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Jan 14, 2004, 07:02 PM
jnkessler
jnkessler
Guest

2 vs 3 blade props


Hello group,

Here's the question; Given similar size/pitch/rpm/etc. , which generates
more thrust, a 2 blade prop or a 3 blade? (I already know to go down an
inch in diameter or pitch when changing from 2 to 3 blade prop).

thanks, JK


Jan 14, 2004, 07:02 PM
M Dennett
M Dennett
Guest

Re: 2 vs 3 blade props


If you use the same blade design, and spin the propeller at the same rpm,
sure you get more thrust from the 3 blade propeller exactly the same way
that three wing panels of identical design moving through the air at the
same speed will generate more lift than two.

However..

What you lose, especially with smaller props and higher rpms, is efficiency.
Therefore you may not get 1.5 times the thrust that you had with two blades.
Each blade flies in the wake of the preceding blade, and with three blades
they are closer together and thus the air is more "disturbed". Some c/l
speed fliers and some free flight fliers spinning tiny props at high rpm use
one blade props for that reason - to gain efficiency. For much the same
reason biplanes generate more lift than monoplanes but the wings do not work
as efficiently unless you have very large separation distances. You also get
more parasitic drag due to additional wing/fuselage junctions etc.

Therefore in general, if you have the ground clearance to run a properly
sized 2 blade propeller, you're better off with that. If you need ground
clearance and thus can benefit from reduced diameter but need the load
factor of a 3 blade prop, then go ahead and use one. Plus, they look cool.

Mike D.

"jnkessler" <jnkessler@kc.rr.com> wrote in message
news:WWiNb.23822$LW.23258@twister.rdc-kc.rr.com...
> Hello group,
>
> Here's the question; Given similar size/pitch/rpm/etc. , which generates
> more thrust, a 2 blade prop or a 3 blade? (I already know to go down an
> inch in diameter or pitch when changing from 2 to 3 blade prop).
>
> thanks, JK
>
>



Jan 15, 2004, 04:00 AM
The Natural Philosopher
The Natural Philosopher
Guest

Re: 2 vs 3 blade props


jnkessler wrote:

> Hello group,
>
> Here's the question; Given similar size/pitch/rpm/etc. , which generates
> more thrust, a 2 blade prop or a 3 blade? (I already know to go down an
> inch in diameter or pitch when changing from 2 to 3 blade prop).
>
> thanks, JK
>
>
>


Its generally accepted that two blades are better than three. And one
blade is better than two. Efficiency wise. however balancing a one
blader is non trivial, and at high RPM the thrust vector roattes round
the shgft line causing vibration.

Resaon being the blades do not like travelling in the wake of the
previous blade.

Jan 15, 2004, 07:01 PM
Sport_Pilot
Sport_Pilot
Guest

Re: 2 vs 3 blade props


"Each blade flies in the wake of the preceding blade,"

While a three blade is slightly less efficient than a two blade prop,
it has nothing to do with the above. This is because the blade
doesn't fly in the wake of the preceding blade. By the time the next
blade comes around the wake is behind the prop on its way to hit the
rudder to cause P factor.
Jan 15, 2004, 07:01 PM
The Natural Philosopher
The Natural Philosopher
Guest

Re: 2 vs 3 blade props


Sport_Pilot wrote:

> "Each blade flies in the wake of the preceding blade,"
>
> While a three blade is slightly less efficient than a two blade prop,
> it has nothing to do with the above. This is because the blade
> doesn't fly in the wake of the preceding blade. By the time the next
> blade comes around the wake is behind the prop on its way to hit the
> rudder to cause P factor.
>


No, it isn't. Especially at slow forward speeds. Think of spiral tip
vortices that don't run with teh main airflow for one thing. Plus stuff
that bounces off cowls.



Jan 15, 2004, 07:01 PM
Bill Sheppard
Bill Sheppard
Guest

Re: 2 vs 3 blade props


>"Each blade flies in the wake of the
>preceding blade,"


In static testing, the wake might possibly factor in. But at speed, one
would think that the advancing blade will have far ourtrun the 'wake' of
the preceding blade.
Bill(oc)

Jan 15, 2004, 07:01 PM
Dan Thomas
Dan Thomas
Guest

Re: 2 vs 3 blade props


hppilot001@cs.com (Sport_Pilot) wrote in message news:<3b550ea2.0401150742.3459ae5b@posting.google. com>...
> "Each blade flies in the wake of the preceding blade,"
>
> While a three blade is slightly less efficient than a two blade prop,
> it has nothing to do with the above. This is because the blade
> doesn't fly in the wake of the preceding blade. By the time the next
> blade comes around the wake is behind the prop on its way to hit the
> rudder to cause P factor.



More misconceptions. Each prop blade does indeed feel an effect
from the previous blade. Disturbances over an airfoil are felt at
considerable distance above that airfoil, and the higher the number of
blades, the closer that interference gets. The effect is more
pronounced at high RPM and low forward speed, such as in takeoff and
climb.
Disturbances caused by airfoils of any sort are spread outward
from the airfoil at the speed of sound. While the propeller blast is
to the rear, there is a disturbance to the sides and front, consisting
of lowered air density and considerable turbulence. The same factors
apply to helicopter rotors, particularly in hover or low forward
speed, where power requirements rise dramatically as the rotor has to
deal with the turbulence.

Dan
Jan 15, 2004, 07:01 PM
M Dennett
M Dennett
Guest

Re: 2 vs 3 blade props


Thanks for backing me up on this one Dan... ;-)

My answer was a simple version of the explanation I have read over and over
again in various references. To me it makes intuitive sense. If there is
something amiss in what I wrote I don't know what it is, but I am relaying
that which I have come to understand as the reasoning. I used the phrase "in
the wake of the preceding blade" which perhaps is a narrow description of
the situation but to me it covers the issue. And in a literal sense it has
to be true to an extent as pointed out by some, in that the prop is
generally flying somewhere below pitch speed except I guess in a dive. And
especially statically or at low airspeeds. I don't think the original
question was seeking a thesis on the situation but just a simple answer.
Biplane wings, which I pointed out as a similar example, of course don't fly
in each other's "wake" as in one behind the other, but each feels the
effect of the pressure disturbances between the two planes, with low
pressure above the lower wing and higher pressure beneath the upper creating
some form of mayhem and corruption to the airflow that would increase as the
gap is reduced. That's enough explanation for me anyhow. I suppose it also
depends how you define "wake"... to me it's a big picture word describing
the pressure disturbances around the blade/wing/whatever due to motion of
one or the other.

Mike D.



"Dan Thomas" <Dan_Thomas_nospam@yahoo.com> wrote in message
news:43cbd38a.0401150948.3362b0fe@posting.google.c om...
> hppilot001@cs.com (Sport_Pilot) wrote in message

news:<3b550ea2.0401150742.3459ae5b@posting.google. com>...
> > "Each blade flies in the wake of the preceding blade,"
> >
> > While a three blade is slightly less efficient than a two blade prop,
> > it has nothing to do with the above. This is because the blade
> > doesn't fly in the wake of the preceding blade. By the time the next
> > blade comes around the wake is behind the prop on its way to hit the
> > rudder to cause P factor.

>
>
> More misconceptions. Each prop blade does indeed feel an effect
> from the previous blade. Disturbances over an airfoil are felt at
> considerable distance above that airfoil, and the higher the number of
> blades, the closer that interference gets. The effect is more
> pronounced at high RPM and low forward speed, such as in takeoff and
> climb.
> Disturbances caused by airfoils of any sort are spread outward
> from the airfoil at the speed of sound. While the propeller blast is
> to the rear, there is a disturbance to the sides and front, consisting
> of lowered air density and considerable turbulence. The same factors
> apply to helicopter rotors, particularly in hover or low forward
> speed, where power requirements rise dramatically as the rotor has to
> deal with the turbulence.
>
> Dan



Jan 16, 2004, 04:00 AM
M Dennett
M Dennett
Guest

Re: 2 vs 3 blade props


Agreed. And the losses are relatively lower with those large diameters and
lower rpms.

Mike D.


"Paul McIntosh" <paul@mcintoshcentral.com> wrote in message
news:bu71tp$b9e$1@sparta.btinternet.com...
> I think most of the IMAC guys are moving to three blade props because they
> are going with larger and larger planes. This means larger and larger
> engines. As the planes approach 1/2 scale, the power requirements tend to
> be a lot greater to get the performance they require. Simply put, two

blade
> props would have to be too large for the performance these engines and
> planes require. Just like full scale, they HAVE to go to multi blade

props
> for ground clearance!
>
> --
> Paul McIntosh
> Desert Sky Model Aviation



Jan 16, 2004, 04:00 AM
The Natural Philosopher
The Natural Philosopher
Guest

Re: 2 vs 3 blade props


Paul McIntosh wrote:

> I think most of the IMAC guys are moving to three blade props because they
> are going with larger and larger planes. This means larger and larger
> engines. As the planes approach 1/2 scale, the power requirements tend to
> be a lot greater to get the performance they require. Simply put, two blade
> props would have to be too large for the performance these engines and
> planes require. Just like full scale, they HAVE to go to multi blade props
> for ground clearance!
>
>


And to absorb power without the tips going supersonic.

Jan 16, 2004, 04:00 AM
jnkessler
jnkessler
Guest

Re: 2 vs 3 blade props


Hello Mike,

Thanks for a very clear and concise answer to my question.

As I understand it; switching from a 2 blade to a comparable 3 blade will
increase thrust but NOT 1.5 times more thrust. Got it.

It also seems 3 blades are more effective at moderate rpm's and larger
diameter.

thanks, JK
opinions will vary.


Jan 16, 2004, 07:01 PM
Dan Thomas
Dan Thomas
Guest

Re: 2 vs 3 blade props


"jnkessler" <jnkessler@kc.rr.com> wrote in message news:<mnGNb.44317$VV4.2376@twister.rdc-kc.rr.com>...
> Hello Mike,
>
> Thanks for a very clear and concise answer to my question.
>
> As I understand it; switching from a 2 blade to a comparable 3 blade will
> increase thrust but NOT 1.5 times more thrust. Got it.
>
> It also seems 3 blades are more effective at moderate rpm's and larger
> diameter.
>
> thanks, JK
> opinions will vary.


You won't get more thrust. At static, you'll likely get less.
Smaller diameter props produce less static thrust because more of the
horsepower is going into drag and turbulence. Fewer, longer blades are
moe efficient. Two ways to get more thrust are to use a larger prop
and gear it down, or add more horsepower.
Three-and four-blade props are used for ground clearance and/or to
absorb more horsepower in a given diameter. Basically the same thing.
Sometimes they're used to reduce tip speeds and therefore noise. And
sometimes to get a smooth, vibration-free ride. In turns, two-bladed
props create vibration.
Modern turboprop airliners have geared props, since the turbine
turns way too fast to attach a prop directly. So they gear them way,
way down and turn large, wide-bladed props running at very high pitch
settings in cruise. Typical RPM on a commuterliner prop might be 800
or so. The WW2 Corsair turned its prop at 1300 for takeoff and 900 for
cruise. Low RPMs were necessary to keep tip speeds low enough that the
prop didn't go supersonic when forward speed was factored in.

Dan
Jan 16, 2004, 08:39 PM
twinster2
twinster2's Avatar

3 bladed pusher props


In following this discussion, I have considered using a 3 bladed pusher prop on my F-18 that I have. I currently use a 9.5X8 two blade, but am considering using a 9.5X6 three blade. The engine is a OS 46. Comments are welcome. Don
Jan 17, 2004, 04:00 AM
Six_O'Clock_High
Six_O'Clock_High
Guest

Re: 2 vs 3 blade props



"DAYFLIER" <dayflier@aol.com> wrote in message
news:20040116204510.21817.00000132@mb-m06.aol.com...
>SNIP


Anyway a three blade prop will not make
> your plane crash....Bobby


ROFLOL! That is good because I sure don't need any more excuses!


Jan 17, 2004, 07:00 PM
Pé Reivers
Pé Reivers
Guest

Re: 2 vs 3 blade props


That is also where the wake theory looses some of it's significance.
These large prop disks require more blades, so the air column is smoothly
accelerated over it's full surface. In windmill and helicopter theory they
call it "blade coverage", and the number of blades tends to increase with
increasing diameter. The next blade in this case is not working in the wake
of the previous one, because the pitch is much larger (function of
diameter), and the flow vortex has advanced sufficently by the time the next
blade arrives.
In areas of stagnant flow, such as in front of cowlings, in this case it may
be of advantage to cuff the blade roots, or locally decrease the pitch in
order to reduce the losses of a too high blade coverage there.

--

With Kind Regards,

http://mvvs-nl.com
http://home.wanadoo.nl/pereivers/
___________

"M Dennett" <dennett@cesaroni.net> wrote in message
news:aFFNb.5194$c1.744126@news20.bellglobal.com...
> Agreed. And the losses are relatively lower with those large diameters and
> lower rpms.
>
> Mike D.
>
>
> "Paul McIntosh" <paul@mcintoshcentral.com> wrote in message
> news:bu71tp$b9e$1@sparta.btinternet.com...
> > I think most of the IMAC guys are moving to three blade props because

they
> > are going with larger and larger planes. This means larger and larger
> > engines. As the planes approach 1/2 scale, the power requirements tend

to
> > be a lot greater to get the performance they require. Simply put, two

> blade
> > props would have to be too large for the performance these engines and
> > planes require. Just like full scale, they HAVE to go to multi blade

> props
> > for ground clearance!
> >
> > --
> > Paul McIntosh
> > Desert Sky Model Aviation

>
>





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