the following:

It just does that to be annoying.