Not Just Another 7 vs 9 Thread

[tomww]

Originally Posted by N941WR

More so when you consider that I wasn't at full throttle on that flight.

So that?s why you were soooooo slow then.

[N941WR]

Originally Posted by tomww

So that?s why you were soooooo slow then.

Yep, just cruising at the speed of a herd of turtles!

Seriously, I have a carb and not fuel injection. With the electronic ignition, P-mags in my case, I can run LoP. Down low I can't do it at WOT but at that altitude I could. When I did run it WOT, I couldn't get it to run smoothly. Bringing the throttle back a little bit smoothed it right out. I suspect that the reason was that having the butterfly valve partially closed caused the air to swirl just enough to improve the fuel distribution.

On that flight, I was trying for max duration. So running at the lower fuel burn was a big help. Typically I would have to stop mid way to top off the tanks but because of the tailwinds and high TAS from being that high, I was able to land with a bunch of fuel left. (Shortly after I took that picture, I started downhill for home.)

Here's a link to my write-up of that trip.

[Ed_Wischmeyer]

Surprised that nobody has brought this up yet...

There are several aspects to short landings: steep descent, low speed, and deceleration. The bigger wing of the -9(A) gives you slow speed, the constant speed prop gives you drag (deceleration), on the approach, in the flare, and on rollout.

On takeoff in my -9A, the % power indicator usually indicates in the high 70s. In other words, I'm taking off with maybe a tad less than 130 HP of the 160 HP that the engine is rated for.

And the Sensenich prop has a 2600 RPM redline, so at cruise, I can't go full throttle, not to mention that in west Texas thermals, I'm always fussing with the throttle.

In my book, constant speed prop is the way to go. I wish my -9A had one.

Ed

[airguy]

Originally Posted by Ed_Wischmeyer

In my book, constant speed prop is the way to go. I wish my -9A had one.

Ed

It is, indeed.

[StuBob]

This thread, combined with further research here, other sites, Van's, etc., brings me to this summary, as relates to the 3000nm AK trip. Assume two airplanes built per Van's recommendations, i.e., 160hp RV9 and 180hp RV7:

1) 100 mi range advantage 7.
2) 100 ft takeoff/landing distance advantage 9.
3) Cruise speed 10+ knots advantage 7.
4) 8 mph stall speed advantage 9.
5) The prices are almost identical, subject to prop choice.

So the 9, with 20hp less, costs the same as the 7. What does the 9 give back in return for that 20ph?

Because I'm a contrarian, I really want to prefer the 9 taildragger. But this morning's math is providing a hurdle.

[Kyle Boatright]

Originally Posted by StuBob

This thread, combined with further research here, other sites, Van's, etc., brings me to this summary, as relates to the 3000nm AK trip. Assume two airplanes built per Van's recommendations, i.e., 160hp RV9 and 180hp RV7:

1) 100 mi range advantage 7.
2) 100 ft takeoff/landing distance advantage 9.
3) Cruise speed 10+ knots advantage 7.
4) 8 mph stall speed advantage 9.
5) The prices are almost identical, subject to prop choice.

So the 9, with 20hp less, costs the same as the 7. What does the 9 give back in return for that 20ph?

Because I'm a contrarian, I really want to prefer the 9 taildragger. But this morning's math is providing a hurdle.

At middle to higher altitudes, cruise speed at the same fuel flow will begin to favor the - 9. IMO, the real differentiatior between the two is whether you'd ever like to do modest aerobatics. If you do, the -7 is your choice.

[Mark_H]

Question: At the altitudes he's going to want to fly over rough country given whatever the rocks reach up to and a likely preference for extra time should it get quiet in the cockpit, how happy is the 7 at 16500?

[terrye]

[quote=StuBob;1235310]
So the 9, with 20hp less, costs the same as the 7. What does the 9 give back in return for that 20ph?[quote]

The 9 gives back efficiency.
Aircraft 7 9
Range 75% 775 710
Range 55% 950 860
Fuel cap USG 42 36
Miles/Gal 75% 18.5 19.7
Miles/Gal 55% 22.6 23.9

I found out long ago you can torture numbers to support one position or another. Try weighting your numbers to give more importance to the performance numbers you value most. Then pick the 9.

[StuBob]

Originally Posted by terrye

I found out long ago you can torture numbers to support one position or another. Try weighting your numbers to give more importance to the performance numbers you value most. Then pick the 9.

That?s how it works, isn?t it? Faced with such a dilemma, someone once told me to flip a coin. ?While the coin is in the air, you?ll wind up hoping for one outcome over another. Take that one, regardless of what the coin does.?

[cajunwings]

Flown them both. Performance overall is pretty much the same when you average out minutes or dollars saved over several hundred hours. Acro of course means the 7. I prefer the way the 6 & 7 respond and handle when flying around for fun, but for long trips or occasional IFR the slightly less sensitive 9 handling is the ticket for me.

Don Broussard
RV9 Rebuild in Progress
57 Pacer