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-9A vs -7A economy

mchargmg

mchargmg

Well Known Member
Hello all,

A friend of mine and I are looking at purchasing a flying RV (both still work full time thus not so interested in building). We have been looking at, and "discussing" the merits of the 7a vs 9a. I like the 9a, he likes the 7a. I think the problem is a mismatch in missions. My wife and I really like traveling together and I want something that will cruise from place A to B. My buddy wants to keep open the acro. possibility. I admit that would be fun, but think it is more than outweighed by the fact we are low time pilots, and the 9a appears to be more friendly to pilots like us.

The other day he said to me. "Is the 9a really more economical?" He was bringing forth the argument I have heard before. For the same speed, a larger engine is about the same as a smaller engine in fuel usage since it is not working as hard.

I pointed out to him the wings are different, and opined I thought the 9a was probably still more efficient at the same speed than a 7a. I freely admit I did not have any real data to support that claim.

So, have you ever traveled in your 9 with a 7 at the same speed over the same route and compared notes at the end of the flight? If so, what are the results, and what engines were involved?

Thanks in advance for any thoughts. This is a truly great site for new folks like us!

cheers

mgm
 
The Rosie gang that just flew to the Caribbean in a large group of RVs would have lots of data points for such a comparison.

I can tell you that I traveled last year from Portland, OR to OSH with an RV-8. I ran WOT and 2300 RPM leaned to 25-50 ROP, he throttled back to what ever it took to stay together. At every fuel stop, we were within 1-2 gal of each other, well within the range of leaning techniques. I have an IO-320, he has an 0-360, both have Hartzell props

The 7 & 9 airframes are so similiar and I'm sure the drag differences between the two wing airfoils is insignificant compared to the overall drag of the airframes. It takes X amount of energy to overcome the drag at any particular speed. Drag the same, the energy (fuel burn) is the same.

The MAIN difference between the 7 & 9 is control stick feel. The 7 is much lighter than the 9, which makes the 7 more sensitive. Both are a joy to fly, but their handling is different.

The two of you need to find someone in your "area" to give you demo rides in each, then decide. Use the "Whitepages" here at VAF to find someone closest to you.
 
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rv9av8tr said:
The Rosie gang that just flew to the Caribbean in a large group of RVs would have lots of data points for such a comparison.

I can tell you that I traveled last year from Portland, OR to OSH with an RV-8. I ran WOT and 2300 RPM leaned to 25-50 ROP, he throttled back to what ever it took to stay together. At every fuel stop, we were within 1-2 gal of each other, well within the range of leaning techniques. I have an IO-320, he has an 0-360, both have Hartzell props

The 7 & 9 airframes are so similiar and I'm sure the drag differences between the two wing airfoils is insignificant compared to the overall drag of the airframes. It takes X amount of energy to overcome the drag at any particular speed. Drag the same, the energy (fuel burn) is the same.

The MAIN difference between the 7 & 9 is control stick feel. The 7 is much lighter than the 9, which makes the 7 more sensitive. Both are a joy to fly, but their handling is different.

The two of you need to find someone in your "area" to give you demo rides in each, then decide. Use the "Whitepages" here at VAF to find someone closest to you.
Click to expand...

Quick off topic, what kind of CHTs do you see running 25-50 ROP?
 
Purely personally

The 7 is not a hard airplane to fly and running lean of peak (which I always do) it makes about 160knots on 7gph of 100LL.. About the same on ethanol free autofuel).. this is at altitude.

The IO360 is no less economical than the IO320.. the Fuel injection and running LOP are the keys to great mpg.. the size of the engine is irrelavent.

The 9 is stiffer and stall speed is lower. For me that is far outweighed by the lightening quick (relatively) response of the 7.. the Stall speed is still only about 55kts with full flaps. which means it will land short, especially with a CS prop.

Drop the hammer on the 360 with a CS and waahoo..:)

Frank
 
And

You really need an autopilot to fly a 7 IFR (read precise flying) but when you do you set it and forget and try not to fall asleep..:)

We were flying 1100 miles Corvallis to Fort Collins Colorado twice a year at one point... didn't even feel tired when we got there..:)
 
I was a bit nervous when I finished my 7A last summer that I may be in over my head with only 150 total hours under my belt. I took 5 hours of transition training and haven't looked back.

I really enjoy the responsiveness of the controls offered by the 7.
 
Yup

Transition training is the key and don't take liberties with it.. I.e keep an eye on the ASI at low altitude and it won't bite.

Delightful airplane.. So why am I selling.. OK don't get me going down that path..:)

Frank
 
mchargmg said:
"Is the 9a really more economical?"
Click to expand...
We're sticking with ... YEP :p and besides the 9 is an awesome cross-country airplane:D We have no problem maintaining altitude and direction without the auto pilot. Trim it up and it just kinda goes where you point it.

And the slower stall speed significantly reduces the energy in an off-field landing environment (it is the 1/2mV^2 thing...).

rv9av8tr said:
...I'm sure the drag differences between the two wing airfoils is insignificant compared to the overall drag of the airframes.
Click to expand...
Not sure I believe that. The wing area is about 250 sq.ft. top and bottom. The 9 wing was designed to specifically be low drag and high lift. Mr. Roncz did an amazing job designing that wing.

Here is what Vans says:
"The RV-9/9A’s efficient airframe does not need a lot of power to perform well. Low overall drag means a smaller engine can pull it along at relatively high top speeds, or, more practically, at useful cruise speeds using little fuel."
​
But what do they know...
 
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The point I was trying to make is that at the same airspeed, fuel burn between the 7, 8, 9 are virtually identical, therefore the overall drag of of these airplanes must be the same. The only difference is the 9 airfoil, so the drag of longer laminar flow 9 wing must have very similar drag of the shorter 7/8 wings.
I don't know the specific aerodynamic coefficients of of these wings, but by simple deduction, equal energy means equal drag.
 
You said it

Sounds like you and your friend have different missions.

All partnerships require, to some extent, cast iron stomachs. Will you be happy knowing that your low time friend is doing aerobatics in "your" airplane?
 
just difficult to beat the 9

rv9av8tr said:
I don't know the specific aerodynamic coefficients of of these wings, but by simple deduction, equal energy means equal drag.
Click to expand...
Don't think it works that way ... remember the 9 wing is "special".
It's just difficult to beat the 9 as an efficient cross-country airplane for 2.
On a good day we can get about 30mpg.

BobTurner said:
Sounds like you and your friend have different missions.
All partnerships require, to some extent, cast iron stomachs. ...
Click to expand...
And ones with different missions probably need Stainless Steel...
 
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I've got a good amount of time in a 6a and a 9a. My friend that flew my 6a for the first time thought his Rv 9a was quick and sporty, till that day. I still think the 9 is way nice but you gotta get what you want- even if that means your acro fix is only a couple aileron rolls :)

Any Rv handles so well you'll have no problem transitioning I don't think the low time excuse is going to go too far.

Efficiency difference ? Meh my buddy with the 9a and I have the same 160 cs and our numbers seem very similar. The bigger difference is the man working the throttle

Yes you would want an AP on the 6 or 7 but who really doesn't want one on a 9
 
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Aspect ratio

I think everyone knows the 9 has a higher aspect ratio wing than the 7.
All else being same, aerodynamic theory says higher aspect ratio will reduce induced drag.

However, induced drag is a miniscule portion of total drag at cruise speed.
The majority at cruise is "skin friction" or "parasite" or "flat plate" (whatever you want to call it). Since the fuselages are essentially the same, and the wing areas are very close, this drag should be nearly identical.

As said earlier, the measurable economy differences at same speeds are likely to be overshadowed by leaning technique.

There is debate about large engine economy vs small engine at same power settings. In the normal engine size ranges for these two craft, I am a believer that again the actual differences are small and can be overshadowed by operation technique. (Therefore the anecdotal arguments)
 
I am flying off the 40 hrs after first flight. I started off with speed and was using about 8 gal/hr moving down the pike. Then Dah! why, all I need is time off the airframe. So I throttled back, still flying about 130, using 4.55 gal/ hr. Not bad I think, RV7A. If I had the choice, I think the 7 is more versatile, and maybe more in demand, that said I am not sure that is a true statement. But I think more 7/A kits have been sold than 9. However, either plane are great!
Dave
 
Depends

All depends on mission. For me, as a lower time pilot the lower landing speed of the 9a and the suggested better performance of the 9 cross country, was the selling point.

Now 300 plus hours in, having flown coast to coast and north to south...I would not change a thing. If aero is your thing, the 7 is it. If cross country like I do, i dont think you can beat the 9a, it is AWESOME. Pretty much a perfect airplane and i get very close to 7 g/hr at full cruise. Throttle back and I am showing 5-7 gal/hr depending on what i do. Thats pretty good numbers for me.

Either plane will be fine, and I love my 9 and would not trade for a 7 even with a cash incentive or a free set of steak knives.:D
 
RickWoodall said:
All depends on mission. For me, as a lower time pilot the lower landing speed of the 9a and the suggested better performance of the 9 cross country, was the selling point.

Now 300 plus hours in, having flown coast to coast and north to south...I would not change a thing. If aero is your thing, the 7 is it. If cross country like I do, i dont think you can beat the 9a, it is AWESOME. Pretty much a perfect airplane and i get very close to 7 g/hr at full cruise. Throttle back and I am showing 5-7 gal/hr depending on what i do. Thats pretty good numbers for me.

Either plane will be fine, and I love my 9 and would not trade for a 7 even with a cash incentive or a free set of steak knives.:D
Click to expand...


I couldn't say better boy! :D
 
Why not compromise and build a 14A (or buy the first one that comes up for sale), then you have cross country comfort and light acro with a power plant that you can manage for fuel burn.
Just a thought. Out of the box of course.


Pat Garboden
Katy, TX
RV9A N942PT
Inspected, certified but not flown yet.
Dues paid 2013
 
My 6A 180 HP, Hartzell C/S............flown by a friend, who flies his 9A (160 HP Hartzell C/S) very economically, was able to get the same fuel economy out of mine. Mine could fly faster & climb faster. His could land slower.
 
I don't have data, but I know that the 9 has slightly better economy than the 7. I don't think it is enough to really influence your decision, though. I think the 9's economy is more evident the higher you cruise, though.

If one of you wants to do acro, I don't think you have any other choice than the 7. You will be happy with either, your partner will only be happy with a 7, so go for the 7.

One of the big benefits of the 9 that doesn't get discussed much is the low stall speed, which I like just due to the safety factor involved in the unlikely event of an off field landing. I still think you need a 7 for your partnership to work though.

Tim
 
If the -14 had been available when I started building, I would have gone that route. Seems to me to be the best of both worlds. Need economy cruise, just throttle back, turn down the RPM and LOP.
 
Well

I fly an 8A and the guy I fly with most has a 9A. Fuel burn is pretty close, although leaning technique has a lot to do with that. On trips, the 6 extra gallons of fuel are pretty nice, he always has to stop before I do. The 50lbs extra useful load is handy too. Not taking anything away from the 9 series, they are fine airplanes. Interestingly enough, my friend is building a 14 now.

Randy
8A
 
NEVER NEVER say NEVER. Particularly if you are low time. When I was low time, my wife asked me if I would ever do aerobatics. I said, "No way, that looks dangerous and scary". Then I purchased a Yak-52 and learnt rolls and loops but could take or leave aerobatics in general. I subsequently sold the Yak and purchased a non-aerobatic plane and it was then that I realised I NEEDED aerobatics. I spent four years with the wrong plane before buying my RV-7. Now I do aerobatics every flight and flying cross country is mind numbingly boring.

My recommendation is to listen to your mate. A -7 is a great cross country machine but a -9 is not a great aerobatic machine. Why would you limit your options for the same money. Remember, you will not be low time pilots forever. When you get bored flying XC (and you will), you will have the option to expand your envelope and become a better/safer pilot.
 
I built my -7 for the extra speed. It is a fun plane local or XC. But to be fair I think I could have been happy with a 9 also.

Since your partnership is going to be a compromise(always), you should both pencil out exactly what you want and how much you are willing to spend. Mission and Budget, then start looking for possible candidates for purchase. you never know what might be out there and find an excellent 7A which is an acceptable XC ride for yourself....or maybe a really sweet 9A that your partner can live with. keep your options open.

and yes you can throttle back to the same fuel burn/speed....but will you? When traveling, I don't.
 
hydroguy2 said:
and yes you can throttle back to the same fuel burn/speed....but will you? When traveling, I don't.
Click to expand...

Same here. I just can't make myself do it on trips.

I can while buzzing around local boring holes in the sky but not on trips.

Climb to altitude and firewall the sucker!
 
All,

Thank you much for the information. Looks like the consensus is the economy between the 7a and 9a are similar. Details vary by pilot and exact plane. Lots of interesting opinions about the relative merits of the two planes, most of which have been brought up in discussions between my partner and I.

One thing not mentioned is the purchase price of flying versions of the two planes. I have been doing a survey, and I find the 7a's go for something like 15k more than the 9a's. Not always, but something like this. Choices, choices.... As always picking a plane is hard, and if you are picking for two it is hard squared!

Thanks again for all the help.

Geoff
 
We were picking for two.....

.... she wanted a 7a so she could do aero's and I wanted a 9 to see Australia.
We compromised, we built a 9a.

Because we were looking at this being our retirement deal we wanted to get about economically. So we built it light and fitted an 0-235. When we are cruising at altitude we cruise to the flow meter. Sitting the flow meter on 20 litres pr hour (5 gph) gives us a cruise speed of 135-140 kts TAS (2400 rpm) and if we really want to stretch the economy, by pulling it back to 110 kts TAS we can see flow figures of 15 lph (3 gph). And this is all at 60% ROP.

Bob
.
 
Because the OP is located in Colorado I would suggest they only consider the larger engine option on the 9, for its better climb performance.
 
I am running a 180 HP IO-340 with a 72x68 Catto 3-blade prop on my 9A. I can just about climb with any fixed pitch 7 out there and even some constant speed 7's as well. As has been stated earlier in this thread, my cruise speed and efficiency is so close to the 7 that it is not worth discussion. Outside of aerobatics, the one advantage I do see in the 7 over the 9 is the size of the fuel tank. The 9 has 36 gallons while the 7 has 42. I would love to have those extra 6 gallons. Not for the increase range per say, as my wife is not happy flying much over 3 hours per leg. No, having the extra 6 gallons gives a little bit more insurance on cross country trips. I flight plan for 8.5 gph on cross country trips. 36/8.5 is still 4.23 hours. During cross country trips we have never burned the fuel down lower than 10 gallons remaining but the truth is those 10 gallons are divided into two tanks. so less than 5 gallons in any given tank is going to get you thinking about it. The additional 3 gallons in each tank that the extra 6 gallon capacity provides changes the perspective of the trip. Plus, on the occasions where you must press on, you can know you have that capability without getting nervous about it.

I have flown up to 4 hours when flying solo but pressing things to that point did indeed make me nervous. When I filled up after that flight I had 2 gallons in the left tank and a bit more than that in the right. 2 gallons @ 8.5 gph . . . ummm . . . that is about 15 minutes. That is cutting it closer than I really feel comfortable with since part of the equation involves managing the fuel, not just burning it as you would if it were coming from a single tank. If the total amount were in one tank I would be assured that I could fly an uneventful 30 minutes more. By having to switch tanks and maintain fuel porting in the active tank the 30 minutes would surely not be uneventful.

I have flown in the 6, 7 8, 12 and my 9A. You cannot go wrong with any of them. I love my 9 and really am not at all interested in changing, unless it were perhaps to a 14. But even that would require some compromising as the price delta would be substantial between it and my 9A. The truth is you will be happy with whatever RV you decide on. Others have commented about the relationship that will exist in a partnership and that will be of much more consequence than which RV you choose. Make the decision to meet as much of both your needs as you can. It will not be the type of plane that will dictate the success of your decision as much as it will be the quality of your relationship within the partnership between the owners that will matter the most.

Good luck on whatever you decide.

Live Long and Prosper!
 
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I know! Who asked me?

....I have considerable experience in all the RVs and our group is about 15 strong, mostly 6 & 7s, 3 9s and my 9-A. My 9-A with a 320 and Whirlwind C/S will run right with them in most all cases and will out climb them with ease. It will land slower, shorter and take off shorter as well. The really big difference is what happens when you go up high, and that is why the 9 is so great as a cross country machine. The 6 & 7s do very good up to about 8500 feet then the 9 wing really starts to shine. At 9000 up there is no contest. At 12,500 & 13,500 where I fly my usual cross country the 9 is king. It runs off and hides from the others on far less fuel. I have documented this for my own edification several times. I use my plane to go places, flying some 550 hr. in the last 24 months. If you want to do aerobatics the 9s aren't the plane, to travel they are. :D
 
RetiredRacer said:
.... she wanted a 7a so she could do aero's and I wanted a 9 to see Australia.
We compromised, we built a 9a.
Click to expand...
Ouch... She really got the shaft on that deal. You could easily see Australia in a 7A, but she can't do any aerobatics in a 9! :) How is that a compromise? :)
 
Shshshshshsh.......

... She must have thought it was a fair compromise ;) . I think she just wanted a nose wheel so she went along with my offer :D .
 
RetiredRacer said:
... She must have thought it was a fair compromise ;) . I think she just wanted a nose wheel so she went along with my offer :D .
Click to expand...
My boss A) wanted the little wheel in the front and B) The slower stall speed and impact energy (we spend a lot of time over not so nice places to land). 7 years prior ... "we aren't going to build one of the "E" things":eek: She was willing to compromise and build, an 8 IF I wanted to sit in the back all the time, so now we have a 9A. That's how our compromise went. Flying around Australia, I could see how a 9 best fits that mission. Filled up the other day almost 2 hours ... 8 gal.
 
RetiredRacer said:
Because we were looking at this being our retirement deal we wanted to get about economically. So we built it light and fitted an 0-235. When we are cruising at altitude we cruise to the flow meter. Sitting the flow meter on 20 litres pr hour (5 gph) gives us a cruise speed of 135-140 kts TAS (2400 rpm) and if we really want to stretch the economy, by pulling it back to 110 kts TAS we can see flow figures of 15 lph (3 gph). And this is all at 60% ROP.

Bob
.
Click to expand...
Can you give altitude and power setting info on this? I am new to the RV-9A world and am seeing 135ktas at 6,500 burning 6, and I don't have the nose gear fairing or wheel pant on yet, and I have a few other draggy things going on, so up higher, I am hoping to do the same numbers as you. I also have the new Sensenich ground adjustable carbon fiber prop set for cruise (20" at 6500 ft gives me 2400rpm, which is where I fly my -10). I have yet to do any speed testing above 6500.
 
The 6 & 7s do very good up to about 8500 feet then the 9 wing really starts to shine. At 9000 up there is no contest. At 12,500 & 13,500 where I fly my usual cross country the 9 is king. It runs off and hides from the others on far less fuel. I have documented this for my own edification several times

Ok. You got any numbers there partner
 
Jesse, Altitude usually is between 6 1/2 & 9 1/2, we don't go over 9 1/2. I don't know the power setting as I work by the flow meter for 20 lph (5 gph). And that usually gives us around 2400 RPM.

We have a Rotec TBI and duel P-Mags on our 0-235 and I believe these items contribute towards our economy.

You will find a major difference when you fit the pants and fairings. After fitting the pants and fairings to our 9a we are now doing around 20 kts faster for the same fuel flow.

Bob
 
We gained 15 knots with the mains faired. Hopefully another 5 with the nose (is that reasonable?). We have a 150 HP O-320. I hope we can end up with the economy your are getting. We have a fair bit of dirty still going on, so tweaking hopefully will give us another 5 knots beyond all the wheel pants. 145ktas at 6 should mean that we can get 135-140ktas at 5. That's insane economy by any standard, especially since the -10 is about as good as I have seen in a 4-place with an average of 15nm/g.
 
I fitted the nose pant and fairings first and picked up 10 kts. then we picked up the other 10 kts when I fitted the mains.
 
Ok, now you've gone and gotten my hopes up for more speed yet. That's just not nice...unless I get it. Are upper and lower intersection fairings standard for the mains on the -9A? We have both. They are stock on the -10, but I'm new to the -9 world and don't know what is standard and what isn't.
 
Vlad said:
I couldn't say better boy! :D
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The Russian knows not what he speaks!

;)

Hahahaha

We were just in my hangar tonight talking about this thread and Vlad and I agreed that you build the plane you want. Figuring that out can be hard!

I can tell you that it feels WEIRD coming over the numbers at 60 in a 9!

I like the responsiveness of the 7. I dont care about a higher landing speed. I am flying a 6 now and it is very close to the 7's I have flown.

Enjoy the build!

;). CJ
 
Captain_John said:
The Russian knows not what he speaks!

;)

Hahahaha

We were just in my hangar tonight talking about this thread and Vlad and I agreed that you build the plane you want. Figuring that out can be hard!

I can tell you that it feels WEIRD coming over the numbers at 60 in a 9!

I like the responsiveness of the 7. I dont care about a higher landing speed. I am flying a 6 now and it is very close to the 7's I have flown.

Enjoy the build!

;). CJ
Click to expand...

... I agree with , "build the plane you want" but that is very difficult if you don't know what that is. I will say this, I have had to make a lot forced landings over the years, most of them off field. When I am flying I am always scanning for the place I will go if need be. It is very reassuring to know that I can put the 9-A down on a football field and stop before I hit the goalpost. Apparently I am becoming more and more respectful of the possibilities or, it's age induced paranoia. I am convinced the 9 is the airplane if you are not planing aerobatics and want the most economy and speed for your $$$$. This is after all just my opinion, but, Everyone is Entitled to My Opinion.:D:D
 
speed, economy, and short field capability...

speed, economy, and short field capability...

I had the same reasons for going with the -9. I'm glad those that are flying confirm the same.
 
VansSpeedData_zps054d9176.png
[/URL]

Plotting the speed data against absolute engine power (read - fuel consumption!) for each model from Van's website doesn't look too flattering for either the RV-9, nor the RV-14.

Of course, the RV-8 comes out best! I must finish building one, one of these days... :rolleyes:
 
Perplexed

Andy - I'm not sure I'm reading your graph right. It looks like it's claiming 210 mph on 150 hp for an RV-8. I can't say that I've ever heard of an RV hitting those speeds on so little hp. What am I missing?
 
skelrad said:
Andy - I'm not sure I'm reading your graph right. It looks like it's claiming 210 mph on 150 hp for an RV-8. I can't say that I've ever heard of an RV hitting those speeds on so little hp. What am I missing?
Click to expand...

... Looks like I am missing something as well? Allan:confused:
 
cool chart

what is amazing on the chart is that the -9 is so close to the -7 at 120 hp in-flight but the -9 can glide. thx for the chart.
 
skelrad said:
Andy - I'm not sure I'm reading your graph right. It looks like it's claiming 210 mph on 150 hp for an RV-8. I can't say that I've ever heard of an RV hitting those speeds on so little hp. What am I missing?
Click to expand...

Well, I checked my calcs, self-doubting soul that I am, but I still think it's correct.

A 200hp RV-8A at 75% power is quoted at 210mph at 8000'

75% of 200hp is 150hp.

Whether it's realistic or not, I dunno, but it's Van's own data.
 
Steve Melton said:
what is amazing on the chart is that the -9 is so close to the -7 at 120 hp in-flight but the -9 can glide. thx for the chart.
Click to expand...

What this chart isn't showing is what happens as the aircraft slows down and how the drag/thrust curve curls back up as induced drag increases. At the cruise speeds shown here, the induced drag is small to negligible, however if you slow down to best glide, or even further to minimum sink speed, you'll see the long wings of the RV-9 shine.
 
Andy_RR said:
Well, I checked my calcs, self-doubting soul that I am, but I still think it's correct.

A 200hp RV-8A at 75% power is quoted at 210mph at 8000'

75% of 200hp is 150hp.

Whether it's realistic or not, I dunno, but it's Van's own data.
Click to expand...

Oh okay, I gotcha. Makes more sense now - must be late and time to go to bed! :rolleyes:
 
Andy_RR said:
What this chart isn't showing is what happens as the aircraft slows down and how the drag/thrust curve curls back up as induced drag increases. At the cruise speeds shown here, the induced drag is small to negligible, however if you slow down to best glide, or even further to minimum sink speed, you'll see the long wings of the RV-9 shine.
Click to expand...

.... I think if you could view the data at 12,500 alt the 9 will really shine speed and fuel burn. Allan...:confused:
 
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