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Home > Archive > Flight simulator > October 2005 > One for the professors - props and drag
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One for the professors - props and drag
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| GeoffC 2005-10-24, 7:33 pm |
| I had a chat with a friend today who suggested (Looking at a DASH 8 going
overhead) that those props must cause a lot of drag. When not powered yes,
but under power (Normal ops) does a prop produce drag as well as thrust?
I would appreciate a pointer to some credible reference on the subject.
Thanks
Geoff
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| "GeoffC" <geoff_noise@hotmail.com> wrote in message
news:b265$435d3e6e$5409c8ce$21160@allthenewsgroups.com...
quote:
> I had a chat with a friend today who suggested (Looking at a DASH 8 going
> overhead) that those props must cause a lot of drag. When not powered yes,
> but under power (Normal ops) does a prop produce drag as well as thrust?
>
> I would appreciate a pointer to some credible reference on the subject.
>
> Thanks
>
> Geoff
Basically you're asking how Efficient a propellor is
in that case.. no a prop ISN"T 100% efficient.
I can go into a more complicated answer. but it's largely a question of
semantics..
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| Dallas 2005-10-24, 7:33 pm |
|
"GeoffC"
quote:
> but under power (Normal ops) does a prop produce drag as well as thrust?
I'm likely not qualified to answer, but I'd like enter my vote then see if
I'm right.
If we are talking about pressure drag I'd say no. Why? Because the prop is
creating a low pressure area in front of the prop. Pressure drag is due to
a combination of the build up of pressure in front of the surfaces, and the
decrease in pressure behind the surfaces. If there is no high pressure in
front and low pressure behind the prop, there is no pressure drag.
There would be induced drag and skin friction drag associated with the
movement of the propeller, but they would occur at vectors not competing
with forward thrust.
Dallas
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| GeoffC 2005-10-24, 7:33 pm |
| Shu,
More complicated would be fine if it provides the indisputable answer.
Thanks also to Dallas although I don't pretend to understand the answer.
Regards
Geoff
"shu" <washu@hiwaay.net> wrote in message
news:bd402$435d56e5$18d6c3f0$8419@KNOLOGY.NET...
quote:
> "GeoffC" <geoff_noise@hotmail.com> wrote in message
> news:b265$435d3e6e$5409c8ce$21160@allthenewsgroups.com...
going[vbcol=seagreen]
yes,[vbcol=seagreen]
>
> Basically you're asking how Efficient a propellor is
> in that case.. no a prop ISN"T 100% efficient.
> I can go into a more complicated answer. but it's largely a question of
> semantics..
>
>
| |
| Gregory 2005-10-24, 11:31 pm |
| On Tue, 25 Oct 2005 00:25:06 +0100, "GeoffC" <geoff_noise@hotmail.com>
brought the following to our attention:
quote:
>Shu,
>
>More complicated would be fine if it provides the indisputable answer.
>Thanks also to Dallas although I don't pretend to understand the answer.
>
>Regards
>Geoff
I thinks what Dallas is saying is.. since the prop `sucks'..
then the pressure out in front of it is low?
-G
quote:
>
>"shu" <washu@hiwaay.net> wrote in message
>
>
| |
| Keith Butcher 2005-10-25, 5:32 am |
|
"GeoffC" <geoff_noise@hotmail.com> wrote in message
news:b265$435d3e6e$5409c8ce$21160@allthenewsgroups.com...
quote:
>I had a chat with a friend today who suggested (Looking at a DASH 8 going
> overhead) that those props must cause a lot of drag. When not powered yes,
> but under power (Normal ops) does a prop produce drag as well as thrust?
>
> I would appreciate a pointer to some credible reference on the subject.
>
> Thanks
>
> Geoff
>
>
Propellers cause drag.
They are aerofoils so the dynamics are pretty much the same as when looking
at those of a wing. However as there is energy being added to the system it
does not appear as an overall drag component for the entire a/c. Until, of
course the power is removed.
So it appears as an 'efficiency' of the prop. The drag/thrust ratios change
with different velocities if the airstream, hence the variable pitch props.
The basic aim is to increase the velocity of a large mass of air by a small
amount, rather than increase the velocity of a small amount of air by a
large amount.
The calculations get complex (beyond me) when you start to look at the 'edge
effects' vorteces produced at the tip. Most easy calculations only look at
a section of a wing and assume and infinitely long aerofoil. The second
complexity is that the tips often approach supersonic speeds. Modern
engines work best at high revs. Gearing the revs down increases weight and
is another component that will fail, so there is a lot of design work to
look at propellor tip design in the supersonic region. This is a seriously
limiting factor in helicopter design and limits the forward speed.
The third complicating factor is the rotational effect that is induced.
Have a look at http://www.ae.su.oz.au/aero/propeller/prop1.html
Even if you can't work through the maths, there is a diagram that shows the
drag vector. This can be resolved into 2 vectors and you will see that the
drag is not perpendicular to the direction of the desired airflow. A great
part of the component is, but some is definately acting against us.
Not for the faint hearted try this one.
http://www.mh-aerotools.de/company/...lerhepperle.htm
| |
| Iain Smith 2005-10-25, 7:33 pm |
| That's amazingly erudite and highly intelligent for someone who watches
paint dry! ;0))
Iain
"Dallas" <Cybnorm@spam_me_not.Hotmail.Com> wrote in message
news:Agd7f.1104$Rl1.34@newsread1.news.pas.earthlink.net...
quote:
>
> "GeoffC"
>
> I'm likely not qualified to answer, but I'd like enter my vote then see if
> I'm right.
>
> If we are talking about pressure drag I'd say no. Why? Because the prop
> is
> creating a low pressure area in front of the prop. Pressure drag is due
> to
> a combination of the build up of pressure in front of the surfaces, and
> the
> decrease in pressure behind the surfaces. If there is no high pressure in
> front and low pressure behind the prop, there is no pressure drag.
>
> There would be induced drag and skin friction drag associated with the
> movement of the propeller, but they would occur at vectors not competing
> with forward thrust.
>
> Dallas
>
>
| |
| GeoffC 2005-10-25, 7:33 pm |
| Keith
Many thanks for the reply, I reckon that the reference sites should keep him
quiet for a while.
Also thanks to the others for their replies.
Regards
Geoff
"Keith Butcher" <keith.butcher@goesnowherewiththis.blueyonder.co.uk> wrote
in message news:%ul7f.136785$G8.66646@text.news.blueyonder.co.uk...
quote:
>
> "GeoffC" <geoff_noise@hotmail.com> wrote in message
> news:b265$435d3e6e$5409c8ce$21160@allthenewsgroups.com...
yes,[vbcol=seagreen]
> Propellers cause drag.
>
> They are aerofoils so the dynamics are pretty much the same as when
looking
quote:
> at those of a wing. However as there is energy being added to the system
it
quote:
> does not appear as an overall drag component for the entire a/c. Until,
of
quote:
> course the power is removed.
>
> So it appears as an 'efficiency' of the prop. The drag/thrust ratios
change
quote:
> with different velocities if the airstream, hence the variable pitch
props.
quote:
> The basic aim is to increase the velocity of a large mass of air by a
small
quote:
> amount, rather than increase the velocity of a small amount of air by a
> large amount.
>
> The calculations get complex (beyond me) when you start to look at the
'edge
quote:
> effects' vorteces produced at the tip. Most easy calculations only look
at
quote:
> a section of a wing and assume and infinitely long aerofoil. The second
> complexity is that the tips often approach supersonic speeds. Modern
> engines work best at high revs. Gearing the revs down increases weight
and
quote:
> is another component that will fail, so there is a lot of design work to
> look at propellor tip design in the supersonic region. This is a
seriously
quote:
> limiting factor in helicopter design and limits the forward speed.
>
> The third complicating factor is the rotational effect that is induced.
>
> Have a look at http://www.ae.su.oz.au/aero/propeller/prop1.html
>
> Even if you can't work through the maths, there is a diagram that shows
the
quote:
> drag vector. This can be resolved into 2 vectors and you will see that
the
quote:
> drag is not perpendicular to the direction of the desired airflow. A
great
quote:
> part of the component is, but some is definately acting against us.
>
> Not for the faint hearted try this one.
>
> http://www.mh-aerotools.de/company/...lerhepperle.htm
>
>
| |
|
| keith butcher said it best
--
**********
shu
"GeoffC" <geoff_noise@hotmail.com> wrote in message
news:a8fe5$435d6d54$5409c8ce$23646@allthenewsgroups.com...
quote:
> Shu,
>
> More complicated would be fine if it provides the indisputable answer.
> Thanks also to Dallas although I don't pretend to understand the answer.
>
> Regards
>
> Geoff
>
>
> "shu" <washu@hiwaay.net> wrote in message
> news:bd402$435d56e5$18d6c3f0$8419@KNOLOGY.NET...
> going
> yes,
thrust?[vbcol=seagreen]
subject.[vbcol=seagreen]
>
>
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