If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Uspring wrote ...
Ash Small wrote:
These two equations then become exactly the same, which implies that the peripheral losses at the interface between the moving column of air and the surrounding, stationary, air completely dominate all of the losses from the system.
These equations describe the kinetic energy that is contained in a moving cylinder of air. For a propeller, the air has to be accelerated in order to generate thrust, so it is not a "lossy" mechanism, but the intent of the prop. Not all of the power put into the prop is converted into air energy, so one usually defines a props efficiency by the ratio of air power to prop power.
In the case of air drag on a car, the idea behind the equation is, that a column of air in front of the car is accelerated to the speed of e.g. the wind shield. So the power lost by the car is the same as the power needed to give the air column its speed. From this a drag force can be calculated.
All of this has nothing to do with vortices around the prop tips. The equations quoted describe an ideal behaviour, where such effects are not taken into account. They can accounted for by adding some fudge factor to the equations.
This it what one would presumably call a 'first order approximation'. I argue that all of the losses come from some form of drag, but it is just a question of semantics I think. You can probably identify several 'distinct' types of loss if you were so inclined, but it all comes down to the resistance of the air to being accelerated, or, in other words, it's viscosity.
I've drawn a quick graph. I assume the x axis (A) is m^2 and the y axis (v^3) is m/s.
I'm not mathematically minded, but I'll try and answer any further questions.
EDIT: Actually I'm still trying to work out what the graph means
(if in doubt, draw a picture!!)
(x axis is 0-10, y axis is 0-100)
I'll sort out the PPT thing later, thanks for the tips.
I argue that all of the losses come from some form of drag, but it is just a question of semantics I think. You can probably identify several 'distinct' types of loss if you were so inclined, but it all comes down to the resistance of the air to being accelerated, or, in other words, it's viscosity.
All of the props energy ends up as kinetic energy of air. Some part of it is useful, i.e. the downward air velocity, which results in an upward thrust. Other energies, e.g. the swirl around the blades and a helical component of the downward stream don't contribute to the upward thrust and are therefore regarded as losses. Being picky: the resistance of air being accelerated is due to its mass, not its viscosity.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Uspring wrote ...
Ash Small wrote:
I argue that all of the losses come from some form of drag, but it is just a question of semantics I think. You can probably identify several 'distinct' types of loss if you were so inclined, but it all comes down to the resistance of the air to being accelerated, or, in other words, it's viscosity.
All of the props energy ends up as kinetic energy of air. Some part of it is useful, i.e. the downward air velocity, which results in an upward thrust. Other energies, e.g. the swirl around the blades and a helical component of the downward stream don't contribute to the upward thrust and are therefore regarded as losses. Being picky: the resistance of air being accelerated is due to its mass, not its viscosity.
All of these losses pretty much follow the same law, ie the 1/2A x v^3 thing, which seems to be common through all of the propeller efficiency/drag stuff.
I think it is something to do with viscosity, especially as the drag forces increase. I seem to remember getting a similar graph when dropping different sized ball bearings into a tube of oil as a student.
There is certainly more than one way to analyse any problem.
All of these losses pretty much follow the same law, ie the 1/2A x v^3 thing, which seems to be common through all of the propeller efficiency/drag stuff.
I think that we have a terminology problem. What in terms of the v^3 law the drag is for e.g. a car, is the thrust for a propeller. For the case of the car I would call it loss, but not for the prop.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
It is somewhat relevant, depending on the Reynolds number.
The Reynolds number for a small drone is going to be a lot lower than a full size helicopter.
The lower Reynolds number means that viscosity is more significant than normal; model aircraft are designed slightly differently to full size aircraft for this reason.
Ash Small wrote ...
BigBad wrote ...
I'm not sure the disadvantage is quite as big as you think it is, otherwise twin rotor helicopters like the Chinook wouldn't exist.
Tip losses are treated fairly well here:
Personally, I think the Chinook is a compromise for a number of reasons.
Well, yeah, all aircraft are a bunch of compromises flying in close proximity, but I think you're completely overstressing the tip loss thing.
Ash Small wrote ...
EDIT: Can't open PPT files, I boycott Microsoft software as much as I'm able to. Open Office doesn't seem to open them.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Novel flying machines
