Newbie with questions

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Trish7
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Newbie with questions

Postby Trish7 » Sat Aug 27, 2011 3:18 pm

Hi all,

I recently purchased a 1978 Piper Tomahawk (despite Jeb's comments in another post). My Tomahawk has 125 hp (112 originally) and droop wing tips. I've been trying to find out how either or both of those changes affects things like V-speeds and other performance parameters. Anyone have any info?

Still very much enjoying the podcast and the meet ups at Oshkosh.

Trish7

Jeb
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Re: Newbie with questions

Postby Jeb » Sun Aug 28, 2011 9:57 am

Trish7:

Congrats on your purchase! I hope it provides many hours of safe enjoyment.

My main gripes with the T-hawk are that it's underpowered and exhibits inconsistent characteristics in the slow-flight regime, and in stalls and in spins. You've resolved one of those issues with the 125-hp engine. The drooped tips certainly will help with the others (which stem, in part if memory serves, from oil-canning wing skins and inconsistent stall-strip placement).

As for revised speeds in consideration of the mods, and presuming there are no numbers accompanying the mods, I'd fly it by the book speeds. Two reasons:

1. The extra HP will give you better takeoff and climb performance, while Vx and Vy won't change that much, if at all. And even if Vx/Yx are different, the difference will be only a knot or two -- well within instrument error. Too, I can't fly it that precisely. The primary consideration is the slightly higher fuel flow from the new engine. Somewhere, someone (Lycoming? The STC holder?) has charts showing the engine's fuel flow at various power settings. I'd get a copy and stick it in the POH and use it for flight planning.

2. The drooped tips will help low-speed handling and might actually delay the stall by a knot or two. The only way to find out the new stall speed would be to put the bird back through some flight testing, which would require a better, temporary pitot tube arrangement and more-accurate ASI. Not worth it (to me, anyway).

Both the more-powerful engine and the drooped tips will work to give you better performance and handling at the low-end. In effect, the mods expand the margins a bit from the stock airplane. I'd fly it by the book, happy in the knowledge you can likely beat book performance a little bit if/when you need to but comfortable knowing what the book says is pretty much what the airplane will do, technique notwithstanding. The only caveat is nailing down the fuel consumption issues. There should be a chart for that.

Hope this helps! Enjoy!

Jeb

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champguy
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Re: Newbie with questions

Postby champguy » Sun Aug 28, 2011 10:37 am

Each of the modifications should have a STC, (Suplimental Type Certificate). The documentation, and the answers you seek, will be in the paperwork with the Log Books. The original Type Certificate, if you don't already have a copy, will be available on line, and individual STCs can be found on line as well.
Having your planes paperwork well organized and understanding it will be time well spent. It is the only way to know if maintenance has been kept up, and where to look on the plane to verify that it was properly done.
I have fond memories of a delivery of a Tomahawk I made for a friend from near Las Vegas to Coastal Oregon. Over high desert, across the Sierra Nevada Mountans and past Yosemite Valley at ten thousand feet, around the Siskiyous and up the Pacific Coast in the clean clear air of the Great North West.
Do the maintenance, fly within the envelope, and you will enjoy a great little bird.
Remember, not all who wander, are lost.
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Jeb
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Re: Newbie with questions

Postby Jeb » Sun Aug 28, 2011 11:20 am

Champguy:

You're right, of course, but a lot of what Trish7 is looking for -- revised performance numbers -- simply may not exist. Back in the day, the STC process was somewhat looser than it is today. Case in point:

My Deb left the factory with a 3300-lb gross weight. Later, I installed tip tanks under an STC initially granted many years ago. Along with them came a 250-lb gross weight increase, to 3550 lbs. There is NO paperwork describing performance changes with the tanks installed, nor are there revised performance charts showing, for example, distance to clear a 50-foot obstacle at 3550 lbs, rate of climb, range/endurance, etc.

Today, if someone wanted an STC for a similar product, they'd probably have to do the testing necessary to publish revised performance charts. I'd guess it's a toss-up whether any performance charts exist for the 125-hp STC for Trish7's T-hawk.

YMMV...

Jeb

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champguy
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Re: Newbie with questions

Postby champguy » Mon Aug 29, 2011 1:21 am

In my world there were never performance figures, and too many changes, rebuilds, and yes the inevitable weight gain.
For those of us who live in a world of "Rules of Thumb" and "Trials by Error" the old standby manual "Takeoff Performance Computer" can give valuable guidance.
Simply take off, known load and conditions, and mark a constant for your plane. Then just slid the scales to adjust for surface, altitude, overload, etc. any you know what to expect. Great little device, with no batteries to run flat from disuse in the bottom of your flight bag.
http://sportys.com/PilotShop/product/9305
Remember, not all who wander, are lost.
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Soccer-Jock
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Re: Newbie with questions

Postby Soccer-Jock » Tue Sep 20, 2011 6:35 pm

Congrats on your buy. The Tomahawk is a solid machine. And in my opinion if you have done your homework and selected one in good maintenance, properly STC'd, with reasonably low frame-time you should have many years of enjoyment. With all said, they can be an exceptional buy for the price. These birds get a bad rap, yet they have so many attractive and positive advantages: Immense visibility, exceptional roominess, good fuel economy, simple operation.


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