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Thrust - Over Rated
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MichaelGibbs(at)cox.net
Guest

Posted: Mon May 08, 2006 11:51 pm Post subject: Thrust - Over Rated

Randy sez:



 	  Quote: 	
		  In short, if you want more thrust, all you have to do is sacrifice 

top speed. Therefore, thrust is irrelevant.


	


If you want more thrust AND more top speed, you get an in-flight 

adjustable prop.  You wouldn't drive your car in one gear all the 

time, why operate your airplane that way?



Mike G.

N728KF

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Tue May 09, 2006 2:59 am Post subject: Thrust - Over Rated

 	  Quote: 	
		   From: Michael Gibbs [MichaelGibbs(at)cox.net]

 You wouldn't drive your car in one gear all the 

 time, why operate your airplane that way?


	


Well ... yes, when you have a Jabiru engine that doesn't like much more than a solid wooded prop, Mike!  But, as I have said before, I love the idea of an in-flight adjustable prop ever since my father put a adjustable pitch prop on our sailboat when I was a child.

In-flight adjustable pitch is my wet dream ... well, my second wet dream! 



Cheers,

Michel



do not archive

_________________
Kitfox 3 - Jabiru 2200

lcfitt(at)sbcglobal.net
Guest

Posted: Tue May 09, 2006 6:08 am Post subject: Thrust - Over Rated

Mike,  I couldn't agree with you more.  However there are airframe - 

engine - prop combinations that will give enough of each with a fixed pitch 

prop that the cost / benefit and complexity just hasn't made me even think 

about yet.



In our airplanes, I think an argument could be made that a cockpit 

adjustable prop is sort of like a bandade for a mismatched engine - 

airframe.



Personally  I like the simplicity in flight and the lack of prop maintenane 

issues of the old fashioned ground adjustable Warp Drive prop.  I can almost 

keep up with the PowerFin in climb and cruise has never been an issue for 

me, I can get plenty.



Lowell



---

alnanarthur(at)sbcglobal.
Guest

Posted: Tue May 09, 2006 6:29 am Post subject: Thrust - Over Rated

Mike,

You are right, everything in life is a tradeoff.

A Harrier Jet takes off with zero ground roll and goes fast!

A ground adjustable has less parts, is more reliable and less expensive.

Also, it's allowed under the sport pilot rule.

I get off the ground in about 200 ft (solo), climb at 1200 ft/min and  

cruise at 105 mph at 5200 rpm.

Allan Arthur,  Sport Pilot

Kitfox 5, N40AA

Rotax 912s, Warpdrive 3 blade

Byron Airport, CA  (C83)  Hanger C8

On May 9, 2006, at 12:48 AM, Michael Gibbs wrote:

 	  Quote: 	

 <MichaelGibbs(at)cox.net>

 Randy sez:

> In short, if you want more thrust, all you have to do is sacrifice

> top speed. Therefore, thrust is irrelevant.

 If you want more thrust AND more top speed, you get an in-flight

 adjustable prop.  You wouldn't drive your car in one gear all the

 time, why operate your airplane that way?

 Mike G.

 N728KF

wingsdown(at)comcast.net
Guest

Posted: Tue May 09, 2006 6:29 am Post subject: Thrust - Over Rated

If you have to turn the engine as fast or nearly so for take off as for

fast flight, then an in-flight adjustable prop has merit. Even slow

turning dinosaur engines for the most part have constant speed props.

Band-Aids come in all shapes and sizes. Some are built right in and you

never see them or for that matter know they are there. And  are quite

subjective as to definition.  



Rick



--

algate(at)attglobal.net
Guest

Posted: Tue May 09, 2006 7:27 am Post subject: Thrust - Over Rated

I don't know that IFA props could be considered bandaids. No matter how well

matched your engine may be to the airframe it is always a compromise to

every other set of circumstances other than the ideal situation the

combination has been "tuned" for. The IFA prop allows the "tuning" for a

wide range of variable conditions.



I'm sure that when you have a fixed or ground adjustable prop there might be

the time you would like the flexibility of a IFA prop ie. When you have

landed on a small lake a long way from home (so you have max fuel) and you

catch a serious number of large fish. All of a sudden the shore line looks a

lot closer and the thought of an additional 300ft/min climb looks pretty

inviting.



Likewise there are times when it's really convenient to be able to maintain

airspeed, lower fuel consumption and noise by simply flicking a switch.



I think sometimes simplicity can be a state of mind that comes with being

comfortable with a technology - I guess even an electric light bulb looked

pretty daunting when it was first introduced.



No flame intended!



GaryA

Lite2



 



--

roncarolnikko(at)hotmail.
Guest

Posted: Tue May 09, 2006 7:39 am Post subject: Thrust - Over Rated

Hi Alan  Yes thrust is a variable with pich and length of prop.  The VW's 

have been limited to small slippery airplanes due to the fact they have  to 

swing a small prop in order to get in the power band.  My goal is to use a 

redrive that allows a large prop suitable for the Kitfox performance range.  

Ron NB Ore

 	  Quote: 	
		  From: alnanarthur <alnanarthur(at)sbcglobal.net>

Reply-To: kitfox-list(at)matronics.com

To: kitfox-list(at)matronics.com

Subject: Re: Re: Thrust - Over Rated

Date: Tue, 9 May 2006 07:28:14 -0700

Mike,

You are right, everything in life is a tradeoff.

A Harrier Jet takes off with zero ground roll and goes fast!

A ground adjustable has less parts, is more reliable and less expensive.

Also, it's allowed under the sport pilot rule.

I get off the ground in about 200 ft (solo), climb at 1200 ft/min and

cruise at 105 mph at 5200 rpm.

Allan Arthur,  Sport Pilot

Kitfox 5, N40AA

Rotax 912s, Warpdrive 3 blade

Byron Airport, CA  (C83)  Hanger C8

On May 9, 2006, at 12:48 AM, Michael Gibbs wrote:

 > 

 > <MichaelGibbs(at)cox.net>

 >

 > Randy sez:

 >

 >> In short, if you want more thrust, all you have to do is sacrifice

 >> top speed. Therefore, thrust is irrelevant.

 >

 > If you want more thrust AND more top speed, you get an in-flight

 > adjustable prop.  You wouldn't drive your car in one gear all the

 > time, why operate your airplane that way?

 >

 > Mike G.

 > N728KF

 >

 >

 >

 >

 >

 >

 >

 >

 >

 >

 >

 >

_________________________________________________________________

Express yourself instantly with MSN Messenger! Download today - it's FREE! 

http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/

janderson412(at)hotmail.c
Guest

Posted: Tue May 09, 2006 2:25 pm Post subject: Thrust - Over Rated

Youre right Rick, fixed pitch are a compromise and some better than others. 

But I have to say, I seem to have stumbled on a beauty...3 blade 72" tapered 

tip Warpdrive. Good acceleration and 97kt cruise, I do have strut fairings, 

gap seals, exit fairing on the radiator and spats so I guess this all helps. 

But impressed as to how well that wee prop does. John A.



From: "wingsdown" <wingsdown(at)comcast.net>

Reply-To: kitfox-list(at)matronics.com

To: <kitfox-list(at)matronics.com>

Subject: RE: Re: Thrust - Over Rated

Date: Tue, 9 May 2006 07:27:04 -0700






If you have to turn the engine as fast or nearly so for take off as for

fast flight, then an in-flight adjustable prop has merit. Even slow

turning dinosaur engines for the most part have constant speed props.

Band-Aids come in all shapes and sizes. Some are built right in and you

never see them or for that matter know they are there. And  are quite

subjective as to definition.



Rick



--

smokey_bear_40220(at)yaho
Guest

Posted: Tue May 09, 2006 4:38 pm Post subject: Thrust - Over Rated

My flame suite on for another long one.....



---------------------------------

 	  Quote: 	
		   Randy sez:



 >In short, if you want more thrust, all you have to

 >do is sacrifice top speed. Therefore, thrust is

 >irrelevant.

 -------------------------

 Mike G.

 N728KF

 If you want more thrust AND more top speed, you get

 an in-flight adjustable prop.  You wouldn't drive

 your car in one gear all the time, why operate your

 airplane that way?


	


---------------------------- 

Good point Mike,



There is a misunderstanding on the list of thrust, HP

and torque.  The whole purpose of the prop is to

convert power into thrust.  Thrust is the most

relevant measure.  It moves the plane.



But we are not talking about static thrust.  You need

the best thrust at the speed you are flying for best

performance.  Max static thrust is best at zero speed

only.  Max takeoff thrust is best somewhere around 1/2

your takeoff speed.  Max climb thrust is best at climb

speed.  Max cruise thrust is best at.... ahhh you get

it.



A fixed pitch prop will give you its max thrust at

only one speed.  You are compromising only in that you

pick the speed you want to have it perform best at. 

At all other speeds, faster or slower, it will give

less then max thrust.



A variable or constant speed prop will still have one

best speed, but the curve will be flatter.  It will

perform almost as well at all speeds as a fixed pitch

prop for its one speed.



Say a cruise fixed pitch gives efficiencies of:

    50% in takeoff

    75% in climb

    85% in cruise



A climb prop might give:

    65% in takeoff

    85% in climb

    80% in cruise



The variable prop might give:

    75% in takeoff

    82% in climb

    83% in cruise



I have a friend who has a Champ product, his 3rd in 2

years.  His 150 HP constant speed Champ would perform

equal to his 180 HP fixed pitch Champ, except around

one speed.  That speed depended upon which fixed pitch

prop he installed.  He might get better climb, but

equal cruise, or better cruise, but equal climb from

the extra 30 HP.



In his case, he used the prop that meant they cruised

equally, but the fixed pitch climbed better with the

30 more HP.  If it had a constant speed prop, it would

have used the 30 extra HP for better performance all

across the speed range.  His fixed pitch got off the

ground better, but burned more for the same cruise

speed.



Another way to say it is that the variable prop was

almost worth the 30 extra HP.



This is my problem with static thrust measurements. 

It only gives you the measurement for the best prop at

no speed.  How useful is that?  I think that is what

Randy was saying.



Static thrust is only good when static.  It is already

losing performance in the takeoff roll, but not so

badly as when at cruise speed.  A cruise prop would be

poor but increasing performance in the takeoff roll,

and continue to get better all the way up to cruise. 

The climb prop is of course in between, getting

better, then getting worse as speed increases.



As to HP and torque, if you are swinging a 2x4 around

with your engine, it will take HP and torque to do,

but produce no thrust.  If you have a feathered prop,

it will take lots of HP and torque to swing and still

produce no thrust.  The efficiency of the prop to

produce thrust is what makes the hp and torque

meaningful.



As a prop increases pitch, it moves more toward a

feathered prop in that the angle of lift of the prop

is more against torque and less forward producing

thrust.  Lift of the prop is always about 90 degrees

to the blade.  A pitch of say 20 degrees means that

some of the prop's lift is pulling air in a circle and

not just pushing it back.  That takes HP.



When I flew the C-130, the pitch could be as high as

55 degrees in cruise.  More than 1/2 the HP went to

spinning the air and less than 1/2 went to thrust. 

Our props don't get that bad due to our less pitch.



But a heavily pitched prop may take the same HP/torque

as a finer pitched prop at higher RPM.  The faster

turning, lower pitched prop will produce more thrust,

to a point.  That point is determined by the most

efficient rpm for the prop compared to mach number,

and the most efficient rpm for the engine.  Those 2

may not match.



A less efficient prop may take more HP or torque to

give the same performance, not more.  A more efficient

prop will give more thrust with the same HP/torque. 

HP and torque don't tell the whole story.  They don't

give thrust, except as the prop allows.



So I am saying this.  The prop that produces the most

thrust will be the one that has its best speed closest

to your engine's favorite RPM, but does it at the

speed you are flying.  Also it has to be the most

aerodynamically efficient.



Max static thrust equals first gear only.  Takeoff

props equal second gear.  Climb props equal third

gear.  Cruise props are overdrive.  Variable props are

automatic trannys.  That is Mike's point.



The actual performance of the plane will be determined

by just how much thrust it can produce today on this

fuel, at this temp, at this density altitude, with

this prop, at this speed, with these sparkplugs, at

this rpm and manifold setting.....  You can measure

all that, or you can just measure thrust.



So thrust is really the key, but it has to be measured

now, at your speed and with all variables.



Static thrust is good for fans, not planes.



Thrust is what happens after all the variables are in

place.  HP and torque are only what you are putting

into a prop, not what is coming out, which changes

with prop efficiency, speed, air density.....



So that is why I wanted a meter that simply measures

the thrust output on board at all speeds, altitudes,

settings, etc.  It will tell you what performance you

are getting now, whatever now is.



And instead of HP, you can simply measure fuel flow if

the engine is tuned right.  That is, if the fuel air

ratio is the same, fuel in equals power out.



That you can compare to whatever you want to change

and see if it made things better or worse.  10% more

thrust with the same fuel flow is 10% better no matter

how you got it.  10% more HP or 10% more torque might

just mean a 10% worse prop is giving equal thrust for

more gas.



Bottom line:  Thrust = performance.  Fuel flow =

energy and $ in to produce the thrust.  The best prop

will be the one that gives you the most thrust per

fuel flow at the speed you want it.



A fixed pitch will peak at a given speed with a strong

curve.  A variable prop will have a broader curve. 

You have to pick the prop that peaks at the speed and

rpm that best matches your engine and most important

performance goal.  This is critical for fixed pitch,

but less critical for variable.



Then you can compare one prop against another to see

which gives the most thrust at the same fuel flow.



Is this too much at once?  Hope it can be understood.



You want thrust.  Thrust moves the plane.  We just

aren't use to measuring it directly.  Fuel flow is

what we pay to get it and max fuel flow is how much

power is available under present conditions.



One day I will simplify this.....

 

Kurt S.  S-5 flying and 1/2 done.



__________________________________________________

smokey_bear_40220(at)yaho
Guest

Posted: Tue May 09, 2006 4:50 pm Post subject: Thrust - Over Rated

Well John,



Rick does understand props and engines.  But you've

done great too, having picked and built all that stuff

yourself.



What engine and prop RPM are you using?  What fuel

flow for takeoff and cruise?  Inquiring minds want to

know.    Especially since some of us have the same

blades, just cockpit adjustable and maybe different

rpms.  Hard to actually compare on different planes, I

know, but it helps a little.



Kurt S.



--- John Anderson <janderson412(at)hotmail.com> wrote:



 	  Quote: 	
		   You're right Rick, fixed pitch are a compromise and

 some better than others. 

 But I have to say, I seem to have stumbled on a

 beauty...3 blade 72" tapered 

 tip Warpdrive. Good acceleration and 97kt cruise, I

 do have strut fairings, 

 gap seals, exit fairing on the radiator and spats so

 I guess this all helps. 

 But impressed as to how well that wee prop does.

 John A.

 

 From: "wingsdown" <wingsdown(at)comcast.net>

 Reply-To: kitfox-list(at)matronics.com

 To: <kitfox-list(at)matronics.com>

 Subject: RE: Re: Thrust - Over Rated

 Date: Tue, 9 May 2006 07:27:04 -0700

 

 

 <wingsdown(at)comcast.net>

 

 If you have to turn the engine as fast or nearly so

 for take off as for

 fast flight, then an in-flight adjustable prop has

 merit. Even slow

 turning dinosaur engines for the most part have

 constant speed props.

 Band-Aids come in all shapes and sizes. Some are

 built right in and you

 never see them or for that matter know they are

 there. And  are quite

 subjective as to definition.

 

 Rick


	


__________________________________________________

janderson412(at)hotmail.c
Guest

Posted: Tue May 09, 2006 7:22 pm Post subject: Thrust - Over Rated

Early days on fuel flow but seems to be around 15 litres per hour and I 

don't have a flow meter. 4500 on t/o, more there if I want with prop setting 

but that's what I use, climb 1200fpm. 97kts at 4300. Just me in it tho at 

this early stage. John



From:  kurt schrader <smokey_bear_40220(at)yahoo.com>

Reply-To:  kitfox-list(at)matronics.com

To:  kitfox-list(at)matronics.com

Subject:  RE: Re: Thrust - Over Rated

Date:  Tue, 9 May 2006 17:49:21 -0700 (PDT)


<smokey_bear_40220(at)yahoo.com>



Well John,



Rick does understand props and engines.  But you've

done great too, having picked and built all that stuff

yourself.



What engine and prop RPM are you using?  What fuel

flow for takeoff and cruise?  Inquiring minds want to

know.    Especially since some of us have the same

blades, just cockpit adjustable and maybe different

rpms.  Hard to actually compare on different planes, I

know, but it helps a little.



Kurt S.



--- John Anderson <janderson412(at)hotmail.com> wrote:



 	  Quote: 	
		  You're right Rick, fixed pitch are a compromise and

some better than others.

But I have to say, I seem to have stumbled on a

beauty...3 blade 72" tapered

tip Warpdrive. Good acceleration and 97kt cruise, I

do have strut fairings,

gap seals, exit fairing on the radiator and spats so

I guess this all helps.

But impressed as to how well that wee prop does.

John A.



From: "wingsdown" <wingsdown(at)comcast.net>

Reply-To: kitfox-list(at)matronics.com

To: <kitfox-list(at)matronics.com>

Subject: RE: Re: Thrust - Over Rated

Date: Tue, 9 May 2006 07:27:04 -0700




<wingsdown(at)comcast.net>



If you have to turn the engine as fast or nearly so

for take off as for

fast flight, then an in-flight adjustable prop has

merit. Even slow

turning dinosaur engines for the most part have

constant speed props.

Band-Aids come in all shapes and sizes. Some are

built right in and you

never see them or for that matter know they are

there. And  are quite

subjective as to definition.



Rick


	


__________________________________________________





_________________________________________________________________

Find the coolest online games (at) http://xtramsn.co.nz/gaming

MichaelGibbs(at)cox.net
Guest

Posted: Tue May 09, 2006 11:37 pm Post subject: Thrust - Over Rated

Kurt sez:



 	  Quote: 	
		    My flame suite on for another long one...

  :

  :

  When I flew the C-130, the pitch could be as high as...

  :

  :

  So I am saying this. The prop that produces the most...

  :

  :

  So that is why I wanted a meter that simply measures...

  :

  :

  Is this too much at once? Hope it can be understood...

  :

  :

  One day I will simplify this...


	


Reminds me of something Mark Twain once wrote: "I apologize for such 

a long letter, I didn't have time to write a short one."  



Mike G.

N728KF

rjdaugh(at)rapidnet.com
Guest

Posted: Wed May 10, 2006 5:32 am Post subject: Thrust - Over Rated

Thanks Kurt!  As usual, another great post.



Maybe you can shed some light on my following comment.



You want to get the rpm's up on takeoff so you can use the power, but you

want to have more pitch for more thrust at speed.  You can approach this

with an in-flight adjustable prop, or you can get close by having a prop

that "slips" during take off, letting the engine rev up, but still has

enough pitch to make some thrust at speed.



I think this is a difference between the skinny blades of the warp drive and

wide, high lift blades of the Powerfin.  My CFI friend (16,000 hours) was

impressed, but not favorably, on how my Powerfin prop was "connected" to the

air.  Engine rpm was not determined as much by throttle position as by

airspeed.  In my plane, he was trying to teach me to set rpm to certain

number at different points in the approach.  This works on his plane with

the Warp drive.  But with the powerfin, you can pull the power off but

unless you slow the plane, the rpm doesn't change.  The powerfin does make a

great brake.



So is "prop slip" part of the formula in getting a prop to best fit torque,

and power?



Randy 



.           



--

janderson412(at)hotmail.c
Guest

Posted: Wed May 10, 2006 3:24 pm Post subject: Thrust - Over Rated

Or you can turbo charge. This sort of does what a VPP does to a degree  in 

that the RPM doesn't have to alter too much to increase power for t/off. You 

can set the prop  at a courser angle and increase boost for t/off. John A.





From:  "Randy Daughenbaugh" <rjdaugh(at)rapidnet.com>

Reply-To:  kitfox-list(at)matronics.com

To:  <kitfox-list(at)matronics.com>

Subject:  RE: Re: Thrust - Over Rated

Date:  Wed, 10 May 2006 07:31:20 -0600


<rjdaugh(at)rapidnet.com>



Thanks Kurt!  As usual, another great post.



Maybe you can shed some light on my following comment.



You want to get the rpm's up on takeoff so you can use the power, but you

want to have more pitch for more thrust at speed.  You can approach this

with an in-flight adjustable prop, or you can get close by having a prop

that "slips" during take off, letting the engine rev up, but still has

enough pitch to make some thrust at speed.



I think this is a difference between the skinny blades of the warp drive and

wide, high lift blades of the Powerfin.  My CFI friend (16,000 hours) was

impressed, but not favorably, on how my Powerfin prop was "connected" to the

air.  Engine rpm was not determined as much by throttle position as by

airspeed.  In my plane, he was trying to teach me to set rpm to certain

number at different points in the approach.  This works on his plane with

the Warp drive.  But with the powerfin, you can pull the power off but

unless you slow the plane, the rpm doesn't change.  The powerfin does make a

great brake.



So is "prop slip" part of the formula in getting a prop to best fit torque,

and power?



Randy

wingsdown(at)comcast.net
Guest

Posted: Wed May 10, 2006 4:41 pm Post subject: Thrust - Over Rated

Excellent point John.



Rick



--

janderson412(at)hotmail.c
Guest

Posted: Wed May 10, 2006 6:18 pm Post subject: Thrust - Over Rated

Yes Rick, I'm sure that's why I'm getting such a good result performance 

wise. John

From: "wingsdown" <wingsdown(at)comcast.net>

Reply-To: kitfox-list(at)matronics.com

To: <kitfox-list(at)matronics.com>

Subject: RE: Re: Thrust - Over Rated

Date: Wed, 10 May 2006 17:40:58 -0700






Excellent point John.



Rick



--

janderson412(at)hotmail.c
Guest

Posted: Wed May 10, 2006 6:25 pm Post subject: Thrust - Over Rated

And re stippage mentioned below, I have the skinny blades on my w/drive and 

when the throttle is closed the RPM dropps off quite quickly and not a lot 

of braking effect. But thrust wise it seems very good. John A.

From: "wingsdown" <wingsdown(at)comcast.net>

Reply-To: kitfox-list(at)matronics.com

To: <kitfox-list(at)matronics.com>

Subject: RE: Re: Thrust - Over Rated

Date: Wed, 10 May 2006 17:40:58 -0700






Excellent point John.



Rick



--

wingsdown(at)comcast.net
Guest

Posted: Wed May 10, 2006 6:44 pm Post subject: Thrust - Over Rated

If anyone else has the CAP 140, get some altitude pull power reverse

pitch , apply power. If you have never been forced to lean forward in

flight it's a trip...watch you air speed, it bleeds off quick. Also

great for that unexpected down wind short field landing. Don't know how

I know that. You can do some pretty fast base to finals and put her on

the numbers, takes some nerve though, the pitch doesn't return that fast

should you need  forward power. I use to love to back up from the fuel

pumps, the looks you get are just amazing. 



Rick



--

smokey_bear_40220(at)yaho
Guest

Posted: Wed May 10, 2006 7:46 pm Post subject: Thrust - Over Rated

Not me Rick 



I know Lance did this with another, bigger plane and

had it down at something like 165 degrees and full

reverse!  It works, but frankly, I am a chicken. 



Maybe in a few more years and 100's more Fox hours....



kurt S.



--- wingsdown <wingsdown(at)comcast.net> wrote:



 	  Quote: 	
		   If anyone else has the CAP 140, get some altitude

 pull power reverse

 pitch , apply power. If you have never been forced

 to lean forward in

 flight it's a trip...watch you air speed, it bleeds

 off quick. Also

 great for that unexpected down wind short field

 landing. Don't know how

 I know that. You can do some pretty fast base to

 finals and put her on

 the numbers, takes some nerve though, the pitch

 doesn't return that fast

 should you need  forward power. I use to love to

 back up from the fuel

 pumps, the looks you get are just amazing. 

 

 Rick


	


__________________________________________________

smokey_bear_40220(at)yaho
Guest

Posted: Wed May 10, 2006 9:14 pm Post subject: Thrust - Over Rated

You guys are too fast for me to get back to you... 

Simple becomes complex, or is it just me lately?  You

are right.  I don't have time to shorten this!



Yes, there is a lot more to prop selection.  The

number of blades and cord of the blade add up to what

is called solidity.  I don't remember all of the prop

facts without researching it again, but here are a few

things:



The prop is thought of as a screw and the plane of

rotation the prop disk.  So if the screw were to fill

just one full revolution with one blade, it would be

100% solid.  That is, no space you could poke 90

degrees thru the disk without hitting blade.



A wide cord blade fills more of the disk than a narrow

cord.  Two blades more than one, three more than two,

etc.   More solidity generally = less slip.  Slip is

greater at low speeds.



The "perfect" prop would theoritically have one (1)

infinately long and infinately thin blade with a

counter weight.  Very little solidity, but what an

aspect ratio!  Spin this as slow as possible to just

barely create thrust and that is as close to 100%

efficient as you can get.  No tip losses or friction

to measure, etc.  Forget ground clearance and gearing

down friction losses though!  



There have been very few one blade props!  Tried? 

Yes, but very few.  None infinate....



There have been a few 3 blade props that outperformed

the 2 blade props they replaced, but I would fault the

2 bladed for not being as efficient as it could have

been.



Other considerations are ground clearance requires

more solidity to absorbe the HP without more length. 

Wider blades and more blades absorbe more HP when

length is restricted.  If you add HP, you probably

will need more solidity by adding another blade, or at

least wider blades.



Engine vibration may favor 3 blades.  Even numbers are

better structurally, but odd are better for vibration

absorbtion.



Quiet operation comes from more blades going slower.



Efficiency comes from running the tips around mach .80

or so, depending on the airfoil and tip design.  A

slanted tip like a swept back wing, seems to help. 

The faster the tips can go without creating a lot of

drag, the closer the rest of the blade comes to its

best speed.



The more leading edges you have, the more air to

divide and the more shockwave drag.  Thus fewer blades

win here.



Longer blades blow more air around and outside of the

plane.  The plane fills less of the slipstream, so it

slows less of it down.



Extending the prop even a few inches away from the

cowl reduces drag measurably.  See how LoPresti opens

up the airflow behind the prop and around the cowl on

all his mods.



Direct drive higher rpm engines require shorter blades

to avoid tips reaching mach one, which cause a lot of

drag and noise.  But shorter blades are less

efficient.



Noise = waste.  There is a direct relationship here. 

Check your kids stereo.  Watts for decibels.  Bell

Helicopter actually figured how much HP it took to do

Wap Wap...  Then they changed the tips.  Noise does

not = thrust or lift.



Twist, airfoil shape, taper, tip shape, flex, material

(wood, metal, composit, combinations)



Props aren't as simple as they look and only good

testing will provide a good design or comparison

between designs.  It is still quite a bit of art since

there are so many factors to compromise on.



So where does this actually get us?



When I lived in KY, I had a hill to climb up to the

runway almost right out of the hangar.  One day I was

on that hill and trying to get it going because I was

about to stop.  I added pitch, then power and rolled

backwards down the hill.  More power = less thrust?



I had stalled the prop!  I was just slinging air. 

More RPM and less pitch and it clawed right up the

hill as it should.



In this case slip was a real bad deal.  I was

surprised at how much power I had put in it (stalled)

and it felt like nothing.  It seemed like at full

power I could have pushed the plane backwards by hand.

 WOW!  I learned from that.  RPM, then pitch. Note: 

That is with narrow cord Warp blades.



Those little racers that are trying to get as much as

possible out of the same size little engine use props

that are way over-pitched for takeoff.  I was told

they were stalled at the beginning of the roll, but

now I don't know.  Isn't someone here a racer who can

tell us?  Maybe they are still light enough to get

going stalled then the prop grabs when it unstalls on

the roll.



So Randy, a little bit backwards.  You should get less

slip with less pitch and more RPM on the same prop,

stalled or not but certainly a lot of slip stalled. 

You are slipping more at higher pitch and low rpm.



But you're correct with the solitity of the blades, I

think.  Wider cord or more blades will work better

slow, and narrow blades will grip better at speed, or

at least waste less.  There is a lot of slip at low

speed and solidity will help here.  Going fast though,

the high aspect ratio thin blades win.



Whew....



Kurt S.



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