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11-02-2010, 09:41 AM
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Join Date: Jan 2005
Location: Atlanta, GA
Posts: 2,295
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Empirical Evidence - Why buy a C/S prop
I am based at a towered airport (LZU) with a 6000ft runway. Yesterday evening I went out for some "therapy" as I like to call it. In spite of a light breeze across the ground and some serious winds at altitude, the air was smooth as silk. Heading back to home base I was northeast of the field and I was told to make a left downwind to runway 7. I was the only one around. When I was abeam the departure end on downwind I was doing 185kts TAS, 195kts groundspeed. Abeam the approach end I was doing 80kts TAS.
That, ladies and gentleman, is why you get a C/S prop. It's also why I sort of chuckle when people talk about putting speed brakes on these airplanes.
BTW: Before someone scolds me about this...I don't make a habit of this. I know it's not 'good' for the engine. Believe it or not, I did pull the power off slowly. Also, I would never do this at a non-towered airport.
__________________
"What kind of man would live where there is no daring? I don't believe in taking foolish chances but nothing can be accomplished without taking any chance at all." - Charles A. Lindbergh
Jamie | RV-7A First Flight: 7/27/2007 (Sold)
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11-02-2010, 10:23 AM
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Join Date: Nov 2009
Location: Sacramento, CA
Posts: 92
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I agree with the nice braking effects of C/S and I'm not scolding but ... Why would you not conform to FAR 91 limitations of 156 knots in the pattern for piston aircraft? And why would you not fly similarly at a non-towered airport?
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11-02-2010, 10:30 AM
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Join Date: Mar 2005
Posts: 5,685
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Well OK, but ...
I have the Hartzell B/A C/S 72" prop on my RV-6A and previously I had a non-blended airfoil prop with F7666 blades. I'm sure there is some drag effect but I don't notice it - yes really. When I come in from a speed run and throttle back and the MAP bleeds off I go to fine pitch max RPM prop setting and I still have to pull back on the stick and trim the nose up to slow the plane to flap speed and it doesn't happen fast even then. I then slip the plane for a short approach and landing. I don't think there should be a lusting for this prop with the thought that it is going to react like a reversible prop to slow the plane down. If I were to do it over I would get a custom fixed pitch prop for maximum speed at sea level or maybe 1,000 ft, and for landing operations, pull the nose up for deceleration to flap speed and slip for the altitude and speed control on landing. I have enough excess power to make a normal takeoff with my stock O-360-A1A from most airports. What the C/S prop gives me I think is assurance that it will perform well under all circumstances.
Bob Axsom
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11-02-2010, 10:42 AM
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Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,587
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Seriously?
Quote:
Originally Posted by Jamie
... I was doing 185kts TAS....
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Do I misremember or is Bob Axom's top speed about 182?
Tell us how you made your airplane that fast, please .
__________________
H. Evan's RV-7A N17HH 240+ hours
"We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
Paid $25.00 "dues" net of PayPal cost for 2015, 2016, 2017 and 2018 (December).
This airplane is for sale: see website. my website
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11-02-2010, 10:48 AM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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Isn't it actually...
Quote:
Originally Posted by tdhanson
I agree with the nice braking effects of C/S and I'm not scolding but ... Why would you not conform to FAR 91 limitations of 156 knots in the pattern for piston aircraft? And why would you not fly similarly at a non-towered airport?
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...200 kts? And it's IAS in the FARs not TAS....
Sec. 91.117 ? Aircraft speed.
(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).
(b) Unless otherwise authorized or required by ATC, no person may operate an aircraft at or below 2,500 feet above the surface within 4 nautical miles of the primary airport of a Class C or Class D airspace area at an indicated airspeed of more than 200 knots (230 mph.). This paragraph (b) does not apply to any operations within a Class B airspace area. Such operations shall comply with paragraph (a) of this section.
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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11-02-2010, 11:16 AM
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Join Date: Jul 2005
Location: North Atlanta
Posts: 1,120
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It's not uncommon for me to enter the pattern at 180 IAS and be at 100 IAS while turning base with my fixed pitch prop. These airplanes slow down just fine if you fly them instead of just sitting there waiting for them to slow down.
I do think CS helps with the short field landings where you get a quick burst of forward speed do to reduced drag in ground effect. Maybe 30 feet or so?
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Sid Lambert
RV-7 Sold
RV-4 - Flying - O-320 Fixed Pitch - Red over Yellow
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11-02-2010, 11:42 AM
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Join Date: Jan 2005
Location: Atlanta, GA
Posts: 2,295
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Quote:
Originally Posted by tdhanson
I agree with the nice braking effects of C/S and I'm not scolding but ... Why would you not conform to FAR 91 limitations of 156 knots in the pattern for piston aircraft? And why would you not fly similarly at a non-towered airport?
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Where is that at? I just searched my FAR/AIM and couldn't find it. I found a reference on some websites to FAR 91.70B but that doesn't appear to exist in my copy. 
__________________
"What kind of man would live where there is no daring? I don't believe in taking foolish chances but nothing can be accomplished without taking any chance at all." - Charles A. Lindbergh
Jamie | RV-7A First Flight: 7/27/2007 (Sold)
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11-02-2010, 11:45 AM
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Join Date: Jan 2005
Location: Lake St. Louis, MO.
Posts: 2,346
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Quote:
Originally Posted by Jamie
......It's also why I sort of chuckle when people talk about putting speed brakes on these airplanes......
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To achieve maximum performance, a constant speed prop is nice, no question about it ....but when it comes to slowing down.....is not all that superior to an efficient fixed pitch prop. Certainly, some RV's do sport fixed pitch props less efficient than others. With various passenger/witnesses onboard my Sensenich equipped -6A, I have often demonstrated just how easy it is to slow down in the pattern.
Here is a (RV-8 builder) first hand eyewitness account as he observed it. See post #112:
http://www.vansairforce.com/communit...ltitude&page=6
Typically indicating 185-200 MPH, I can easily cross over the field at pattern altitude, fly a tight pattern and without excessively chopping the power can and do slow to Vfe somewhere in the circled area of downwind and base leg.
If a given fixed pitch RV cannot fly a profile as outlined above, I submit their fixed pitch prop is not nearly as efficient as my faster than a constant speed and nearly bulletproof Sensenich.

__________________
Rick Galati
RV6A N307R"Darla!"
RV-8 N308R "LuLu"
EAA Technical Counselor
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11-02-2010, 11:48 AM
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Join Date: Jan 2005
Location: Atlanta, GA
Posts: 2,295
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Quote:
Originally Posted by hevansrv7a
Do I misremember or is Bob Axom's top speed about 182?
Tell us how you made your airplane that fast, please .
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No doubt Bob can beat me at straight and level flight. At WOT at 8000FT I do 178kts when light. I was descending to the airport and was doing about 190 TAS before leveling out so I had momentum on my side and my speed was already bleeding off.
__________________
"What kind of man would live where there is no daring? I don't believe in taking foolish chances but nothing can be accomplished without taking any chance at all." - Charles A. Lindbergh
Jamie | RV-7A First Flight: 7/27/2007 (Sold)
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