Feb 08, 2013, 03:39 PM
I hate waiting for parts
United States, NC, Garner
Joined Apr 2001
7,349 Posts
Quote:
 Originally Posted by C₄H₁₀ Myth #18: You can fly that jet since you play a lot of videogames
Exact opposite for me... I can fly many different typed of fixed- and rotary-wing R/C aircraft but I can't fly planes in video games worth a crap. Plus I got tired of being owned by 12-year olds so I gave up online gaming years ago.
Feb 08, 2013, 03:46 PM
Wake up, feel pulse, be happy!
United States, AK, Fairbanks
Joined Aug 2009
12,737 Posts
Quote:
 OK you got me interested... how's this work?
The pitch values found on props normally refer to their geometric pitch, which is akin to the "thread of the screw", so to speak. If you put the prop in a solid substance and turned it, the pitch is how far it would move for each revolution.

The trick is that the prop also has an airfoil, and principles of fluid dynamics are ignored in the screw analogy. Even if you're moving at 100% of your screw-thread geometric pitchspeed, that airfoil still produces some thrust. If your model is slippery enough, you can continue accelerating on thrust from that airfoil alone.
 Feb 08, 2013, 04:09 PM Illinois Joined Sep 2001 28,294 Posts I think Rare Bear is the poster child for "faster than pitch speed" performance. The P-51 also gained several MPH from the exhaust from the radiator. Andy
Feb 08, 2013, 04:20 PM
Registered User
Bishopville S.C.
Joined May 2003
4,147 Posts
Quote:
 Originally Posted by C₄H₁₀ The pitch values found on props normally refer to their geometric pitch, which is akin to the "thread of the screw", so to speak. If you put the prop in a solid substance and turned it, the pitch is how far it would move for each revolution. The trick is that the prop also has an airfoil, and principles of fluid dynamics are ignored in the screw analogy. Even if you're moving at 100% of your screw-thread geometric pitchspeed, that airfoil still produces some thrust. If your model is slippery enough, you can continue accelerating on thrust from that airfoil alone.
Well said. You can also waste a lot of power trying to accelerate beyond that speed using a higher pitched prop. Some planes are faster with less pitch.
Feb 08, 2013, 04:34 PM
A disaster in the making.
United Kingdom, England, Selby
Joined Jan 2013
383 Posts
Quote:
 Originally Posted by Mike_Then I understand that some high-lift airplanes will climb when power is applied but most airplanes are neutral in that they go straight and level whether they are at 50% power or 100% power. My experience has been that a neutral airplane which climbs severely under full power has a thrust angle issue. Just like a plane that dips severely when the throttle is released has too much up thrust (needs too much trim to maintain level flight under power).
Looks like my old flying instructors have been teaching me all wrong then. Any cambered wing will produce more lift as speed increases. The only one that will not is a symmetrical airfoil that uses AofA as the means of producing lift.
And the tech term for your thrust issues is the thrust/drag couple.

Finally, an aircraft just flies in a certain way, it is for the controller to ensure you fly it within its flight envelope. Nothing needs too much trim, the controller must supply the airspeed and trim in suitable quantities to stay aloft or crash. It is the one stirring the stick not the aircraft's fault if an unplanned aircraft and ground interface occurs.
Feb 08, 2013, 04:43 PM
A disaster in the making.
United Kingdom, England, Selby
Joined Jan 2013
383 Posts
Quote:
 Originally Posted by C₄H₁₀ The pitch values found on props normally refer to their geometric pitch, which is akin to the "thread of the screw", so to speak. If you put the prop in a solid substance and turned it, the pitch is how far it would move for each revolution. The trick is that the prop also has an airfoil, and principles of fluid dynamics are ignored in the screw analogy. Even if you're moving at 100% of your screw-thread geometric pitchspeed, that airfoil still produces some thrust. If your model is slippery enough, you can continue accelerating on thrust from that airfoil alone.
Forget the thread thought. The prop IS an airfoil.
Feb 08, 2013, 04:54 PM
Member
Edina, MN, USA
Joined Oct 1999
11,580 Posts
Props add velocity to the approach air -- so why would it not be the case that flight speed could/would be faster than "pitch" speed?

See the NASA chart.

Also see how different calculated pitch speed is to measure pitch speed off the back of the prop as shown on my chart.

My Mustang is near 100; but pitch speed is over 60!

 Jim's electric RC P51 @ 95mph (2 min 18 sec)

.

Images

 Feb 08, 2013, 05:45 PM ARFs make me BARF United States, MI, Roseville Joined Dec 2000 9,674 Posts No dihedral?
Feb 08, 2013, 05:51 PM
ARFs make me BARF
United States, MI, Roseville
Joined Dec 2000
9,674 Posts
LOUD supersonic prop tips!
 The Republic F-84H Thunderstreak - Propeller variant (3 min 41 sec)
 Feb 08, 2013, 06:02 PM Ascended Master Palmdale, CA Joined Oct 2000 13,600 Posts One of my bosses in Flight Test worked on the XF-84H... he said the Edwards base commander prohibited running the motor at all except at specific times of the day, the noise was so obnoxious. At Pax River, listening to an E2-C run up its motors was really awful to anyone near it, or even in the flight test building.
Feb 08, 2013, 06:24 PM
Grumpy old git.. Who me?
Aberdeen
Joined Mar 2006
12,941 Posts
Quote:
 Originally Posted by Damnandblastit Looks like my old flying instructors have been teaching me all wrong then. Any cambered wing will produce more lift as speed increases. The only one that will not is a symmetrical airfoil that uses AofA as the means of producing lift.
Yep, they were wrong..

ALL AIRFOILS produce more lift as speed increases, cambered, symmetrical, flat plate.. it makes no difference. Lift increases in proportion to the square of airspeed, for any and all airfoils.
Think about it.... If symmetrical airfoils made the same lift regardless of airspeed then how come they need adequate airspeed to fly?

the only time lift would not increase is if the airfoil was at an angle of attack at which it made no lift in the firstplace, in that case lift is zero for all airspeeds (again this applies to all airfoils)
Feb 08, 2013, 07:03 PM
Member
Edina, MN, USA
Joined Oct 1999
11,580 Posts
It's not easy to full mother nature; haven't found a time when something quieter was less efficient!

My tests of the exact same prop profiles/blade config of 2 vs. 4 blade always should the 4 to be quieter.

Look how all the new Turbo props have 6 or 8 scimitar blades.

The XF-84H's prop as reported had super sonic tips.

I tend to prop for large and slow -- use 60 BL vs. 46 w/bigger prop!

100mph Toledo Special w/60 BL, 14x12 APCE, A123-6S, @ < 30amps WOT:

 Jim's RC Electric Toledo near 100MPH (2 min 6 sec)

!
Feb 08, 2013, 07:09 PM
Fueled by Arabica Beans
United States, NY, Syracuse
Joined Oct 2008
4,233 Posts
Quote:
 Originally Posted by C₄H₁₀ The pitch values found on props normally refer to their geometric pitch, which is akin to the "thread of the screw", so to speak. If you put the prop in a solid substance and turned it, the pitch is how far it would move for each revolution. The trick is that the prop also has an airfoil, and principles of fluid dynamics are ignored in the screw analogy. Even if you're moving at 100% of your screw-thread geometric pitchspeed, that airfoil still produces some thrust. If your model is slippery enough, you can continue accelerating on thrust from that airfoil alone.
Gotcha, I also forget the airfoil aspect of props. I can see how this works out in some cases.
Feb 08, 2013, 07:14 PM
Illinois
Joined Sep 2001
28,294 Posts
Quote:
 Originally Posted by mrittinger No dihedral?

The P-51 I built for Ian from your plans has a flat top spar (built-up wing, no foam), so it's what, 1/4" dihedral. Flies great!

Andy
Feb 08, 2013, 07:26 PM
Registered User
Australia, NSW, Sydney
Joined Mar 2012
1,099 Posts
Quote:
 Originally Posted by rcmaverick I am going to start this mini series of myths - TV series mythbuster style. We find a myth - Confirm the myth then verify/ bust/ plausible it. This is an easy one - but anyone who has verified it through practical tests using any means is more than welcome to post their findings. You can even propose an accepted engineering formulae from a scientific journal or wiki (?) Feel free to add your own myth but for the sake of continuity please number them sequentially.
You can look at it in 2 ways

Will a 2 blade prop with the same pitch and diameter be more efficient than a 3 blade prop?

Given the same input power would a 3 blade prop produce more thrust than a 2 blade one?

Both these questions mean the same thing.

"Propellers are similar in aerofoil section to a low-drag wing and as such are poor in operation when at other than their optimum angle of attack. Therefore some propellers use a variable pitch mechanism to alter the blades' pitch angle as engine speed and aircraft velocity are changed.

A further consideration is the number and the shape of the blades used. Increasing the aspect ratio of the blades reduces drag but the amount of thrust produced depends on blade area, so using high-aspect blades can result in an excessive propeller diameter. A further balance is that using a smaller number of blades reduces interference effects between the blades, but to have sufficient blade area to transmit the available power within a set diameter means a compromise is needed. Increasing the number of blades also decreases the amount of work each blade is required to perform, limiting the local Mach number - a significant performance limit on propellers."

So we do have to assume that we are dealing with a fixed pitch prop and there is sufficient blade area and strength to transmit the power.

I was hoping to find a clear cut formula that directly links efficiency to the number of blades but as per page 88 and 89 of the "A treatise on air screw design" link below clearly states that a 2 blade prop is significantly more efficient that a 4 blade one. Due to practical considerations however a 4 blade prop can be made thinner and of less diameter.

So based on that I would still call it BUSTED.

[REFERENCES]

1. A Treatise on air screws

http://ia700300.us.archive.org/33/it...00parkuoft.pdf

2. http://masterairscrew.com/manualinst...ropellers2.pdf

3. http://en.wikipedia.org/wiki/Propeller_(aircraft)