Weasel

Weasel

Well Known Member
Can someone tell me why the air coming in the standard cabin vents is hotter than ambient?

Before you speculate....take a temperature probe with you and stick it in the vent while flying and compare it against a known outside air temperature.

Is it heat of compression alone? boundary layer ingested air from the cowling? heat leaking from around the cowling hinge (mine is fairly tight but i guess it could still leak some)?
 
prop air friction and engine heat

my guess is both 1) prop air friction 2) engine heat escaping from around the cowl
 
Historically the fresh air intake from a NACA scoop from the side of an RV fuse is ~8degF warmer than ambient under full open. In a partially open state, its probably double that or more. Prop friction contributes little to this. The warm cowl, slow moving boundary layer, and some air loss from the cowl fit (usually from where hinges meet or from skybolt fasterners where there is cowl bulge between) are the causes.
 
Yep!

And this is why those with the temp probes in the inlets will have false higher True Airspeeds and want to tell you how fast they are. :)

Vic
 
When I bought my -10 the OAT probe was in the NACA duct and read higher than ambient (based on temps reported at airports and altitudes). I moved it under the wing out from the prop wash etc. I see about a 5-8 degree difference between the OAT temp now and a thermometer placed in the vent full open. In the hanger they report the same temp.

Lesson learned, don't put your OAT probe in the NACA duct.

TJ
 
So there we have it . . .

20,000 hours of experience and a definitive answer! It should be documented in the Book of RV Wisdom, by VAF.
 
What I did

Same issue on NACA vents under cowl cheeks in my -4...I used the caveman test approach, climb at full power, air gets hot when CHT is up, Idle dive to VNE, air is nice and cool. I put the fiberglass backed aluminum self adhesive insulation on inside of cowling to reduce the heat transfer along the sides, and now it is much better. As Kahuna mentioned the hinge leakage may also contribute, and I'm going to add a seal to that next time cowl if off...its getting warm soon and I got to stay cool! I have a NACA duct in my R/H lower wing skin for the rear seat, and it is always cool.
 
Kahuna

is exactly right on this, in that my observations agree with his to the letter (degree). Taping the cowl seam hinge line significantly reduces the heat gain and drops the NACA air temp about 3 degrees in my 6A, leaving it still 5 degrees above ambient. Lots of heat is transferred from the cowl to the boundary layer by conduction - even after the convection leak is mitigated.

This has significant implications not only for OAT measurement but for maximizing cockpit and people-cooling on hot days. I wish an elegantly simple solution existed for bringing in under-wing ambient air to the RV cockpit presented itself, but I'm not aware any.

Always willing to learn, though.

-Stormy
 
an old study on prop heating

Some interesting reading on prop temperatures from an old study, circa 1940. I would notion more prop heat transferred to the air per foot of fwd travel from high power setting and slow speed, such as a climb.

http://naca.central.cranfield.ac.uk/reports/1941/naca-tn-799.pdf

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prop%2Bheat%2B2.bmp


prop%2Bheat%2B3.bmp
 
probes measure total temp, not static temp. There is no way to directly measure static ambient temperature from an airplane, even the big ones measure TAT. At 170 kts, 1000 ft the total temp (TAT) will be 4 deg warmer (deg C - about 7F) than the static temp (SAT). The difference is a function of Mach and even at low Mach (0.2) where we fly it makes a difference.

http://en.wikipedia.org/wiki/Total_air_temperature
 
Kahuna said:
Historically the fresh air intake from a NACA scoop from the side of an RV fuse is ~8degF warmer than ambient under full open. In a partially open state, its probably double that or more. Prop friction contributes little to this. The warm cowl, slow moving boundary layer, and some air loss from the cowl fit (usually from where hinges meet or from skybolt fasterners where there is cowl bulge between) are the causes.
Click to expand...

I will test mine tomorrow when I go flying with vent open and close to see if the temp varies. I have two probs, one in the NACA scoop and one under the wing in the wing root area and they are often within one degree F at most. But I had not paid attention if I have been comaring them with the vent open or closed.
 
T ref

I was comparing temps at the NACA duct with a digital probe under the wing well out of the prop arc when I said my NACA scoop air temps were 8F above ambient. So I guess it was a fair "TAT for TAT" comparison, but not an absolute static air temp value.

It was still a lot warmer than it had to be.:mad:

Too bad those fuel tanks and the spar carry-through are in the way of a nice SCAT run from under the wing to the panel vents. I suppose we could pull cockpit vent air from the cowl inlet and insulate the SCAT really well. Seems like a draggy configuration that would tend to compromise engine cooling, though.

-Stormy
 
200 MPH = 7.17 F rise.

According to equation 2 in the reference Steve linked, at 200 mph, in complete stagnation at the leading edge of the wing, no heat transfer, then the temp would be increased by 7.17 Deg F.

A lot more than I would have thought. Interesting! But, with minimal stagnation, then the temp measured by the probe should be much closer to static ambient.

I have learned enough here for now, back to plugging pinholes.
 
1 mph difference

Got interested in this and looked it up on a TAS calculator. At 7500 ft, 180mph indicated and OAT of 65 deg f vs OAT of 73 deg f the difference in TAS calculated is 1 mph. Not enough to worry much about.
 
FWIW, my Rocket had NACA scoops in the front fuselage for the pilot vents and a NACA scoop under the wing for the passenger vent.

The air temps coming from the vents were typically 11 deg. F. hotter from the front vents. This was measured using a calibrated temp gauge placed in the air vent for several minutes, entirely inside the cockpit.

I added wing root vents (that looked like machine guns) and they were also about 11 deg. F hotter, apparently still being influenced by engine heat. These were outboard from the fuselage sides by an inch or so.

I gave up and concluded that the engine was heating the air enough to make the difference. I don't recall ever checking during a power off glide (because Rockets aren't made to glide), but can attest that there were many days when I wished I'd put an extra NACA vent under the other wing to help cool the pilot!

I also concluded that NACA vents in the fuselage sides aren't worth the time it takes to install them. Underwing vents are far superior. It's getting the air to the front that is a problem! Next time....
 
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vic syracuse said:
And this is why those with the temp probes in the inlets will have false higher True Airspeeds and want to tell you how fast they are. :)

Vic
Click to expand...

That is only part of the problem Vic... most OAT probes will have the majority of the probe body exposed to the cockpit environment when it is mounted in a NACA vent inlet. That means only about 10% of the probe thermal mass is exposed to the air needing to be measured. I discovered this many years ago when uninformed builders keep installing them in the NACA's. I helped some builders insulated the probe from the cockpit by putting a small paper cup over the probe and injecting it with expanding foam insulation. It always made teh temp readings much more believable. The best choice is somewhere out in the wing though, so that there is no influence from any of teh warmer temps produced by the fuselage and engine compartment.
 
Turbulater?

From the old model sailplane days, stirring the boundary layer to get Re up was pretty common. Would that not work here? My assumption is no, because there really is no uneffected air in the local region wasn't at the elevated temp- you can mix, but you're mixing a relatively homogeneous air mass. ???
 
air intake in the leading edge of the Vertical Stabilizer

I'm not sure where I have seen it but I have in the past, but how about installing a fresh air intake in the leading edge of the Vertical Stabilizer and then run the scat tubing forward to the cockpit? It might be easier to do during construction then a fix after the fact.

Tim
 
sblack said:
probes measure total temp, not static temp. There is no way to directly measure static ambient temperature from an airplane, even the big ones measure TAT. At 170 kts, 1000 ft the total temp (TAT) will be 4 deg warmer (deg C - about 7F) than the static temp (SAT). The difference is a function of Mach and even at low Mach (0.2) where we fly it makes a difference.

http://en.wikipedia.org/wiki/Total_air_temperature
Click to expand...

A little off topic but;
About 4 years ago I wanted to really accurately track speed. In the process of trying to accurately plot speed I found that it really became an obsession with accurate instrument readings. The first issue was temperature (now you know why I used the word obsession because temperature maybe adds a knot or 2 of inaccuracy) I had the probe located on the right hand side of the fuselage near the NACA cabin air scoop. The temps seem to be off by 10-15 degrees. Here is a comparison plot with a wing mount temp probe
RV-6A%2Bperformance%2Bcharts.jpg

I moved the OAT sensor back under the right hand horizontal tail like many have suggested. To prove it provided good data, I flew 6 flights (dedicated $300+ of gas; I now call that obsession) with a second OAT sensor taped to the wing. The figure below shows the test set up and a plot of the recorded OAT on the probe under the tail and points of the data from the wing. In flight I had only about 1 degree F different between the two probes and they both trended exactly the same. Orange curve is from my data recorder and green dots are points checked looking at wing probe.
Figures%2Bfor%2Barticle%2BQuest%2BArticle.jpg

Notice the 2 deg F of temp rise for 80 kts of air speed increase.
Back to original post, I think anything around the NACA scopes (at least on a Rv-6) is 15 deg F warmer then true OAT.
 
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FWIW, During my flight yesterday, I tested to compare the temp with the vent open and closed and it did not seem to make any difference in the reported temp. The temp kept track exactly as my other prob that is mounted in the wing root area under the wing so if they are wrong, they track the same error rate. These are two different probs with to different system (Garmin and GRT)
 
rvbuilder2002 said:
The best choice is somewhere out in the wing though, so that there is no influence from any of teh warmer temps produced by the fuselage and engine compartment.
Click to expand...
Do you have any knowledge of relative difference between measurements out on the wing, and at the back of the fuselage, say under the horizontal stab?

A number of builders here have used the location under the horizontal stab for their temperature probes... Handy as it puts them close to the AHARS units that people seem to like mounting just ahead of the horizontal stab in the aft fuselage. That still puts it inline with the engine and prop, but 20' further back maybe it's far enough?
 
Snowflake said:
Do you have any knowledge of relative difference between measurements out on the wing, and at the back of the fuselage, say under the horizontal stab?
Click to expand...

See post 22, second plot. The green dots are taken below the wing on the aileron bell crank access cover. The yellow curve in the temp taken on the rudder push tube access cover on the lower RH side of fuselage below the horizontal tail.
 
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