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  #1  
Old 11-24-2017, 08:37 PM
Vansconvert Vansconvert is offline
 
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Default Rates of climb, 9 vs 14 vs 7

So the 7 is the king of climb. I get that. The 9 at 160 horse and the 14 at 210 horse are within 100 feet per minute of each other per Vans published figures; stating that is with a fixed pitch on the 9, and assuming a constant speed on the 14. Why is the rate of climb that close with 50 horsepower less and a fixed pitch at that?
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  #2  
Old 11-24-2017, 09:01 PM
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grubbat grubbat is offline
 
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Default King of climb?

The -7 king of climb? Really?
I would have thought that the additional 3ft of wing would give the -9 the edge, all things considered. Guess you learn something everyday.


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Originally Posted by Vansconvert View Post
So the 7 is the king of climb. I get that. The 9 at 160 horse and the 14 at 210 horse are within 100 feet per minute of each other per Vans published figures; stating that is with a fixed pitch on the 9, and assuming a constant speed on the 14. Why is the rate of climb that close with 50 horsepower less and a fixed pitch at that?
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Old 11-24-2017, 09:05 PM
Kyle Boatright Kyle Boatright is offline
 
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Quote:
Originally Posted by Vansconvert View Post
So the 7 is the king of climb. I get that. The 9 at 160 horse and the 14 at 210 horse are within 100 feet per minute of each other per Vans published figures; stating that is with a fixed pitch on the 9, and assuming a constant speed on the 14. Why is the rate of climb that close with 50 horsepower less and a fixed pitch at that?
The two biggest factors are span loading and excess power. The -9 stacks up well on both counts.
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Old 11-24-2017, 09:20 PM
Vansconvert Vansconvert is offline
 
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2450 fpm advertised on the 7A with a 200 horse and constant speed. The 14 with 210 horse is at 1900, about the same as a 9 , fixed pitch, 160 horse. Those are the specs on Vans website.
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Old 11-24-2017, 09:24 PM
Kyle Boatright Kyle Boatright is offline
 
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Originally Posted by Vansconvert View Post
2450 fpm advertised on the 7A with a 200 horse and constant speed. The 14 with 210 horse is at 1900, about the same as a 9 , fixed pitch, 160 horse. Those are the specs on Vans website.
Also consider the weights of the three and estimate the power to weight ratio and the excess power available at Vy.
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Old 11-24-2017, 09:42 PM
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Ed_Wischmeyer Ed_Wischmeyer is offline
 
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Default

As for fixed pitch vs constant speed — you want the engine to give max power which usually means max RPM. You can get that with either a c/s or a fixed pitch. But the you need to convert that power to thrust, and there’s a zillion factors, including prop diameter. There’s also helix angle as a major factor (sorry, go look it up.)

As in so many other things, it depends.
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  #7  
Old 11-25-2017, 08:37 AM
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Hartstoc Hartstoc is offline
 
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Default Thinking apples to apples-

The 9 and the 7 share everything except the wing, so a meaningful comparison could be made with identically equipped airplanes carrying identical loads. I’m betting the 9 would decisivly outclimb the 7 in that contest, with the 9’s advantage increasing with altitude.

That said, I chose the 7 because it has so many more “hot-rodding” and maneuvering options. I love the thrill of a composite Hartzell pulling 190 HP at max ROC and I love boring odd-shaped holes in the sky, but I also have deep respect the wing John Ronce designed for the 9 and the efficiency and economy it imparts to Van’s airplane.
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Old 11-25-2017, 09:07 AM
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RVbySDI RVbySDI is offline
 
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Quote:
Originally Posted by Hartstoc View Post
The 9 and the 7 share everything except the wing, so a meaningful comparison could be made with identically equipped airplanes carrying identical loads. I’m betting the 9 would decisivly outclimb the 7 in that contest, with the 9’s advantage increasing with altitude.

That said, I chose the 7 because it has so many more “hot-rodding” and maneuvering options. I love the thrill of a composite Hartzell pulling 190 HP at max ROC and I love boring odd-shaped holes in the sky, but I also have deep respect the wing John Ronce designed for the 9 and the efficiency and economy it imparts to Van’s airplane.
Well there is the rudder, the elevator, oh yes, and the flaps. All very different too. Quite a few important differences actually.
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Old 11-25-2017, 09:26 AM
Kyle Boatright Kyle Boatright is offline
 
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Originally Posted by RVbySDI View Post
Well there is the rudder, the elevator, oh yes, and the flaps. All very different too. Quite a few important differences actually.
Not meaningful to climb rate, which is the discussion topic.
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Old 11-25-2017, 09:28 AM
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Default I believe rudder and elevator are same 9 and 7, no?

And flaps are definitely part of the wing, but not used during such climb comparisons. Minor differences in empannage would have little influence on climb in any event. Differences in power, prop, and weight are what need to be controlled to make a meaningful comparison.
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