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07-20-2016, 04:30 PM
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Join Date: Feb 2007
Location: colorado
Posts: 873
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For the aero engineers and wanna bes
I see several RVs entered in the Airventure Cup Race. Mostly in the blue class (360 cid). And three in the Red class (320 cid)
I have entered my RV4 with 2 RV6s. In the red class.
In the past I have run my catto prop but this year I am going to fly my sensi GA prop. Here is the question:
I have the pitch dialed in for max speed at about 8000da at 2700 rpm
What can I expect to happen if I decide to fly at say 1000 da?
More hp, thicker air,
Should I bring my pitch pins to make an adjustment?
Cm
__________________
RV-4 "Mr. Twister"
Pitts S1S "Mexican Red" sold and missed
Mr. Twister Airshows
Rocky Mountain Renegades
the mission... have fun.
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07-21-2016, 07:55 AM
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Join Date: Feb 2010
Location: Gardnerville Nv.
Posts: 2,828
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No expert here, but lower altitude will make more HP, so higher RPM, so maybe you will be able to run more pitch? the brain can go crazy thinking about thicker air and more drag ect.....so maybe an expert will chime in.
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7A Slider, EFII Angle 360, CS, SJ.
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07-21-2016, 09:15 AM
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Join Date: Jun 2012
Location: Hillsboro, OR
Posts: 63
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Test!
Test it! Your manifold pressure will be much higher at 1000DA vs 8000DA; more power available. So a small increase in prop pitch should work. Of course if you are comfortable running your engine over 2700rpm (big IF right there), you could leave the pitch where it is at now and run above 2700rpm. My advice would be to test, test ,test.
__________________
RV addict for life.
NOTE: The information, ideas, and opinions presented in this post are my own and not those of my employer. Build, test, and fly at your own risk.
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07-21-2016, 09:41 AM
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Join Date: Jun 2014
Location: Ottawa, ON, Canada
Posts: 353
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I'm in the "wanna be" category:
Does the denser air not also mean the prop has to "cut through more air", reducing some of the gains in horsepower?
Or is this influence negligible?
__________________
J.F.
Sling 4 empennage kit on order!
Future EAA 245 Member (Hopefully)
Current Piper Warrior PA-28-151 Owner/Pilot
Ottawa, Ontario, Canada
http://www.sling4.ninja
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07-21-2016, 09:51 AM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by kamikaze
I'm in the "wanna be" category:
Does the denser air not also mean the prop has to "cut through more air", reducing some of the gains in horsepower?
Or is this influence negligible?
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The influence is not negligible, but it is also not linear with the increase in HP at lower altitudes so WOT and 2700 RPM at 8000 will result in an over speed at WOT at 1000 ft.
If the throttle is closed enough at 1000, to get the same MP as was attained at 8000 then the RPM will be close but still likely a bit higher.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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07-21-2016, 09:51 AM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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Pretty sure that if you fly that course in the summer at 1000' DA, your progress is going to be pretty slow, no matter how you pitch your prop.
It would probably be pretty slow in winter, too.
Charlie
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07-21-2016, 10:00 AM
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Join Date: Feb 2007
Location: colorado
Posts: 873
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Quote:
Originally Posted by RVTrumpet
Test it! Your manifold pressure will be much higher at 1000DA vs 8000DA; more power available. So a small increase in prop pitch should work. Of course if you are comfortable running your engine over 2700rpm (big IF right there), you could leave the pitch where it is at now and run above 2700rpm. My advice would be to test, test ,test.
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I know this much
The prop is 4 inches bigger dia than my catto. Unlike the catto that came to life at or above 2700 rpm this prop does not perform better above 2700.
Cm
__________________
RV-4 "Mr. Twister"
Pitts S1S "Mexican Red" sold and missed
Mr. Twister Airshows
Rocky Mountain Renegades
the mission... have fun.
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07-21-2016, 10:48 AM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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If it's a ground adjustable composite Sensenich, the reason it doesn't perform above 2700 is likely more about airspeed than rpm. According to Sensenich, they don't make blades for the ground adjustable hub that are twisted for RV speeds (excepting the -12). You'll hit a 'wall' much earlier, when the inner portion of the blade doesn't have enough pitch & starts blocking airflow, instead of contributing thrust.
Call Sensenich & ask them about it; I did when I was considering a prop using their blades.
Charlie
(Hate to repeat myself, but please don't try to fly that race at 1000' DA.)
:-)
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07-21-2016, 10:51 AM
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Join Date: Feb 2007
Location: colorado
Posts: 873
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Quote:
Originally Posted by rv7charlie
Pretty sure that if you fly that course in the summer at 1000' DA, your progress is going to be pretty slow, no matter how you pitch your prop.
It would probably be pretty slow in winter, too.
Charlie
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Is this some competitive trash talk? Lol Umm ive won this class 4 times and set the class speed record of 214mph in 2010 flying much of the race at reduced power..
Since im based at 5100 msl. The only testing i can do is on the flight into KMHE. Im thinking the increase in hp will be offset by the more dense air but my gut tells me i will need more pitch..
Cm
__________________
RV-4 "Mr. Twister"
Pitts S1S "Mexican Red" sold and missed
Mr. Twister Airshows
Rocky Mountain Renegades
the mission... have fun.
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07-21-2016, 11:06 AM
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Join Date: Dec 2009
Location: Montreal
Posts: 1,456
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Blade pitch has to consider rpm and true airspeed. Rpm is related to input power. TAS will be higher for a given IAS at higher alt which would require more pitch, all other things being equal. But they aren't equal. Power will be down so IAS will be down. Which one wins? TAS effect or effect of power? I would only be guessing. Better off testing.
__________________
Scott Black
Old school simple VFR RV 4, O-320, wood prop, MGL iEfis Lite
VAF dues 2020
Instagram @sblack2154
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