R

RV8R999

Well Known Member
Issue: Air flowing through the fuselage NACA inlet, located just aft of the cowling along the left side, is significantly hotter than ambient temp whenever airspeed is below 130mph or during descents. I suspect the slow moving boundary layer air is being heated through conduction of the engine compartment via the cowling. I added a small vortex generator upstream from the NACA vent in an effort to entrain cooler air outside the boundary layer. It has been suggested heated air spilling from the engine compartment through the cowl inlets maybe the cause or possibly air flowing from the cowl and exhaust exit area from under the fuselage. This TUFT test was conducted to understand the flow path from cowl inlet to fuselage NACA inlet throughout the full airspeed envelope during level flight as well as typical climb and descent profiles.

Test and Test Conditions:
A/S: 0-175 mias
ALT: 20-2500ft PA
DA: 2000-4100ft DA
GW: 1550lbs CG 80.3"
OAT 86-72F
RPM range - 450-2700rpm
Climbs @ Vy (110 mias) max RPM, clean
Descents @ 130 mias 1650 RPM
Landing configuration: Flap 20 deg, gear down :)

Sequence
T/O Climbout @ 110 imph full power (2400 rpm)
Level off at 1000 ft PA (2900ft DA) accel to 130 mias stabilize
accel to 175 mias
climb to 2500 ft PA (4000ft DA) @ 110 mias (Vy) full power approx 1600 fpm
descend to 1000 ft @130 imph 1650 rpm approx 800 fpm
Level off at 1000 ft PA, decel to 80 mias, flaps @ 20 deg
decel to 70 mias, flaps 20 deg
descent to landing @ 80 mias, power idle from 180 to touch-down

Results: flow remains mostly longitudinally aligned with AOA. Some perturbations noted just aft of the VG. Prop wash appears to have little affect from idle to full RPM while in flight. Flow around cowl inlet appears to flow parallel with the flow further aft and above the NACA inlet suggesting this isn't contributing to heating. No flow noted from cowl or exhaust exit area.


Please see pics and youtube video.

111oyf7.jpg

x4nltt.jpg

rk7f9d.jpg


youtube link:

http://www.youtube.com/watch?v=QsdkD6fLzOE

camera was a CONTOUR HD set at 135deg FOV @720p, 30FPS frame rate.
 
Very cool!

I like how you did this! Very cool! That is what it's all about; learning and experimenting with these super-fun machines!
 
Whatever the reason may be, locating an OAT probe in or near the NACA vent contributes to higher than actual OAT readings (We have MANY customers that have this issue). Maybe it's because the sun hits it. Under the wing is probably the most suitable location for accurate OAT indication.
Nonetheless, great testing!!
 
Ken,

If your getting bored in retirement...

I got an 8 with plenty to do 15 minutes North of you!;)

Great video either way! I also saw the one from your tail-cam (very cool).

Scott
 
Scott - Funny! Nah not bored and I'm only semi-retired. Start work for Sikorsky in October as a XP.

Where are you based? I'll fly up on a weekend morning.

Ken
 
Its no surprise the airstream is well attached around the cowling and up to the NACA vent. There might be a very small amount of conductive heating occurring through the cowl, but my guess is you've got a little hot air leakage through the cowl seams that's getting into the vent.

Try taping up the seams (upper-lower cowl and cowl-firewall) and see how that affects your NACA air inlet temp... might be a contributor. Nice video work!
 
Bill - this is what I thought even before I conducted the VG and tuft test. I taped ALL the seams very thoroughly and didn't notice any difference at all - which was a big surprise and what led me to believe this is conductive heating. Of note - during the first take-off the air is much cooler than it is during subsequent take-offs or slow speed further suggesting the cowl is heating up and transferring the heat to the inlet air.
 
FYI for you Ken, I found the exact same thermal behavior in my old Grumman Yankee when I re-engined it from 115 HP to a 160 HP - the air coming in the NACA scoops was considerably warmed by the bigger engine. I always blamed it on leakage from the cowl, but never tried taping it up - sounds like your experiment indicates it wouldn't have helped.

Anyways, another data point!

Paul
 
good data point Paul - thx. The real solution is to minimize heat transfer which means insulating the left side of the cowling. Not sure alum foil is going to be enough so maybe a light thermal blanket of some sort....

any suggestions???
 
If you had a still camera with a relatively fast shutter speed looking at the cowl from about the same location, and taking a series of still photos, you might be able to capture the influence of the prop.

The prop generates a pulse of aft-wash that's typically significantly different in direction and turbulence than the average flow which the video camera captures. But its duration is much lower than that of the free-stream air passing between the blades, because the passage of a single blade only occurs for about 2% or 3% of the time. It will take a number of photographs to see it.

Alternatively, a very high speed video camera might be able to do it. But you'd still need multiple frames during a blade pass. If my quick calc is at all right, a blade pass will occur in a period of about .6 milliseconds.

Dave
 
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Hot air from rv8 Naca duct

I was having the same issue and just lived with it for the last 2 years. But......I live in Phoenix and I had just had it last week so.....I realized that my firewall mounted heater box was just dumping the heat from the heat muff in the engine compartment. The heat box was located low on the left center of the firewall and I just assumed that the heat would flow out the bottom of the cowl. WRONG.... I added a 2 in. Flange to the heat box, clamped scat tube to it and routed the scat tube real close to the lower cowl opening. Then just for fun, I used metalized tape on the vertical cowl seam at the left inside edge of the firewall sealing any air leakage between the cowl and firewall... So simple... I flew today and finally I have cool outside air to the pilot. The rear vent is still cooler, I dont know why, dont care, don't sit there, but the front is soooooo much better now. Give that a try it worked for me.

Lou Pappas RV-8. N231D
 
I did the same thing awhile ago with 1" scat running from the heater box along side my oil breather line to the cowl exit. No change to NACA temp. Glad it worked for you.
 
heat shield

In an effort to solve the high NACA vent temperature issue most RV-8's have I previously determined air was being heated as it flows along the left side cowl into the vent (see previous posts).

I ordered this stuff:

1 "HeatShield 711002 HP Heatshield Mat 0.018" Thick x 24" x 24" Heat Shield Barrier Cloth"
Automotive; $37.42
In Stock
Sold by: Amazon.com LLC

cut in half and applied to the left side lower cowl from the split joint running from front to back covering a 12" section from the cowl seam downward - which corresponds to flow path of air entering the NACA vent.

Conducted a penalty turn on the ground for 20 minutes (was also checking newly added baffle material added to the prop flange)

after 20 minutes of ground ops allowing CHTs to rise into the 330-350 range I shut down. The temperature on the outside of the cowl where the heat shield was installed was remarkably cooler (no surprise) than all other stations. Compared to the same spot on the right side I'd guess it was 50-75 deg cooler. I can't keep my hand on the right side near the #1 cyl for more than a second or two but next to #2 cyl all day long. It is barely warm to the touch.

Wx is horrible in Southern FL today so probably will not get a chance to op-check in flight but if I am right about why the air is so hot I'm confident this will really help. More to follow...
 
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