C

Christopher Murphy

Well Known Member
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
 
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.
 
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.
 
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?
 
kamikaze said:
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?
Click to expand...

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.
 
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
 
RVTrumpet said:
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.
Click to expand...


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
 
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.)
:)
 
rv7charlie said:
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
Click to expand...

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
 
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.
 
Christopher Murphy said:
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
Click to expand...

I think he's just pointing out that at 1000'DA in summer you will be underground even in the midwest...
 
1000ft da

rv7charlie said:
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.)
:)
Click to expand...

Charlie. The 1000 ft da was just an arbitrary number. Most of the course is above 1000msl. It is my hope that i can get a big tailwind higher up but sometimes the fastest altitudes are lower. If I were very serious about winning the race the catto prop would be installed. Im just very curious about how this prop will perform at lower da's

Ive been talking to sensenich since the day they first showed this prop. Ive tried to get them to offer a heavier spacer and the prop has been back to them for some tweaking of the index pins.

It performs very similar to the metal prop without the rpm restriction and it seems very smooth. It has much better acceleration on take off too.

Cm
 
Interesting. We must be talking to different people.

About a month ago, I asked about performance on a ~180HP -7 using the GA blades, and the guy I spoke with was very specific in saying that the blades are intended for Light Sport speeds, and would be sub optimal for faster RV's because of inadequate twist. (He said that it isn't power issue; the prop I was looking at had 3 blades & could absorb the power.)

I also asked specifically about whether they had considered making blades optimized for the 3/4/6/8 speeds, and he said that they'd discussed it but nothing was actually in the works.

Charlie
 
Interesting

This prop was made especially for faster RV types

Here is a link to their site concerning this propellor

http://www.sensenich.com/products/item/92

It is a very light prop and for the rv4 thats a problem. I had sabre make me an 11lb spacer to help the cg.

So far I am very happy with the prop. I have flown much acro, formation, and several airshows with it. I dont think too many rv4s are using this prop and the issue i had was not enough pitch available so we re indexed the pins.

Cm
 
Chris,

Come down to visit and we'll do a side by side speed and climb comparison. I'm really curious about how that prop compares to mine, I'll buy lunch at PUB.

On the other hand, I could meet you at KBJC and you buy lunch. I only have several more years to fly in that airspace before the ADS-B---stuff kicks in.

Cheers, Hans
 
Christopher Murphy said:
This prop was made especially for faster RV types

Here is a link to their site concerning this propellor

http://www.sensenich.com/products/item/92

It is a very light prop and for the rv4 thats a problem. I had sabre make me an 11lb spacer to help the cg.

So far I am very happy with the prop. I have flown much acro, formation, and several airshows with it. I dont think too many rv4s are using this prop and the issue i had was not enough pitch available so we re indexed the pins.

Cm
Click to expand...

Thanks for the link; that's very good news to see a Sensenich adjustable composite prop for RVs. I can't imagine why the guy I spoke with didn't tell me about this prop, because I specifically asked him about composite blades for RVs.

Charlie
 
Here is what I observed

I ran the AVC race. First leg 208mph second leg 204mph. Ended up flying at 9500/7500 msl at 2650 rpm. Not full power. I found I could get about 2850 rpm at 2000ft msl with the prop pitched the way I had it for higher altitude.

I got second place by .11mph...

Cm
 
You must log in or register to reply here.