Naca scoop evaluation: expert opinions desired!
 

After three flights in my RV-7, I am still having issues with the oil temperature. It just keeps climbing slowly. I can only fly for about 15 minutes before it rises to the point where I have to land. We double checked the indicated temps with a infrared temp probe after landing and they match pretty well. We will be checking the vernatherm in boiling water, among other items, but for the third flight we taped some yarn to the cowl to see if the scoop for the cooler was getting enough air.

My question for those familiar with how a Naca scoop works is: does it look like it is working as it should? At lower airspeeds it looks as if the third bit of yarn aft of the front of the opening, underneath it, is being sucked into the vent, at higher speeds only the front two seem to be. The first part of the video is taxi and takeoff and climbout at about 100-110 KIAS, the middle portion is at about 140-145 KIAS (the wheel pants and gear fairings are not on yet), the last part is approach and landing, over the fence at around 70-75 KIAS.

The engine is a brand new Aerosport IO-375 with 9.5-1 pistons, the prop is an MTV-9 3 blade, I have a Christen inverted oil system installed, the oil cooler is a 9 row NDM. The power setting during the cruise portion was 27" MP at 2500 RPM. Lower settings don't help the cooling, until the power is back for descent and landing the temp continues to rise slowly. Cylinder head temps are kept under 400 during climb, at level cruise they are running between 360 and 380.

Not my best landing - of course being the one I put up for public viewing -but, hey, it's only my third in the airplane, and at least it was on the CL! ;)

thanks,
Damon

http://www.youtube.com/watch?v=SSAOjvzXnus

First a few questions:
1. What temp's are you seeing?

2. Why are you using the NACA scoop rather than picking the air off the back of #4?

3. What are your CHT's doing?

That is a VERY large NACA scoop, and assuming you made it correctly (proper dimensions, sharp edges, etc.) it should put a LOT of air into your cowl. However, you can only get air in, if you can get air out. The exit opening is sized for the two inlets and you just added another big air inlet. It is possible that you are not getting air through the cowling even if the NACA inlet is made correctly.

NACA vents will flow ALOT of air if there is no restriction on the discharge side. Your cockpit vents fed by a NACA duct will seem like it is passing alot of forced air through it, and it is.... Take that same set up and try to push air through a oil cooler with it's restrictive fins and the system will not flow any where enough air to carry away the oil heat.... BTDT....

NACA scoops are poor for feeding thick heat exchangers. Rake angle is critical as well to get the best possible pressure recovery. Also, you have placed them in a low pressure part of the cowling according to CFD plots. Finally, you need high delta P from cooler inlet to outlet so as mentioned above, the air needs to get out somehow or you'll have low mass flow through the cooler.

What Ross said - NACA ducts really have to be shaped exactly to work well. To get air into the cowling really needs a scoop that sticks out into the airflow - the NACA shape doesn't work well without a lot of work on the internal ducting. An external scoop looks ugly and will take a while to mould. I would suggest you take the oil cooler air from the back of the baffle - you will get plenty of flow, no ugly holes in the cowl, and no discernible influence on the CHTs.

Pete

You need some data. It will tell you what you need to know, or provide that here and other can provide some better assistance.

What cooler,
what delta P across the cooler, with airspeed of course.
delta t across the cooler for the air and oil.

This will tell you if the airflow is low, and narrow the field to the cause. Look at some of DanH's posts for help. An electronic manometer with piccolo tubes will be very helpful for gathering the data. Omega has some good handheld thermocouple meters, or you could retask some of your other instrumentation for a while to avoid the costs. Shop well and you can find the thermocouple meter for under $80 with two inputs, and handle K or J metals. This is less work that you have already invested, don't give up without data.

Enjoy your educational adventure. Cool (no pun) design. Keep us posted.

PS. Look up Alan Judy posts, and he has many pictures posted on a yahoo group too. Maybe some tips for your externally fed cooler. He has a VERY slick 6.

It will be educational to gather data through actual measurement but in the end, it is already well known (read NACAs original conclusions here) that NACA ducts don't work well to feed heat exchangers. Use plenum air or a ram duct to feed your cooler and you'll instantly get better results. Even with positive rake, a perfect shape and location in a high pressure zone, the NACA duct is quite inferior in this application to a proper ram duct.

I have pretty much have the same set up on my 7A. IO-360, standard compression and using the 7 row cooler Vans sells. The system did not work until I routed the exit air out the bottom of the cowl. I think this even creates a suction which helps pull air into the NACA.

Naca
 

My friend has a Glasair 2 with that set up it works well. He never sees more than 190 degree oil.

Thanks Chris, I was thinking of doing just that. It's good to have some validation of that idea, thanks! The reason we tried the Naca scoop in the first place was it was done before quite successfully on an -8 with a Lycon 10-1 compression, using the seven row cooler here in FL. He had to block off part of his cooler in the winter, the installation worked so well. Perhaps the airflow around the -7 cowling is different enough so the scoop is not as effective.

I am going to try adding the tube to exit the air, if that doesn't work I will get a bigger cooler and probably put it in the regular spot and get rid of the scoop, it may add more drag than I want anyway. I haven't been able to test my top speed yet because I can't fly it long enough before the oil gets too hot and I land!

I will report back what I find out!

Damon