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-9A vs -7A economy

Vans performance figures for the 7A, 8A and 14A are with a Hartzell CS prop whereas the figures for the 9A are with a unknown brand fixed pitch prop.

A farer comparison would be the CAFE measured performance of Vans 160 hp 9A with the MT 3 blade CS prop. At 8,500 ft density altitude (75% power) CAFE (page 3) measured 192.7 mph for the 9A which is 3.7 mph faster than the 160 hp 7A (at 75% and gross weight) and only 0.3 mph slower than the 160 hp 8A (at 75% and gross weight).

If I recall correctly the MT prop is slower than the Hartzell so put a Hartzell on a 9A and it may even be faster than an 8A with the same hp :cool:

Fin 9A
 
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Jesse said:
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.
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Jesse, I got the wheel pants, leg fairings and intersection fairings from Vans. I used Vans nose wheel upper intersection fairing but threw the mains upper and lower intersection fairings in the bin and bought those African made intersection fairings from Aircraft Spruce (cannot remember the manufacture).

I fitted a AntiSplat "Nose Job" to the front leg and fitted the AntiSplat leg fairing that came with the Nose Job. The Vans upper intersection fairing fitted nicely with AntiSplat's larger leg fairing.

Bob
 
I plan to fit the nose fairings tomorrow and do the test again at 6,500 then take it up to 8,500, 10,500 and 12,500 to test speeds. Then we will pick an altitude for the prop tests later on this week if Smokey can make it up.
 
Finley Atherton said:
Vans performance figures for the 7A, 8A and 14A are with a Hartzell CS prop whereas the figures for the 9A are with a unknown brand fixed pitch prop.

A farer comparison would be the CAFE measured performance of Vans 160 hp 9A with the MT 3 blade CS prop. At 8,500 ft density altitude (75% power) ...
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Well, now you're starting to cherry pick data to suit the results you're after...

For starters, Van's quotes at 8000' (DA and close to ISA, presumably) so your data already has an uncorrected 500' advantage...

Power is power and fixed-pitch propellers are not generally noted for their propulsion efficiency deficit compared with CS props, so I'd argue that unless Van's data is rubbish (I would tend not to argue this point...), it's more comparable than random data from everywhere else on the internet.

That's not to say you couldn't optimize the airframe and propellor to achieve a better result than published data.
 
Andy_RR said:
Well, now you're starting to cherry pick data to suit the results you're after...

For starters, Van's quotes at 8000' (DA and close to ISA, presumably) so your data already has an uncorrected 500' advantage...

Power is power and fixed-pitch propellers are not generally noted for their propulsion efficiency deficit compared with CS props, so I'd argue that unless Van's data is rubbish (I would tend not to argue this point...), it's more comparable than random data from everywhere else on the internet.

That's not to say you couldn't optimize the airframe and propellor to achieve a better result than published data.
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Propellers can make a difference.
Ok,knock a couple of mph off the 9A speeds for the 500 ft difference but then again maybe Vans is using 8,000 ft and WOT in which case more % hp is being produced than at the 8,500 ft used by CAFE?? Whatever the case the real point I wish to make is that the 9A is no slouch and has at least similar speeds to the 7A with the same HP and propeller.

I am not trying to cherry pick data rather I am attempting to "compare apples to apples" and I hardly think that data from the CAFE Foundation can be written off as "random data from everywhere else on the internet".

Fin
9A
 
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I got my pants all on today and test flew. Here is my data after doing the speed runs at different altitudes and power settings. I didn't have density altitude info, but I think it was about 1,600 ft or so on the surface, which is 65 MSL. These numbers are all at pitch setting 4 on the Sensenich prop with a 150HP O-320 running 100LL.

6500
20"
2400
6gph
139KTAS

8500
WOT 21.6"
2640
7.3gph
153KTAS

8500
20"
2500
6gph
143KTAS

10500
WOT 20"
2600
6.5gph
152KTAS

10500
20"
2500
5.5gph (more aggressive leaning and high egt's)
142KTAS

12500
WOT 18.5"
2600
5.8gph
149KTAS (that's 25.7 nm/gal, a big WOW for this -10 driver)

The high egt's bring up another issue. In the -10, I don't think I can get egt's over 1420 or so at 20". These egt's were reading in the high 1400's. anybody want to weigh in on whether this could be an instrument reading high perhaps (GRT EIS 4000). If so, could the CHT's also be reading high if it's an instrument calibration issue? I don't know much of the history of the EIS except it was in a Zenith 801 own floats in the Bahamas for 3 years. May have no bearing on the instrument, but maybe it could.
 
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PerfTech said:
...Well Fin, Looks like you put the brakes on all the chatter once and for all. At least on this thread. Thanks, Allan...:D
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Allan,

Maybe I can reignite the debate :)

The typical 7A has 180hp and the 9A 160hp so the accepted wisdom is that the 7A has much higher possible cruise speed. The 0.5 g/h difference for the same cruise speed as stated by Vans equates to 7 to 8 extra hp required by the 7A to match the cruise speed of the 9A (using the formula hp = FF in g/h * 14.9 when LOP). So the 180hp 7A ends up with only a 12 to 13 hp advantage for the cruise. Admittedly a guess but I would think that this would be unlikely to transtate to any more that about 5kts?? extra and with a significantly higher FF.

Put 180hp on the 9A and you would almost have to start feeling sorry for the 7A in comparision :rolleyes::rolleyes:

Fin
Unbiased 9A owner!
 
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Finley Atherton said:
Allan,

Maybe I can reignite the debate :)

The typical 7A has 180hp and the 9A 160hp so the accepted wisdom is that the 7A has much higher possible cruise speed. The 0.5 g/h difference for the same cruise speed as stated by Vans equates to 7 to 8 extra hp required by the 7A to match the cruise speed of the 9A (using the formula hp = FF in g/h * 14.9 when LOP). So the 180hp 7A ends up with only a 12 to 13 hp advantage for the cruise. Admittedly a guess but I would think that this would be unlikely to transtate to any more that about 5kts?? extra and with a significantly higher FF.

Put 180hp on the 9A and you would almost have to start feeling sorry for the 7A in comparision :rolleyes::rolleyes:

Fin
Unbiased 9A owner!
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You need to take a good look at the drag polars for both airframes before you can reach a conclusion like this without misleading yourself...

A
 
Apologies, my analysis in post #59 was not realistic as I was assuming 100% power. If we are emphasising economy then a more realistic power setting would be say 65% - about the maximum recommended for LOP cruise.

At 65% the 160hp 9A pumps out 104hp and the 180hp 7A, 117hp - a 13hp difference. The 7A needs to use somewhere around an extra 0.5 g/h (Vans figures) or 7 to 8hp to match the cruise speed of the 9A leaving only about 5 to 6hp extra available which is not going to translate to a lot more knots.

Fin
9A
 
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Are we disputing Vans own numbers from his website?

160 hp 9A 8000 75% 187 mph
160 hp 7A 8000 75% 190 mph

Same for top speed with a 5 mph split favoring the 7A.

Since I am trained in it, I get all the aerodynamic arguments, but as I mentioned much earlier in this thread, I dispute whether the craft speed performances are different enough to measure. The differences appear to be within the variation that occurs in builder quality.
 
rzbill said:
Are we disputing Vans own numbers from his website?

160 hp 9A 8000 75% 187 mph
160 hp 7A 8000 75% 190 mph

Same for top speed with a 5 mph split favoring the 7A.
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I am using Van's data referenced in Post #57.
As stated earlier in this thread the website numbers are for an unknown brand FP prop for the 9A and a Hartzell CS prop for the 7A. Van's 9A now has a CS prop and I presume this partially accounts for the better 9A performance than stated on the website. The comprehensive CAFE test of Vans 9A also supports the better performance figures.

Fin
9A
 
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To throw in another perspective...

Van's data for the RV-6A, using a 160HP engine + fixed pitch prop lists a maximum speed of 200mph at 8000' while the 160HP RV-90A has a corresponding speed of 195mph. A difference here is that the -9A has the new style "pressure recovery" wheel pants, which are known to be more efficient than the older "skinny" wheel pants that shipped standard with the -6A for much of it's existence.

RV6A Performance
RV9A Performance

What do you make of that?
 
The nearly 400Lb difference would account for a few MPH I reckon. The 6A weighed 985Lbs and did 200MPH, the 9A weighed 1,350Lbs and did 195. This is also reflected in the initial climb rates, and you could also chalk up a couple of MPH to prop differences too.

There's really only one set of numbers that really bother me though - those that I get from my -9A. I'm not going to lose any sleep wondering if I can fly more efficiently in a -7 when I've built a -9A. :D
 
Given that the difference between "solo" weight and gross weight results in only 1mph difference in top speed, I don't think weight makes all that much difference. A gross weight -6A still beats the -9A at solo weight by 4mph, according to Van's numbers, and the -6A has draggier wheel pants.
 
ChiefPilot said:
A difference here is that the -9A has the new style "pressure recovery" wheel pants, which are known to be more efficient than the older "skinny" wheel pants that shipped standard with the -6A for much of it's existence.
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I've read here on VAF a number of times that changing the wheelpants out on a -6 is good for 5mph in cruise. A number of people have supplied data from gps-backed speed runs that seem to verify it as well.

Haven't dropped the money yet on the parts, but I plan on replacing my older pants with the newer ones. I'll have to try and do some speed runs myself to compare the before and after.
 
KRviator said:
The nearly 400Lb difference would account for a few MPH I reckon. The 6A weighed 985Lbs and did 200MPH, the 9A weighed 1,350Lbs and did 195.
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There's a typo there somewhere on the website. The *empty* weight of the -6A is listed at 985 lb too, and that's a lot more realistic. I suspect the -6A was flown somewhere in the 1300-1350 lb. range similar to the -9A.
 
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