Capflyer
Well Known Member
I have embarked on a project that I hope will yield some lost speed. I read through just about every post related to cooling and cooling drag issues along with extensive web searches including links others have posted here.
A little history:
Currently there is about 260 hrs on my fuel injected, dual EI ignition plane from the last 20 months. Initially I had terrible but typical RV7A cooling issues. Last Spring I retrofitted a SJ plenum along with completely rebuilding the front of my stock Van's cowling for the 5.25" round inlets hoping this would help with lowering my CHT's. It ended up lowering my CHT's an average of 15deg. After Oshkosh I replaced my stock oil cooler with a SW which lowered my oil temps 40 deg. All the while I had tinkered with all the other pre-requisite fixes like double checking for leaks and sealing them up, cleaning up any debris between the cylinder fins, etc. Toward the end of last Summer I added louvers to the bottom cowl. Overall I saw my climb and cruise CHT's drop but still running in the 375 deg range in cruise. While helping a friend install an engine monitoring system in his Tiger I noticed that on his lower baffles wrapped around the bottom of the cylinders the sides were sealed with RTV to help with the Tiger's cooling problem. What the heck, I had tried everything else so why not this. Spent a good 10 minutes and a $5 tube of high temp Copper RTV and sealed the sides of mine. CHT's dropped an average of 25 deg!
Fast forward a few months. It's cold and I do not fly any XC's, that is until April when I flew to GA and SC. I found that in cruise flight my IAS and TAS were lower than expected and compared to XC's from last year. My compression check in December for my condition inspection yielded 78's and 79's, fuel and oil consumption all normal, etc. The plane is now polished and painted so a much smoother surface than last year.
Earlier this week I did some speed tests and compared them to ones done last year. The ones from last year were prior to the SJ retrofit and without the intersection fairings on the plane and the ones from this week were done with the louvers covered up.
Tests were done at 8K DA at Peak EGT and WOT and 2600 rpm, speed is in Kts.
Last Year: 149 IAS / 168 TAS / 22.4 MAP
This Year: 143 IAS / 160 TAS / 23.1 MAP
Interestingly, my manifold pressure increased and still yielded slower speed.
The only thing I can now attribute it to is excessive cooling drag. Here are some measurements:
My buddies stock RV8 has a 1:1.1 ratio inlet to outlet size
Mine with louvers closed off has a 1:1.4 inlet to outlet ratio, actual measurements are inlet 35.4"sq and outlet (exhaust pipes deducted) 51.7"sq.
For those not familiar with the nose gear jungle located at the firewall, between the engine mount, nose gear brace and mount, and bottom cowling brace, there is a lot of items internally that I believe create turbulence from the exiting air flow. This combined with tests others have done showing the airflow actually going back into the exit area bring me to my non-engineering degree conclusion that this could be another part of the problem.
I am going to try blocking off some of the exit area opening around the brace between the exhaust pipes and see if both reducing the exit area more along with cleaning up the exiting air will help.
All suggestions, comments, ideas, etc on this will be graciously accepted. My goal is to get something accomplished before flying to Oshkosh in a few weeks.
Here is a picture of the initial stages. Using cardboard to make templates for sheet aluminum to cover up the jungle and close off the center exit area.
A little history:
Currently there is about 260 hrs on my fuel injected, dual EI ignition plane from the last 20 months. Initially I had terrible but typical RV7A cooling issues. Last Spring I retrofitted a SJ plenum along with completely rebuilding the front of my stock Van's cowling for the 5.25" round inlets hoping this would help with lowering my CHT's. It ended up lowering my CHT's an average of 15deg. After Oshkosh I replaced my stock oil cooler with a SW which lowered my oil temps 40 deg. All the while I had tinkered with all the other pre-requisite fixes like double checking for leaks and sealing them up, cleaning up any debris between the cylinder fins, etc. Toward the end of last Summer I added louvers to the bottom cowl. Overall I saw my climb and cruise CHT's drop but still running in the 375 deg range in cruise. While helping a friend install an engine monitoring system in his Tiger I noticed that on his lower baffles wrapped around the bottom of the cylinders the sides were sealed with RTV to help with the Tiger's cooling problem. What the heck, I had tried everything else so why not this. Spent a good 10 minutes and a $5 tube of high temp Copper RTV and sealed the sides of mine. CHT's dropped an average of 25 deg!
Fast forward a few months. It's cold and I do not fly any XC's, that is until April when I flew to GA and SC. I found that in cruise flight my IAS and TAS were lower than expected and compared to XC's from last year. My compression check in December for my condition inspection yielded 78's and 79's, fuel and oil consumption all normal, etc. The plane is now polished and painted so a much smoother surface than last year.
Earlier this week I did some speed tests and compared them to ones done last year. The ones from last year were prior to the SJ retrofit and without the intersection fairings on the plane and the ones from this week were done with the louvers covered up.
Tests were done at 8K DA at Peak EGT and WOT and 2600 rpm, speed is in Kts.
Last Year: 149 IAS / 168 TAS / 22.4 MAP
This Year: 143 IAS / 160 TAS / 23.1 MAP
Interestingly, my manifold pressure increased and still yielded slower speed.
The only thing I can now attribute it to is excessive cooling drag. Here are some measurements:
My buddies stock RV8 has a 1:1.1 ratio inlet to outlet size
Mine with louvers closed off has a 1:1.4 inlet to outlet ratio, actual measurements are inlet 35.4"sq and outlet (exhaust pipes deducted) 51.7"sq.
For those not familiar with the nose gear jungle located at the firewall, between the engine mount, nose gear brace and mount, and bottom cowling brace, there is a lot of items internally that I believe create turbulence from the exiting air flow. This combined with tests others have done showing the airflow actually going back into the exit area bring me to my non-engineering degree conclusion that this could be another part of the problem.
I am going to try blocking off some of the exit area opening around the brace between the exhaust pipes and see if both reducing the exit area more along with cleaning up the exiting air will help.
All suggestions, comments, ideas, etc on this will be graciously accepted. My goal is to get something accomplished before flying to Oshkosh in a few weeks.
Here is a picture of the initial stages. Using cardboard to make templates for sheet aluminum to cover up the jungle and close off the center exit area.
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