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Phoenix 7 - Too Heavy?

Old 09-29-2020, 09:40 AM
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misterv47
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Default Phoenix 7 - Too Heavy?

Hi,

I recently completed a build of a Horizon Hobby Phoenix 7. I live in Denver, CO at 5400 feet, so that's where most of my problems with this airplane originate.

It has Robart 510 series air retracts, and an OS MAX 75 AX with JettStream muffler for power.

The problem is its weight. Right now, it is 8.75 lbs dry. That is 0.25 lbs more than what is suggested by Horizon as the highest weight in the manual.

I need to know: is 8.75 lbs too heavy for this airplane?

This plane has flown once, earlier in the summer. At that time, it had a Rossi 60 with a muffled tuned pipe. On its first and only flight it seemed very under-powered, it barely lumbered into the air after a long take-off run that used up most of the concrete runway we have. I flew it around and trimmed it out, making gentle turns as it struggled to stay airborne.

Re-engining with the current OS 75 AX came after this one and only flight, after I realized how under-powered this airplane was. I have broken in the OS, but have not yet flown the Phoenix again, I've been dragging my feet because I'm afraid that 8.75 lbs dry is just too heavy.

Is it too heavy? I COULD pull out the retracts and reduce the dry weight by a little under 12 ounces. But I'd like to keep the retracts if I could make 8.75 lbs work..

Any advice appreciated.
Old 09-30-2020, 06:05 AM
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big_G
 
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My Curare clone tips the scale at 9 lbs. I have a Rossi 61re on a pipe and an 11-7.5 prop., turning 15,800 on the ground. It flies VERY fast and has almost unlimited vertical. Something else is not right on your plane . It's not a weight issue. What props have you tried and what rpm's are you achieving?

Last edited by big_G; 09-30-2020 at 06:07 AM.
Old 09-30-2020, 08:50 AM
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misterv47
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Default Engine Performance Info

Big_g,

Thanks so much for your reply. It is beyond helpful to know that my weight is in the same realm as your Tipo.

I agree with you that power, or lack thereof, may be the problem. Here at Denver altitude there is about a 15% reduction in IC engine performance across the board.

I did some driveway testing to understand where I'm at with a few of my engines:

Jett 46 w/JettStream muffler:
Pitch: 7
RPM: 14600
Static thrust: 6.5 lbs
Computed Horsepower: 1.7

Rossi 60 w/Novarossi Tuned Pipe #50400:
Pitch: 8
RPM: 13000
Static thrust: 7.25 lbs
Computed Horsepower: 1.9

OS 75 AX w/JettStream muffler
Pitch: 8
RPM: 13450
Static thrust: 8.25 lbs
Computed Horsepower: 2.3

Notice that even with the piped Rossi 60 I am nowhere close to the RPM's that you have. The "computed horsepower" is based on a web site I found, and takes pitch, RPM, and static thrust as inputs.

I find it very odd that Jett 46 produces so much more power (as evidenced by thrust/RPM) for its displacement than the Rossi 60.

Anyway, my plan is to shave about 4 ounces off the current weight and try to get to 8.5 pounds or so, then I think I'll try to fly it again.

Thanks so much for you response, and please let me know if you have any other ideas.
Old 09-30-2020, 08:55 AM
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Ooops that was supposed to read "Curare".. sorry for the typo.
Old 10-01-2020, 10:04 AM
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Something is wrong with the Rossi. Even in thin air, the rpm's should be closer to 15,000 with a 11-7.5 prop...a little less with an 11-8. That is why the thrust is so low.

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