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-   -   Converting a -9(a) to -7(a)? (https://vansairforce.net/community/showthread.php?t=80931)

longranger 01-06-2012 04:20 PM

Quote:

Originally Posted by N941WR (Post 614293)
What numbers are you using?...

I like to plan on a hour reserve, even VFR. 3.5 hours and bit of headwind and you're down to 600nm range. Yeah, you can power back to increase endurance, but regardless of how you fly it, or wherever you set your reserves, the 42 gallons in a -7 will get you farther than 36 in a -9. That made the difference for me, but I'm sure it wouldn't for everyone.

Vlad 01-06-2012 04:30 PM

Quote:

Originally Posted by sbalmos (Post 614190)




Soooo... I just faxed in the order for the -9a tail. N112SB is now officially on its way. As Gene Wilder says in Young Frankenstein... What have I done? My God man, WHAT... HAVE... I... DONE??? :D

Yes, I'll be fine. My wallet won't. Let the mayhem begin.



WHAT? 2 Sierra Bravo cancel your order IMMEDIATELY! You don't even know what you got yourself into :D 9A is big and slow get the 8 forget the 7.

diamond 01-06-2012 04:39 PM

Quote:

Originally Posted by longranger (Post 614320)
I like to plan on a hour reserve, even VFR. 3.5 hours and bit of headwind and you're down to 600nm range. Yeah, you can power back to increase endurance, but regardless of how you fly it, or wherever you set your reserves, the 42 gallons in a -7 will get you farther than 36 in a -9. That made the difference for me, but I'm sure it wouldn't for everyone.

It may very well make the difference for me, because one of the cross country trips that I plan on making frequently is 920 statute miles. At 55% power in a -9 with Van's numbers, that doesn't happen without a stop. With a -7, it might work with good conditions. You can argue that it's too long a trip without a break, but I might like to have the option to make it in one hop if ever I want. So for me I weigh the advantages of the -7s longer range with the advantages of the -9s slower stall speed and flight characteristics. I'm not interested in acro, so that doesn't factor in.

hydroguy2 01-06-2012 05:26 PM

Let's see a show of hands...how many experienced RV pilots fly there plane 5 hours non-stop and land with legal reserves?

longest time in the seat was 3.5hrs is enough for me, 650miles, still had 11gals on board. I've only flown mine 6.5hrs in one day, OSH-8U8, 975nm with headwinds and one stop. From a safety standpoint, I would think a trip that length would be a major mental & physical drain for most pilots.

I built a 7, but could have easily been happy with a 9. My daughter is starting flight lessons so a 9a would have been perfect. Also, the only time I run 75% or above is racing or testing. other times I'm at 6-7gph puttin' around the valley.

skelrad 01-06-2012 05:30 PM

Quote:

Originally Posted by diamond (Post 614327)
It may very well make the difference for me, because one of the cross country trips that I plan on making frequently is 920 statute miles. At 55% power in a -9 with Van's numbers, that doesn't happen without a stop. With a -7, it might work with good conditions. You can argue that it's too long a trip without a break, but I might like to have the option to make it in one hop if ever I want. So for me I weigh the advantages of the -7s longer range with the advantages of the -9s slower stall speed and flight characteristics. I'm not interested in acro, so that doesn't factor in.

If speed/range is your main factor, and there is no other reason for you to go with the 7 over the 9, just put in extended range tanks. There are some great options that are easy to install that will give you an extra 10 gallons or so total. That doesn't seem like much, but it's an extra 1+ hours of flight, which means another 180 miles or so. Line up a 7 and a 9, and if the 7 has to stop for fuel but the 9 doesn't, all of the sudden the "slow" 9 has a much faster trip speed.

diamond 01-06-2012 05:40 PM

Quote:

Originally Posted by skelrad (Post 614342)
If speed/range is your main factor, and there is no other reason for you to go with the 7 over the 9, just put in extended range tanks. There are some great options that are easy to install that will give you an extra 10 gallons or so total. That doesn't seem like much, but it's an extra 1+ hours of flight, which means another 180 miles or so. Line up a 7 and a 9, and if the 7 has to stop for fuel but the 9 doesn't, all of the sudden the "slow" 9 has a much faster trip speed.

I considered the extended tank option until I had a discussion about it with Vans personnel at OSH last year. They convinced me that this option carries risks that I'm not willing to take; not the least of which is untested spin characteristics.

RickWoodall 01-06-2012 06:23 PM

Choice is yours.
 
I went through the same decision process 4 years ago. Didnt even have my ppl when for the first year or two of building. The slower land, better glide and arguably better cross country platform sold me. I went 9a, have loved every single hour would not change a thing. I like all rvs, but a side by side is way better with a child or wife in my opinino. A tip up is just awesome for views in flight and you just will be amazed at how easy and fun flying in one is. Build what you want, enjoy the stuffin out of it.

After any rv flight a cessna trip is just painful. :D Enjoy the build.

RV7ator 01-06-2012 08:16 PM

Lightin' The Fuse
 
Why oh why do some 9 apologists try to make the 7 equal to the 9 IF.... This usually is manifested in speeds. The two airframes are different and Van's design intents are different.

No way does a 9 equal a 7 in top end.

The 7 carries more fuel, has greater range - which can often be useful for tankering for price/availability. Cheek/bladder endurance are not the only metric.

The 7 airframe is stronger. Aerobatic abilities aside, Van's limits 9 baggage to 75 pounds, 100 in the 7. That's a very useful difference.

The 9 can fly slower than the 7. It's an efficient climber with a tad higher spec'd service ceiling (take a deeper breath before crossing 14,500). The handling is more docile.

They are different. A lot of the number differences are not great percentage-wise, but they exist.

You pays your money and takes your choice. Yet they're both delightful in their own way.

John Siebold

alpinelakespilot2000 01-06-2012 10:40 PM

Quote:

Originally Posted by RV7ator (Post 614434)
Van's limits 9 baggage to 75 pounds, 100 in the 7. That's a very useful difference.

Not correct. The build manual confirms 100# for the -9. (See p. 14-2 "Maximum Baggage Weight") The structure is built for up to 100#. What you placard it at is dependent on weight and balance numbers. I used some weight on the crush plate to be able to use the full 100#. According to Bruce Reynolds, the 75# Van's originally had advertised was due to the low weight of the very smallest engine.

Finley Atherton 01-07-2012 03:25 PM

Of course I am not trying to stir the pot but going on the best figures I can find it appears that a 9A may cruise slightly faster than a similarly equipped 7A.

Vans figures compare the 7A with a Hartzell C/S prop and the 9A with a FP prop.

CAFE figures for the 160 hp factory 9A with a MT C/S prop show about 192 mph TAS at 8,500 ft DA with the engine at 2,605 rpm and 23.7".

Vans gives 189/190 mph TAS for a 160 hp 7A at 8,000 ft and 75%. So maybe the 9A is faster at 75%??? Also factor in that the MT is considered slower than the Hartzell so the 9A with a Hartzell may be even faster again. This is borne out by my 9A with a Hartzell which does 194 mph TAS at 8,500 DA WOT and 2,700 rpm (4 leg GPS spread sheet method).

Much is made of the fact that the 7A with 180 hp can cruise faster than a 9A with 160 hp.
Using the CAFE figures above for the 160 hp 9A and Vans figures for the 180 hp 7A at 75% the difference is about 5 or 6 mph (or about 3 or 4 mph using my figures). So lets be generous and say maybe 5 mph faster for almost an extra gal/hr. :eek:

Fin
9A


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