Novel flying machines / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Novel flying machines

first previous 11 12 13 14 next last

Move Thread

LAN_403

Patrick

Thu Jun 12 2014, 12:21AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

BigBad wrote ...

...it's less efficient; the most efficient is quite a small pitch

yes, so far all data has shown this to be the case. but why? is it air mass?

im still looking at paul lipps propeller explanation and conventional slippage/pitch propeller theory.

BigBad

Thu Jun 12 2014, 12:34AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

It's just standard wing theory, propellers are mostly just wings spinning around in circles.

As you increase AOA the wing progressively turns from an edge-on thin section to a flat-ish plate with a comparatively huge cross-section, and the amount of air you're deflecting downwards, starts off being small, goes up, but eventually starts going back down again (the air doesn't stick to the rear of the wing, the flow separates.)

LiftCurve

Meanwhile the drag is just going up.

So we say the wing has 'stalled'.

Ash Small

Thu Jun 12 2014, 01:12AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

BigBad wrote ...

(the air doesn't stick to the rear of the wing, the flow separates.)

With marine props this is referred to as 'cavitation'. This is generally undesirable, but so called 'supercavitating propellers' make advantage of this, as it can reduce friction (drag?) (I think that's the easiest way to explain it). I don't think it would be desirable in an 'aviation' prop, but it can be used to advantage in marine applications, although efficiency never seems to be above ~55%, but it can be used to advantage when boat speed is above ~100mph (when no prop is very efficient, and these become the most efficient)

For efficiency you require a much closer approximation to the 'ideal propeller'.

Maybe look at some of the 'human powered' flying machines that look like they're made from balsa and clingfilm (ceran wrap).

(Not sure if you need a 'proxy server' to view this Stateside of 'the pond'. If so, try googling 'Canadian team's human-powered helicopter takes flight'. The machine is called 'Atlas'.)

Patrick

Thu Jun 12 2014, 02:54AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

is the shallow pitch capturing more mass, but imparting less velocity change? (than a higher pitch, all else being equal)

Ash Small wrote ...

Maybe look at some of the 'human powered' flying machines that look like they're made from balsa and clingfilm (ceran wrap).

(Not sure if you need a 'proxy server' to view this Stateside of 'the pond'. If so, try googling 'Canadian team's human-powered helicopter takes flight'. The machine is called 'Atlas'.)

i have seen this, there very large, slow turning, ok so all my experiments show larger diameter shallow pitch props are best, but a 12 or 13 inch prop is getting quite big, any gains would be eaten up by supporting structure of the duct. the small diameter props are clearly better at lower speeds, but only while producing low thrust, hence the octo-copters.

what about a 5 blade shallow pitch prop? would adding more blades make up for the lesser diameter and angular velcity at the circumference?

Dr. Slack

Thu Jun 12 2014, 05:58AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Patrick wrote ...

is the shallow pitch capturing more mass, but imparting less velocity change? (than a higher pitch, all else being equal)

Ash Small wrote ...

Maybe look at some of the 'human powered' flying machines that look like they're made from balsa and clingfilm (ceran wrap).

(Not sure if you need a 'proxy server' to view this Stateside of 'the pond'. If so, try googling 'Canadian team's human-powered helicopter takes flight'. The machine is called 'Atlas'.)

egads! To mix metaphors, by lettting the tail wag the dog, you're chasing your tail round and round in a circle. Concentrate on what's dog, ie the physics, acceleration, power etc, and what's tail, practical engineering things like weight of the duct, or weight of the thing you're trying to fly. Comparing like with like is important. But if I have this advantage of gettign cheap ESCs from my brother-in-law, doesn't that mean I should use more of them? Well, yes, but that's not physics.

Zoom right back out, until you have a 'thing' that has to be levitated by acclerating a fluid flow. For flow impulse equal to craft weight, a narrow jet has high energy, and in the limit, an infinitely wide down draught has zero energy. So the physics of generating lift says use as wide a down draft as possible.

However you've got to drive that down draught with blades that have profile drag (friction) and tip drag (induced, pressure difference ameliorated by a duct), which influence the length of blade chosen. Happy the designer who has to make a 15m sailplane or a 2ft wide quad, because that compromise decision is taken out of his hands.

If you have a very long blade and a very light craft, you don't need much fluid speed, so a shallow pitch is what's required. *Keeping the down-draugt size constant*, a heavier craft needs more impulse per second, so more mass flow, so a faster prop, or a higher pitch. Everything flows from the demands of the craft weight, and the choice of down-draft diameter.

Reformulate the problem so it's clear what are arbitrary choices, what's demanded from physics, what are engineering optimisarions, and keep your eye on the donut, not the hole

Patrick

Thu Jun 12 2014, 06:20AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im still trying to figure out where the donut is... Much less keep my eye on it, so far ive really taken a kick to the head with all this.

and i havent even figured out what the profile of a duct should be. Should it flare the fluid flow out?
But im greatful for all the help ive gotten so far.

heres the motor:

1400 k sub V. 150 watts continuous, it seems.
0.051 ohms, 3 phase brushless.
120 degrees F, for ~10 amps.
150 degrees F, for ~15+amps.

Data for the "windsor" master airscrew 10x4 prop.

Tip mach number:
10x3.14 =31.4" per rev
7,000rpm = 116rps
31.4 x 116 = 3,642 in/s = 303 ft/s
mach 1 at 800 ft = 1100 ft/s
303 / 1100 = 0.275 M
Well at least i caught a break with it being 0.27M, sub-sonic.

ellippse prop, from Paul Lipps plane.

again a unique shape. There was a article that explained his reasoning and how to dimension a prop, but now i cant find it.

BigBad

Thu Jun 12 2014, 02:25PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

I think that the duct should contract on the outflow side because the air sausage you sucked in each second at the top is moving faster, and so is thinner.

But you don't want to overdo it; if you bring the duct outlet in too much you're building a jet engine, the pressure in the duct will go above ambient, and the airflow speed will increase; BUT OBVIOUSLY YOU DON'T WANT THE AIRFLOW SPEED TO INCREASE!!! ;)

If you don't bring the duct in at all, you'll get vortexing within the duct, and that will waste energy.

Uspring

Thu Jun 12 2014, 03:51PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Patrick wrote:

and i havent even figured out what the profile of a duct should be

Looking around for ducts in wiki etc. the effect of reducing vortices is mentioned, but I think another effect is more important (really sticking my neck out).
The slip stream around the prop constricts after it leaves the prop as shown here

That is (strangely) due to the effect of the pressure the prop creates behind it. The pressure will accelerate the air behind the prop. At the prop, air speed is v/2 and at the end, when the pressure has equalized to the ambient pressure, it will have accelerated to v. (I'm looking at the special case of a prop not moving relative to the ambient air). Since the volume of air going into the prop is the same as the one going out at the end of the stream, the cross section of the stream end is just half as that of the prop.

The amount of energy required is determined by the mass flow rate and its velocity at the end of the stream. It will increase with v^2 (mass rate assumed to be constant). Thrust will be proportional to v. Thus a low v will increase the thrust to energy consumption ratio.

With a e.g. cylindrical duct, the constriction of flow behind the prop is avoided. For a given mass flow that implies, that the speed of air exiting the duct is just half of that of the unducted case. For a given power input the result is 26% more thrust.

I believe that this can be even carried further by ducts which expand behind the prop. But probably, if that space is available, a bigger prop should be used.

Ash Small

Thu Jun 12 2014, 05:32PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I seem to remember that the duct cross-section should be (approximately) the section of a wing, with the inside of the duct being shaped like the upper side of the wing, and the outside being cylindrical.

I can sketch it later, but have to go out soon.

I think this is pretty much in accordance with Udo, post above.

EDIT: This is a very quick, rough approximation of what I mean, and is not necessarily to scale, etc:

The prop, I think, goes pretty much at the narrowest point, I think.

Patrick

Thu Jun 12 2014, 08:45PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

so i want to open the downstream funnel abit. this was my goal more than a year ago, but its a hard shape to make. so i got to get the mold right in one to three attempts.

a two plug mold seems to be the solution, with an NACA cross section. The question then becomes, 10 inch in diameter at the prop circumference. what diameter do i taper out to at what distance behind the prop face? would visualizing with saw dust be useful? then graph in cad a exponential curve?

Edit: saw dust and flour are useless, I guess I'll use my Anemometer at different stations to visualize the pressure boundary.

NACA calculator:

first previous 11 12 13 14 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob