The performance improvement was very small.  There was a small increase
in manifold pressure over the updraft without filter.  I can make 19.5"
MAP at 12.5K feet.  At best, my TAS increased 5 knots.  Quite an expense
and effort for so little of a performance gain.

Tom,

 

You got me thinking about whether or not you are finished with the "experimental" mods, yet.  Then I looked at some material Chris Zavatson had provided - a chart showing a comparison between his old airbox and the improvements gained from his new ram air charted against altitude derived static pressure.

 

I went up to 12,500 and at WOT, 2500 RPM, level flight at 163 IAS (193 TAS), I saw a 20.2" MAP against a 18.66" standard day altitude derived static pressure.  BUT, it wasn't a standard day since the baro setting was 30.55 and the OAT was -8C (17F) at altitude.  This yielded a palt of 11960 and a dalt of 11550 (about 19.4") - maybe. 

 

Hmmmmm....  How can we compare our ram systems?  The "Aerodynamics of Naval Aviators" pub says you can only make comparisons at the same density altitude.  But, there are a few problems with a few things.  To wit:

 

1. The method for computing the true static pressure is troublesome since it is affected by temperature, humidity, the mean temperature at the various altitudes below you, baro setting, etc.  The temperature is also affected by compressibility since we are going faster than 150 Kts.  Of course, converting this all to "standard day" readings.  Note, however, that there is a rule of thumb that says for every 10 degrees temperature difference from standard there is a 4% change in altitude.  I didn't get the TAT (True Air Temp), but it was probably a few degrees cooler, say -11C, which would then only be a difference of -1C from the standard temp for 12,500 MSL.

 

2. Remember that the pure rise in ram pressure is approximately (in " HG):

100 KIAS - .5"

150 KIAS - 1.1"

200 KIAS - 2"

After all, that is how our pitot/static airspeed indicator works.  So, at my KIAS of 163, I could only expect a max of 1.2" rise.  What is it exactly? Who knows?

 

3. The measurement of MAP:  Remember that MAP is Manifold Absolute Pressure - actually, a measurement of vacuum at anything less than WOT.  Everybody's MAP instrument may be a bit different - kinda like body temperature.  With the engine off, my readout is usually about 29.6 at my airport elevation of 700 MSL (standard day should be about 29.2).  Of course, on the test day I didn't look at that readout on my VM EPI 800 system. My buddy's VM 1000 said 28.6.  So, one must also know the MAP instrument offset.

 

4. Assuming all things being equal (like don't drain off anymore air), the pressure was about .6" Hg (about 8" H2O) higher than static in the upper cowling cooling plenum.  Hmmmmm.... Next time I will re-plumb my pitot driven injector air system and see what it says. 

 

Where does that lead us?  See you at the next Air Venture Cup Race.  Now that's the way to compare things.

 

Consider this:

 

RAM-Air STC Now Available For Grumman AA-5 Traveler And AA-5A Cheetah

Alt Air Without The Filter

Approach Aviation Development Thursday said it has received FAA STC SA00206BO for an Alternate Air system for all Grumman AA-5 Traveler and AA-A Cheetahs.

The new “RAM-Air” Alternate Air System is designed to provide an alternate source of induction air to the engine without the restriction of the standard induction air filter. This system allows the pilot to bypass the induction air filter in-flight without the reduction in available engine power caused by resorting to carburetor heat. A cable control in the cockpit opens/closes a valve into the airbox that supplies unfiltered, high-pressure air from the available air in front of the forward engine baffling.

The high-pressure location of the inlet provides a source of "Ram-Air" which has been tested to increase manifold pressure by approximately 1/2+" Hg. This equates to 4-5 HP more horsepower at altitudes where normally aspirated aircraft engines typically produce 75% power or less. The system also increases the service ceiling and effectively reduces the aircraft’s pressure altitude by about 750 feet MSL.

"The “RAM-Air” Alternate Air System is an outstanding and economical performance addition to the Grumman Traveler and Cheetah." said Jeff Simon, President of Approach Aviation Development, Inc. "Aircraft induction filters are always a compromise of engine protection vs. performance. This system preserves the integrity of the filter on the ground, while allowing the pilot to take advantage of the increased power naturally available in flight.”

The “RAM-Air” Alternate Air System, complete with all necessary hardware, is available at the introductory price of $750. Installation is very straightforward with a typical install time of approximately 4-6 hours.

FMI: www.ApproachAviation.com

 

From Aero-news.net

 

Scott Krueger AKA Grayhawk
N92EX IO320 Aurora, IL (KARR)

Some Assembly Required
Using Common Hand Tools

And the Air We Breathe.