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

gbrasch 01-06-2012 11:30 AM

Quote:

Originally Posted by sbalmos (Post 614190)
the sheer speed increase of the -7a.

Using Van's numbers on 160 HP models at 75% power/8000', the -9 does 189 mph, the -7 192 mph. So unless your pulling a -7 with a 200 HP, there is not much difference, especially since many people are using the 180 HP motor in their -9's.

RVbySDI 01-06-2012 12:35 PM

Quote:

Originally Posted by gbrasch (Post 614217)
Using Van's numbers on 160 HP models at 75% power/8000', the -9 does 189 mph, the -7 192 mph. So unless your pulling a -7 with a 200 HP, there is not much difference, especially since many people are using the 180 HP motor in their -9's.

Agree with this. The difference in speed is really marginal, at least in the practical sense of things. My 9 has the IO-340 that is rated for 180 HP (same as the 360). Perhaps the 200 HP equipped 7's may outrun me but if I choose to run the throttle wide open and forget about the fuel burn My 9A can run just as fast as any other standard configured 7 out there.

Given that though, the great majority of my flying is cruising LOP with the throttle at 70-75% HP. At this setting I see anywhere from 150-165 MPH and burn around 7.0-7.5 GPH. I really think a 7 comparably configured will not perform a whole lot differently.

Ralph Inkster 01-06-2012 12:56 PM

Quote:

Since the -7 is, essentially, a -9's fuselage with -8s wings, has anyone ever considered or done a conversion of a -9 to a -7 by swapping out the wings? Just from a theoretical design perspective, what else is there to change? Mods to the tail, the fuselage side skins and/or bulkheads where the wings are mounted? etc

Food for thought?
Although not impossible, it would be very impracticle to attempt a 'conversion' 9 to 7. There are multiple differences in wing & emp attach points that would amount to 30%(?) of a fuse kit panels that would have to be replaced.

sbalmos 01-06-2012 01:08 PM

Yup. So, ultimately, the airspeed numbers are even closer than I remembered and forgot to check on. In reality, I'm giving up aerobatic abilities by going with the -9a. Big deal. ;) Gross simplification probably, but I'll take the -9a's better glide and slow-speed characteristics over aerobatic abilities.

Thanks again everyone! Now for the long wait until the tail kit arrives at my door.

diamond 01-06-2012 01:38 PM

I like the larger tanks on the -7 vs -9. I know you can put extra wing tanks on the -9, but after a stern warning not to do so from a reputable Vans person, I would never do that. If long-range cross country was a major part of my mission, that would be an important factor for me.

RVbySDI 01-06-2012 01:44 PM

Quote:

Originally Posted by diamond (Post 614255)
I like the larger tanks on the -7 vs -9. I know you can put extra wing tanks on the -9, but after a stern warning not to do so from a reputable Vans person, I would never do that. If long-range cross country was a major part of my mission, that would be an important factor for me.

I agree that it would be nice to have the extra 8 gallons that the 7 tanks provides. If it were up to me I would go for extending the fuel tanks. However, on every trip to date that I have flown with my wife she is not interested in flying for more than 3 hours without landing. At my fuel burn that gives me a 1.5+ hour reserve.

N941WR 01-06-2012 01:54 PM

Quote:

Originally Posted by sbalmos (Post 614190)
... and the sheer speed increase of the -7a...

I guess that one has already been answered.

Quote:

Originally Posted by diamond (Post 614255)
I like the larger tanks on the -7 vs -9...

This is not even an issue because they are sized for the IO-360 with 200 HP. Since you are most likely to put in a 160 HP O-320, 36 gallons is more than you will ever need.

It sounds like you have figured out what your mission will be, that's a good thing. The -9 is a great flying plane and doesn't give up much to the -7. Best of luck with your project.

longranger 01-06-2012 02:04 PM

Quote:

Originally Posted by N941WR (Post 614264)
..This is not even an issue because they are sized for the IO-360 with 200 HP. Since you are most likely to put in a 160 HP O-320, 36 gallons is more than you will ever need.

But that same O-320 on a -7 (or an (I)O-360 at 55%;)) will get you another 45-minutes in the air (6 gallons at 8 gph). Even using 75% of 180hp on both airplanes, it's 30 minutes or more. Yes, if you're limited to 3-hour VFR legs, it won't make much difference, but if your mission is 600nm or more, or IFR, that extra 6 gallons might save you a stop, assuming you have the "personal range" for it. I'm not a big acro guy (yet?), so fuel capacity put me over the edge toward the -7.

MauiLvrs 01-06-2012 02:21 PM

Quote:

Originally Posted by N941WR (Post 614264)
a 160 HP O-320, 36 gallons is more than you will ever need.

Simply not true... The 36 gal in our 9 doesn't come close to what we need to get to Hawaii :eek: ... But the extra gas in the 7 doesn't either :D

N941WR 01-06-2012 03:06 PM

Quote:

Originally Posted by longranger (Post 614267)
But that same O-320 on a -7 (or an (I)O-360 at 55%;)) will get you another 45-minutes in the air (6 gallons at 8 gph). Even using 75% of 180hp on both airplanes, it's 30 minutes or more. Yes, if you're limited to 3-hour VFR legs, it won't make much difference, but if your mission is 600nm or more, or IFR, that extra 6 gallons might save you a stop, assuming you have the "personal range" for it. I'm not a big acro guy (yet?), so fuel capacity put me over the edge toward the -7.

What numbers are you using?
36 gallons divided by 8 GPH gives you 4.5 hour range. Call it 4 hours with a half hour VFR reserve. If you get that low, you can power back even more and stretch that half an hour even longer.

Using Van's numbers you can pull back to 55% power and still fly at over 140 knots for 730 Nautical miles. That is plenty long enough. If you power up to 75%, then the range is 600 Nautical miles (710 statute miles) until dry tanks.

Truth is, very few people fly their RV?s at 75% power, most throttle back smile at the lower fuel burn.

Either way, everyone will have some reason to justify their choice and they are all valid reasons for the decision maker to drop some major coin on the aircraft of THEIR choice.

If you are still undecided in your 7 vs 9 choice, ask Van?s to hold your order until you can arrange a ride in both. I?m not going to tell you the -9 or the -7 is better because it is all about YOUR mission. They are both fine aircraft.


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