Prop Pitch question

[rv9aviator]

I have an RV-9A with a Lycoming 0-320 E3D 150 HP engine with a Ed Sterba 68-74 prop. My engine redline is 2700 RPM. I read somewhere you should pitch your prop for max RPM at 8000 Feet Density Altitude with wide open throttle. My engine only turns 2500 RPM at that altitude WOT. My airport elevation in 1360 feet and I have good climb and cruise performance. One of my pilot friends that has probably over 10,000 hours asked why I wanted to max the rpm at that altitude. He said all I would do is burn more fuel for little gain in speed. He said he would leave well enough alone. I need some more opinions and a better understanding of why! Thanks

[Greg Arehart]

Jim,

I'm no expert, but I would wonder why one would pitch for maximum RPM at any altitude. The easiest way to do that would be (I think) to have the flattest pitch prop you could find (i.e., the least drag). Perhaps you mean pitch the prop so that it makes redline (approximately) at 8000 DA? I think this would allow you to get close to optimal cruise performance out of your airplane, since 8000 is a reasonable "average" cruising altitude. My prop yields about 2750 rpm WOT at ~8000 DA and gives me great cruise performance (top speed ~200 mph, cruise speed ~165 mph @ 6.5-7.5 gph). I do lack in takeoff and climb performance compared to some RVs, but I spend more of my time in cruise than anything else so I'm happy with that compromise.

I would suggest that your prop might be a bit overpitched if you cannot get more than 2500 rpm at 8000 DA. The reason for trying for redline (in my understanding) is that the engine puts out the most horsepower there so should (?) be more efficient. Maybe someone else who knows more than me will add to this discussion or correct my errors.

cheers,
greg

[gasman]

First, purchase one of these..... http://www.aircraftspruce.com/catalo...s/onetouch.php and make sure that your tach is correct.
Then make sure that you are leaning at your 8k DA. A rich mixture will rob a couple hundred rpm.

Check manifold pressure to see what % power you are getting at 8k. Add the first 2 numbers of rpm and mp...... total should be 48 and that would be about 75% power.

Try this.... at 8k da, wot, dive the aircraft to get the rpm up to 2700. Then level off and see what the rpm settles in at level flight. If all of your compressions are good, and the timing is set right, then you either need to clean up the airframe, or re-pitch the prop.
If the prop is over pitched, you are losing some climb ability. I would rather see 2800 rpm at 8k da than 2500 rpm.

ps..... Jim, you do have all of your fairings on, don't you?

[gasman]

Jim. I just viewed this post....... http://www.vansairforce.com/communit...03&postcount=3

Please finish the aircraft before asknig any questions about performance.

You just waisted my time!!!!

[pierre smith]

Originally Posted by gasman

Jim. I just viewed this post....... http://www.vansairforce.com/communit...03&postcount=3

Please finish the aircraft before asknig any questions about performance.

You just waisted my time!!!!

Jim, wheelpants and gear leg fairings gave us another 16 MPH and quite a bit more revs.

Best,

[rv9aviator]

I didn't associate the added speed with fairings to more RPM's. Sorry I wasted your time Gasman!

[PIN 37]

You just waisted my time!!!!

Bit rich isn't it?

By the way, its wasted (spelling)

[RVbySDI]

Gasman,
Your statement seems to be a tad bit harsh. You have provided some valuable information to many people. Regardless of whether the original poster can immediately apply your advice should be irrelevant to whether you provide some benefit with your post. You have provided valuable information in a learning environment. In essence, you have taught me something by your post so be assured in my mind your time was not wasted in posting your reply.

[pierre smith]

It's been a few years since we flew the -6A after adding wheelpants, leg fairings and upper and lower intersection fairings, but 2600 RPM comes to mind as our max without all that, then 2750 and 202 MPH TAS after.

Yep, they make a big difference.

[gasman]

Originally Posted by rv9aviator

I didn't associate the added speed with fairings to more RPM's. Sorry I wasted your time Gasman!

I guess you didn't really waste my time.

I did find out that firefox spell checker can't correct for meaning. But if all posts had to be without errors then PIN 37 would have his work cut out for him and he would never have the time to build his dream 7.

Jim, you gave so much detail in your first post and then you left off the most important item. Maybe the post should have read like this........

At 8000 ft. DA I can only get 2500 rpm. Should I re-pitch the prop or install the wheel pants?

Answer......... Install the wheel pants first.

You will find that with a properly pitched FP prop and an average built airframe (the cleaner, the faster) at 8k da, the last 100 rpm will take a while to get to. The last bit of speed comes on slowly. That's why I like to fly past my target alt and then drop down to it.

Go enjoy your RV and don't worry about performance numbers till it is completely finished.