Propeller Physics and VABs Program. / General Science and Electronics / Forums

Forums

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

« Previous topic | Next topic »

Propeller Physics and VABs Program.

first previous 8 9 10 11 next last

Move Thread

LAN_403

Ash Small

Sat Dec 13 2014, 06:25PM

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

Each prop has 0.456m^2 disc area. For 10kg all up weight, load per prop is 2.5kg.

This equates to 5.48kg/m^2 disc loading, which is higher than I was hoping for, but this also depends on how much of the total mass is batteries.

Next thing, I think, is to consider the propertires of the CF. Ultimate tensile strength and bending strength should give us a CSA. I'm thinking some taper towards the tip would probably be ok, although we do need to consider the bending stresses.

I'll have to look up some figures.

I'm assuming around 2000rpm for hover, and max rpm of 5k, or somewhere close to this. Maybe we could push the hover to 3k?

Patrick

Sun Dec 14 2014, 07:30AM

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

i was thinking 2k would be more effcient than 3k rpm, right? as we want to accelerate the most air by the least amount. or can a higher RPM still do this?

this is useful, it shows a great crabon layup with tension carying the force in a way i never would have thought of.

Ash Small

Sun Dec 14 2014, 02:45PM

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

I think it comes down to maximum rate of ascent. If max rpm is 5k, and we want twice hover thrust, we set hover thrust for 2.5k.

If we set hover thrust for 2k, then at 5k it's producing 2.5 times hover thrust, so needs to be stronger, so therefore has more drag.

If we set hover thrust for 3k, then at 5k it's only producing 1.66 times hover thrust, so less drag.

Obviously, as rate of ascent increases, thrust decreases, but I don't think that will be a big issue here. We just need a starting point to lay up a prop for some static tests.

We do need to get some figures for tensile strength, etc., then we'll need a safety factor. I'm also just assuming max tip speed of ~200m/s based on full size copter design using CF, but the tensile strength figures should be useful here.

It could just be that the tip speed used on full size copters is 200m/s to prevent the tips from going supersonic, I've not done the maths, but I assume 200m/s is a good point to start.

Dr. Slack

Sun Dec 14 2014, 06:31PM

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

at 5k it's producing 6.25 times the 2k thrust

Ash Small

Sun Dec 14 2014, 07:10PM

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

Dr. Slack wrote ...

at 5k it's producing 6.25 times the 2k thrust

Thanks Neil. Where do you get your figures from? Any cjhance of a link?

I was rather hoping you or Uspring or someone would get involved at this point

It sounds, from what you're saying that we may want to set the hover at around 4k rpm.

Patrick

Mon Dec 15 2014, 12:09AM

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

if we double the speed we get 4 times the lift right? just like a forward wing...

Ash Small

Mon Dec 15 2014, 12:16AM

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

Patrick wrote ...

if we double the speed we get 4 times the lift right? just like a forward wing...

I've not checked the maths yet. I know about picking the right pitch for HP and RPM. I'm hoping for a bit of input from others, but I can look it up if necessary.

Patrick

Mon Dec 15 2014, 12:48AM

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

i guess we need to know the slip to figure on pitch, right?

Ash Small

Mon Dec 15 2014, 01:35AM

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

Patrick wrote ...

i guess we need to know the slip to figure on pitch, right?

We certainly need to check that we're using the 'right maths'

Ash Small

Mon Dec 15 2014, 02:28PM

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

Well, I've done some more googling this morning, and ascertained that most 'copters have a blade angle of around 5 degrees when hovering.

I've found various other equations, etc., but none that seem to be appropriate here,

Supersonic speed is 320m/s, so the 200m/s figure assumed above is well within that, although the propeller shape does cause some air to move faster than this, so 200m/s is probably a sensible limit.

A 5 degree angle of attack is much less than any of the other prop designs posted thus far, and will have a lot less drag.

I still need to google a bit more maths.

EDIT: I've discovered that a typical cambered airfoil wath a 5 degree angle of attack has a lift coefficient of 1.

Found something with chord length, seem to be getting somewhere. Pretty busy tomorrow, though.

first previous 8 9 10 11 next last

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