R

RV8R999

Well Known Member
A recent thread about air vents prompted this question:

Anyone else notice the air coming out of the pilot vent being VERY warm at lower speeds (below 130)? At first I thought this might be due to cowl leakage along the sides. I used some 200mph tape to completely seal the seam between cowl and fuse with no change. Maybe the cowl itself is conducting enough heat to cause higher temps through the vent? I do not have thermal barriers installed in my cowl - maybe this will help?

At higher speeds the air is much closer to ambient (perceived anyway).

Thoughts?
 
Yup - I don't know how the speed dependency really works Ken, but this is why the fuselage NACA scoop is such a bad place for OAT probes - completely bogus.

One explanation for the speed dependency is that we pretty much only slow down in the pattern, and it's bloody hot down low in the pattern - especially this year! ;)

Paul
 
RV8R999 said:
A recent thread about air vents prompted this question:

Anyone else notice the air coming out of the pilot vent being VERY warm at lower speeds (below 130)? At first I thought this might be due to cowl leakage along the sides. I used some 200mph tape to completely seal the seam between cowl and fuse with no change. Maybe the cowl itself is conducting enough heat to cause higher temps through the vent? I do not have thermal barriers installed in my cowl - maybe this will help?

At higher speeds the air is much closer to ambient (perceived anyway).

Thoughts?
Click to expand...
Pretty much the reason I modified the aft fuselage to fly with the canopy open in the pattern!
 
Paul - I was thinking the same thing, it was more of an altitude-ambient air temp issue than speed so I went out this AM and flew through entire speed range at 8K with OAT at 70F. Noticeable temp difference at A/S below 130 with a very steep gradient around the 135-125 range. Of course I was using my highly un-calibrated temp probe (my left hand) so my data could be out-to-lunch. Despite questionable gradient data it is clear the air is being heated above ambient as I was able to reach back and feel the air through the wing vent which is discernibly cooler at all A/S.

No sun out today so have to rule that out. Hard to believe air flow around the cowl could xfer so much heat, unless because the NACA trips the slow moving boundary layer this air has plenty of time to xfer the energy vice the faster moving flow just outside the layer???

Maybe an air dam upstream from the NACA Far enough ahead to allow the NACA to still function but to mix the hotter boundary layer with the cooler slip stream air? Granted the air behind the prop is a mess but I'll try it.
 
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vortex generator

Decided a vortex generator upstream the NACA vent might work.

quick fab with some scrap alum and double sided tape:
35avzlx.jpg


then installed forward of the inlet. Wanted to position such that at higher AOA (lower speeds generally) the VG would be at about 30deg to the flow. Trouble is this is in the prop wash which will be difficult to estimate therefore I'll just guess and see what happens.

I'll fly later today if the the TS clear up and see how it goes...
2vir0c0.jpg


not sure why the photos are upside down sorry :(
 
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test flew this configuration this morning and qualitatively speaking the air through the vent was markedly cooler at low airspeeds. Still warmer than the wing vent but the air wasn't "hot" as it normally is OAT was 85F. I may move the VG farther forward on the next run.

next step is to measure PVT (Pilot Vent Temp) and compare to OAT both with and without the VG through entire speed envelope at constant ALT.

I might add some cowl insulation along the left side too.
 
Sometimes at high angles of attack such as flaring to land I get exhaust fumes in the cabin. My brother suggested putting a fence under the duct because the air flowes from under the plane up over the top. He said he has experianced this in other planes he has flown and that has fixed the hot air coming in. Our Sport Cruiser has a scoop on the NACA duct that works pretty good as well.
 
cowl inlet spillage?

I doubt very much you are getting conductive heating of the flow by flowing over the cowl. Much more likely that you are getting warm air spilling out of the cowl inlet. There is probably more spillage at high speed than low, but the trajectory of the flow would change, and it may be that the stream of spilled hot air just happens to hit the vent at 130 kts.

A vortex generator is going to encourage mixing in the flow, so whatever the source of heat, it will likely be diluted with cooler air in the mixing process.
 
Intlet/Vent Geometry?

maybe but the inlets are above the vent which is on the left side of the A/C with the upward moving prop blade. I'd think at higher AOA any inlet spillage would flow above the vent exacerbated by clockwise prop rotation (view from rear).

Exhaust and/or Cowl exit air would really have to get wrapped up tight to make the flow angle work but not saying it isn't possible.

whatever the source, the VG APPEARS to improve cooling through mixing as was the intent - I think moving the VG forward may provide better results as the vortex should be entrain a greater volume of cooler air. We shall see.

Thanks for your input Steve!
 
More VG tests

Moved the VG about 15deg and 15" below and forward of the vent. Much improved during power off descent but worthless in cruise as the air temp was significantly heated at all A/S in this configuration.

Move the VG in line the with Vent and about 15" forward resulted in remarkable improvement at all airspeeds above 110 and qualitatively was the same temp as the wing inlet at speeds above 130. Low power descents and high power slow climbs still allow significant heated air to pass through the vent.

I suspect an 1 VG will not work in all flight regimes (go figure). I don't really like how VG's look on airplanes so I'll probably tweak this location a bit more by moving it more forward and will settle with just 1.

There is definitely a relationship between inlet air temp/airspeed/RPM

Need to find my digital thermometer for recording real data...
 
Try a strip of 60 grit sand paper

ran vertically to trip the boundary layer.

Mount a video camera or have a buddy fly beside you and attach some yarn 24" or so in front and below so we can all watch the flow.
 
not sure about the 80 grit as I do not believe the flow would be organized enough to entrain the cooler air outside the BL.. The idea is to develop an organized vortex which entrains cooler air flowing into the inlet - hence the VG.

The tuft test is a great idea however and easily doable. I can mount my CONTOUR HD on the left wing and video the flow patterns.
 
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