running a 9.25 pitch prop ?  
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All Forums >> RC Airplanes >> Pylon Universe - RC Pylon Racing >> Q-500 Racing >> running a 9.25 pitch prop ?
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running a 9.25 pitch prop ? - 5/20/2003 11:14:14 PM   
daven



Posts: 7026
Joined: 12/7/2001
From: Andover, MN, USA
Status: online 		It seems like a lot of people just loving running these apc 9.25 props, and I've really struggled getting them needled properly.

I either don't get them rich enough, and the engine bogs down on take off, or I get it too rich, and it never really unloads properly.

I can need.e a Nelson with the 8.8x9 quite well, but these 9.25s have caused me nothing but problems.

Do you set the needle with a different prop, and then just switch, is their a certain RPM level to be at? Any help would be appreciated.

Thanks,

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Dave Norman

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running a 9.25 pitch prop ? - 5/20/2003 11:35:38 PM   
splatt



Posts: 651
Joined: 1/30/2002
From: splattsville, MN,
Status: offline 		chuck told me the same thing you said at the end of your statement. Needle it with a 9 or a 8.75, then put on the big one and open it up a 1/4 turn and try it.

My guess is to wait for hot weather and try again

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running a 9.25 pitch prop ? - 5/21/2003 12:17:24 AM   
luv to race


 

Posts: 490
Joined: 12/10/2002
From: Liquored, FL,
Status: offline 		Dave.. What's the 9.25 turning for you? when your having these problems..

RB

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9.25 - 5/21/2003 12:24:35 AM   
daven



Posts: 7026
Joined: 12/7/2001
From: Andover, MN, USA
Status: online 		peaks at about 19,000-19,200-

Don't have an engine that will turn it any higher. Maybe the new one will.

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Dave Norman

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running a 9.25 pitch prop ? - 5/21/2003 2:35:41 PM   
Ed Smith


 

Posts: 2875
Joined: 12/29/2001
From: Brantford, ON, CANADA
Status: online 		I can only pass on my experience with the heavier pitch props.

I set the needle in the pits. I never touch it on the line. To get the setting I bring the revs up slowly using a tach. Do not pinch the line you will overheat the engine and never get a setting. Once the peak is reached on the tach do not let the engine dwell there. Shut it off or it will overheat.

On the line I start the engine then throttle it back so that it sits there running rich. Just before the flag drops I go to full throttle.

Ed S

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running a 9.25 pitch prop ? - 5/21/2003 4:47:22 PM   
luv to race


 

Posts: 490
Joined: 12/10/2002
From: Liquored, FL,
Status: offline 		Dave,

Ed has the solution. Start the motor throttled back and that should keep it from over heating. And 19.2K is not bad, sounds like a good horse to ride! Recognize your air conditions, and only run that 9.25 when the perfect day arrives.

Oh! If the motor continues to be touchy.... change the needle, it may have a flat spot in it.. just a thought..

Randy B.

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       Post #: 6

Fools rush in where angels fear to tread - 5/21/2003 8:43:59 PM   
DHG


 

Posts: 896
Joined: 4/24/2002
From: Arvada, CO, USA
Status: offline 		Dave,

All I know about engines is what I learn from the guys who beat me ... but you might want to try raising the head .002" and see what happens. Touchy needling can be caused by overcompression, and/or too much load (i.e., too "heavy" of a prop), and/or "falling off the pipe" (i.e., pipe not resonating in its upper power band, or only resonating intermittently as you turn the needle to chase the proper mixture). 19K should be enough rpm for the pipe, but that still leaves the first two factors. Raising the head a little generally will cure either or both, assuming the engine is otherwise in good shape ... sounds like it is.

If that doesn't fix it, my advice is to forget what they're wearing in Paris this year and run what's been working for you. There's a lot of "psych" value in Ed's set-it-and-forget-it method ... folks may even get the notion you know what you're doing.

Beetle Bomb

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       Post #: 7

running a 9.25 pitch prop ? - 5/21/2003 9:39:37 PM   
daven



Posts: 7026
Joined: 12/7/2001
From: Andover, MN, USA
Status: online 		I'll admit it, I'm a fuel line pincher... I've never gotten the knack of peaking it, and backing it off with the tach.

I do set the needle in the pits the morning of the race, and back it off about 1/8" turn before the first heat. I'll start the engine on the line, and pinch the line. At that point, I'll screw it in a hair, or leave it alone. After the Heat, I'll check the plug, and adjust the needle approx 1/8" (in or out) depending on how it looks, and the color of the oil on the bottom of the plane.

As to the head, most of my engines have ended up around .012 thousandths raised. I've never had much luck down around .010 or lower.

Its not just one of my engines I've had this problem with, its all three. It must be the pinching method getting things too hot with the 9.25 pitch prop.

Thanks

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Dave Norman

klasskote.com
supertrc.com

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running a 9.25 pitch prop ? - 5/22/2003 7:12:41 AM   
MikeyD


 

Posts: 72
Joined: 4/19/2002
From: Indianapolis
Status: offline 		Dave
For what it's worth, 9.25 is one of my favorite props. I do not claim to be an engine guru, but this is what I do consistently everytime I go to the line. First, I will assume you have already test flown for the morning, or have found the needle setting.

When I go to the line, I do not do anything with the needle, I leave it set for the day. I start the motor at 3/4 throttle. With my tach on the blade, I than go to full throttle and watch the RPM go until the meter no longer is escalating (Analog Tach) usually about 19.2 - 19.5. The second the my analog tach stops escalating up the scale, I flip a pre-programmed switch on my JR8103 radio that mixes Aux3 to my spoiler channel. This guarantees me a fixed position on the throttle usually at 3/4 throttle. I than flip the switch for full throttle literally a second or so before the 60 second clock expires.

BTW, there are days when the weather just isn't there. If I cannot get over 19K, I like to drop down a 9.0 or even 8.75

Mikey D

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running a 9.25 pitch prop ? - 5/26/2003 11:00:00 PM   
PylonWorld



Posts: 1346
Joined: 12/29/2001
From: Monroe, NC, USA
Status: offline 		These tips about using the air valve shutoff to keep the engine cool prior to takeoff started me thinking about something.

Since Q-40's don't have any carb or venturi limitations, could you use an air valve to keep the engine cool until just before takeoff? Or is it better that the engine gets to temp? It would seem that keeping the engine cool could reduce plug damage.

The plus side is that carb or venturi modifications are allowed, so you could open the air valve to the max allowed if it is undersized.

16.1.2. Powerplant

3) Intake: A single carburetor or venturi with a maximum inner diameter of 9 mm (0.3543 inch). Carburetors or venturis may be modified and are not subject to availability rules.

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Don Stegall
RCPRO Chairman of the Board

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running a 9.25 pitch prop ? - 5/27/2003 4:03:21 AM   
Ed Smith


 

Posts: 2875
Joined: 12/29/2001
From: Brantford, ON, CANADA
Status: online 		Don,

It could be doe that way but my partner and I have what I believe is a simpler solution.


Again we get the setting in the pits. On the line we start the engine 3/4 turn rich. The pilot starts the engine and walks away. The caller adjusts the needle to 1/2 turn rich. Just befoe the flag the caller sets the needle at the pre-determined position. Works every time, we never overheat on the line.

This is why I can never understand the go when ready syndrome. A team should develop their own 60 sec start procedure. Why people are willing to give up control of their own start procedur to three other competitors is totally beyond me.

Ed S

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running a 9.25 pitch prop ? - 5/27/2003 5:57:07 AM   
PylonWorld



Posts: 1346
Joined: 12/29/2001
From: Monroe, NC, USA
Status: offline 		Ed,

Is there an advantage of using a venturi over an air valve with the same intake dimensions? Airflow, etc?

Are you coming to Bowie? Flying or driving?

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Don Stegall
RCPRO Chairman of the Board

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running a 9.25 pitch prop ? - 5/27/2003 4:07:11 PM   
Ed Smith


 

Posts: 2875
Joined: 12/29/2001
From: Brantford, ON, CANADA
Status: online 		Don, I would doubt it.

For decades we were convinced that the venturi had to be trumpet shaped for the most efficient fuel/air flow. I am sure there are others but the first one I can recall is the Nelson F1 engine. The rear intake had a great big kidney shaped hole. No trumpet shape venturi here.

When the Q40 event first started the Q40 engine had the back plate mount and the short spinner backplate. Unless you wanted to remove half of the fuselage at the front end it was almost impossible to get the engine in and out with the back plate mount on it. The big venturi got in the way. My partner at the time solved this by machining off the whole trumpet shape He left just enough of the plastic in the crankcase to meet the 9mm rule. It made no difference to the running of the engine whatsoever. All this leads me to believe that the value of the trumpet shape is a bit of a myth. I cannot deny though, it sure looks aggressive.

I am going to Bowie. I will be driving there on Thursday, intending to arrive late Thursday afternoon.

Ed S

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       Post #: 13

Tea and trumpets - 5/27/2003 10:56:16 PM   
DHG


 

Posts: 896
Joined: 4/24/2002
From: Arvada, CO, USA
Status: offline 		OK, I'm exactly half a second ahead of youse guys, and no doubt destined to blow my lead as usual ... I spent most of Saturday modifying a Q500 carb for Q40 to do exactly that (start up rich & cool with a big prop), plus also allow me to close the air intake completely for landing in our famous Colorado offroad singletrack dustbowl 80-grit moonscape. I'd been covering the venturi with a piece of nylon stocking, held in place with a large O-ring ... this works fine, but costs 200-300 rpm. The stock Q500 carb also costs 200 rpm. I was thinking, Why not have the best of both worlds?

Well, here's one word of caution: By the time you get done hogging out the straight cylindrical, 0.3500" Q500 air channel to a nicely flared, 0.354" Q40 air channel, the hole in the rotating barrel is so big that it won't close completely. So, forget about Task 2 (dirt protection on landing). Task 1 should still be attainable, though. I didn't get a chance to put it on the test bench and see if I got my 200 rpm back, but I'll do that soon & then we'll know.

Right now, with the barrel open, the inside contours of the air channel look almost identical to those in the stock venturi. It's a little taller, and there are still a few machining marks in there which I'll polish out later if the experiment works. I'm hoping that when I close the barrel as much as possible, the mixture will richen up enough to allow the engine to slowly die out rich and cool. That has to be better for it than to pinch it off lean (ask any free-flighter), plus there's less strain on the servo.

Maybe I won't have to worry so much about getting dirt in it if I learn to keep it on the runway. :

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       Post #: 14

running a 9.25 pitch prop ? - 5/28/2003 9:32:33 AM   
Bill Vargas