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I mentioned in the Virtual Fly-in today that you can estimate the power loss of using warm under-cowl air by calculating the density altitude of the warmed air and turning that into power loss. Here's how that would work.

Let's assume relatively standard atmosphere conditions in this example - Sea Level, 59 deg F, pressure = 29.92", dew point = 45 deg F. In this case, the density altitude is about 150 ft. (https://www.weather.gov/epz/wxcalc_densityaltitude). This is approximately the same density of air that would be ingested into the engine if equipped with cold air induction.

Now let's assume that warm under-cowl air is ingested instead of cold outside air. We put a temp sensor next to the air filter and measure the air temp as it enters the carb at 100 deg F (41 deg F above ambient temp - this is pretty typical from my own testing). Plugging this back into the Density Altitude calculator gives us a new DA of 2700 ft.

Using the rule of thumb that atmospheric pressure decreases approximately 3.5% per 1000 ft of altitude (https://www.faa.gov/regulations_policie ... ak_ch4.pdf ; the rule is actually 1" Hg per thousand feet, so it's 1"/29.92" = 3.35%). From this, we see that the 100 deg air is 9% less dense (3.5% * [(2700-150)/1000] = 8.9%).

It might seem like this is a high price to pay when the power is free for the taking, but there are some advantages to drawing under-cowl air: it's simpler mechanically, lighter in weight, and the likelihood of carb ice is reduced because you've got "full time partial carb heat". Is this loss worth recovering? You're the pilot in command - you be the judge.

Jeff

Thanx Jeff. Good explanation

Just to add a bit more into the equation, how "pressurized" is the air inside the cowling versus a "static" cold air intake? Since the air is flowing through the cowling and out the cowling vents, it stands to reason that the "hot" air being taken in to the carb inside the cowling is actually pressurized to a degree above ambient (I could be wrong about this, venturi effect and all.)

This is probably insignificant, and anyone with actual carb-heat knows that more power is to be had with the carb-heat off. There are some clear benefits to cold ram-air intakes, but I am curious if anyone has measured the static air pressure of the post-baffle region under the cowling.

Hi Guys,

While talking about cold air induction and power increase it's fun to talk about increases in top speed. Most of us like to talk about how fast our airplanes are. The truth is, aside from a few WOT runs to try and secure bragging rights, top speed just isn't that important. What *is* important is climb rate.

You will seldom hear an AeroVee pilot complain about cruise performance. The complaint is usually anemic climb rates on hot and high days. During climb the throttle is opened up, the CHTs start increasing, the air speed drops causing airflow through the cowl to decrease, ... So I would argue that during climb the under cowl temperature increase will be maximized. Perhaps the 9% Jeff calculated goes to 12%. In any event, the power will be least exactly when you want/need it the most.

I think a lot of folks (I will) climb at approximately 100 mph to keep the CHTs in line. A 100 mph air speed consumes a considerable percentage of available power. For the sake of argument lets just say 50% of "potentially available" power is used just to maintain flight. There is nothing you can do about that, it is what it is. That leaves 50% of "potentially available" power to generate climb. But, by giving away 12% you give away maybe 24% of available climb power. I ain't doin' it. I will have cold air induction.

Slightly off topic:

I wonder if anyone has ever calculated the temperature rise the turbo guys get while running 5 in. of boost. It's been over 40 years since thermo and I have no desire to review. As I recall turbocharging without an inter-cooler, and ignoring temperature rise due to "beating up" the air. is close to an adiabatic process. That would be temperature rise on top of temperature rise ;-)

Wes

The lost power from temperature is actually very easy to calculate. Air density is inversely proportional to absolute temperature, so just take the ratio of the temperatures in Kelvin. In Jeff's example it would be 289/311 = 0.93 or 93% power.

WesRagle wrote:You will seldom hear an AeroVee pilot complain about cruise performance. The complaint is usually anemic climb rates on hot and high days. During climb the throttle is opened up, the CHTs start increasing, the air speed drops causing airflow through the cowl to decrease, ... So I would argue that during climb the under cowl temperature increase will be maximized. Perhaps the 9% Jeff calculated goes to 12%. In any event, the power will be least exactly when you want/need it the most.

Wes,

When you're short on power to begin with (and really, when does anyone ever have ENOUGH power anyway!), it just doesn't seem right to give it away. I get it completely! For my own purposes, a Jabiru has enough power on tap to waste a bit here and there. An AeroVee has less, and so you might not want to lose anything.

Here's what I did on my engine, and I loved the way it worked out. One thing I did not mention earlier is that the engine will run better (smother, lower CHTs and oil temp, better detonation margins, etc) when the induction air is cooler, and on a hot day during WOT climb, that's worth something in it's own right.

http://sonex604.com/cold_air_induction.html

Jeff

Hi Jeff,

For me a lot of the fun of experimental aviation is trying to do more with less. So, I get a little worked up about "giving away" anything.

sonex1374 wrote:The bottom of the coffee can is cut at an angle to match the slope of the cowling, while the sides stop approximately 1/2 inch above the cowling to provide a method of air escape to prevent pressure buildup.

Question for you: Why the concern about ram rise?

The examples of cold air induction I have seen by folks using AeroInjectors have always allow a path for air to escape the air box. I'm sure you have looked at impact pressure as a function of airspeed. At the speeds we fly ram rise will only provide the equivalent of approximately 1000' of density altitude (much less during climb). I think I understand that the AeroInjector meters fuel by throttle position rather than air flow which would cause ram rise to lean the mixture a little. But 1000' doesn't seem like *very* much.

Thanks,

Wes

If I understand correctly, the concern is differential pressure between the intake area and the top of the tank. We only have so much head pressure and this could be overwhelmed by a large static pressure at the fuel exit in the aerocarb. So it is not just a 1000' equivalent difference, it is delta P impeding the flow of fuel.

Hi Bryan,

Bryan Cotton wrote:If I understand correctly, the concern is differential pressure between the intake area and the top of the tank. We only have so much head pressure and this could be overwhelmed by a large static pressure at the fuel exit in the aerocarb. So it is not just a 1000' equivalent difference, it is delta P impeding the flow of fuel.

That makes sense. I can understand that.

From the Ellison site (ref. http://ellison-fluid-systems.com/faq-gravity_system.shtml):

Gravity feed will provide about 1.0 PSI for every 30 inches of head.

So, were only running about 1/2 PSI of fuel pressure on a low tank. At Sonex speeds ram rise approaches that.

Edit: I guess if the fuel cap is tight and the fuel vent is facing into the air stream it would be mox nix. Hum ...

Thanks,

Wes

Or if the vent terminated by the carb to equalize the pressure.