AeroVee Performance (Cold Air Induction)

[WesRagle]

Hey Folks,

I am building a Onex from a third hand kit. Things are going pretty well so I've been thinking more about engines lately. I have considered just about every engine I thought might work, but in the end, only a VW conversion makes sense for me. Since I have just about settled on a VW I started thinking about Sonex #664 I built years ago.

My Sonex was pretty much a stock AeroVee setup with one exceptions. I fed the AeroInjector outside air rather than "under the cowl" air. I also used a K&N filter that was much larger than the standard paper filter.

I can still remember the basic numbers.

Solo climb 700 ft. per min @ 100 MPH
WOT 150 mph @ 3750 RPM
Typical Cruise 135 mph @ 3500 RPM. (Just felt good there :-)

According to published data, at 3750 RPM I had already fallen of the back of the power curve. Anyway, those numbers were significantly higher than what other AeroVee pilots were reporting at the time. BTW, standard W54JV5L-44G prop.

Also, I was looking at some the data that John M. has gathered while trouble shooting the turbo issues (Ref. "https://www.sonexaircraft.com/wp-content/uploads/2018/03/Turbo_Cooling_System-Ground_vs_Flight_Comparison-030218.pdf").

If I am looking at the data correctly, while cruising at 29" MAP on a 56 Deg. F day the under the cowl temperature was 119 Deg. F. If you plug those numbers into a density altitude calculation you will see that the difference between 56 Deg. F and 119 Deg. F at sea level is about 3000 ft. I assume the temp rise due to cylinder head cooling is about the same for a normally aspirated engine at 29" (about one atmosphere). Poof, about a 10% decrease in air density.

So, unless there is something I don't understand, taking induction air from under the cowl robs the engine of about 10% of the oxygen (and thus 10% of the power) that is available using cold air induction.

Questions:

Does anyone else use cold air induction with the Aerovee?
Does it seem strange to anyone else that a lot of folks just toss 8 HP without giving it a second thought?

Just curious,

P.S. There is another thread where we can discuss carb ice.

[Onex107]

I'd like to know more about how you managed the fresh air feed w/ a K&N filter. Was your oil cooler also on the bottom? So the cowl had an opening in the front? What do you plan to do on the Onex?

[WesRagle]

Hi Onex 107?

Check out the NACA on the side of the cowl. The NACA feeds directly into a plenum box which was hinged to the firewall and then sealed against the cowl side and held there with a couple of camlocs. The camlocs <sp> allowed the the cowl to be removed.

The plenum supplied cool air to a firewall mounted oil cooler and an induction air box. The oil cooler outlet was plumed to a common air selector and provided cabin heat as well (the pipes were coated by JetHot). Worked just fine. Also allowed me to eliminate the smile on the front of the cowl.

The air box was fabricated using 0.025 6061 and surrounded the K&N which was attached to the bottom of the AeroInjector. I did use closed end rivets for the air box to preclude the possibility of sucking up a lose mandrel. Also, weep holes were drilled in the lower corners of the air box. Without them I would end up with a box full of fuel if I forgot to pull mixture. The K&N was large enough that the paper filter would fit inside.

I believe there are some pics of the setup in the old AeroVee Yahoo group. However, I unsubscribed from that group and once you unsubscribe they won't let you back in :-(

As far as the Onex is concerned I would love to replicate the setup but until the engine is hung I won't know if there is enough room. I just can't stand to leave that much power laying on the table. I can promise that I will do my best to come up with some way to get cool air to the carb.

[Sonex1517]

I copied a lot of the Yahoo files here

http://www.sonexfoundation.com/Yahoo_files.html

[radfordc]

Bob Mika uses a duct from the oil cooler intake area to a shroud wrapped around the carb air filter. This allows cooler air to reach the carb but still doesn't require any form of carb heat. It seems to improve the running characteristics of the engine.

[Jerry09w]

One thing to think about with cold induction air is the possibility of carb/intake ice. I made some measurements on my standare AeroVee installation, and I found the air going into the air filter was 70-90 degrees warmer than the outside air temp. I also found that the temperature drop in my aerocarb was about 70 degrees. If you add a cold air intake you might want to be able to shut off the cold air and go back to under cowl air in case of ice.

[WesRagle]

Hi Jerry,

I understand your point. I suppose the debate as to whether POSA style carbs are susceptible to carb ice will go on forever. I didn't have carb heat and never had an issue. However, I only flew the plane for maybe 150 hours. I did fly in the heart of the icing curve a few times and so was on the lookout, but never had an issue.

Having said that, I'm older now and not nearly as invulnerable as I was back then. I would much prefer to have carb heat.

I guess the point I would like to make is this:

The physics behind the relationship between cold air induction and increased engine performance is indisputable. It is demonstrated in practice every time a C-150 pilot turns off carb heat for a go around on a balked landing. Now, for those of us that chose a Sonex design to get the maximum in bang for the buck a VW is the obvious choice for an engine (my opinion). Consider also that the VW is the lowest powered engine of the possible choices. This brings me right back to my original question: "Does it seem strange to anyone else that a lot of folks just toss 8 HP without giving it a second thought?"

I owned a Sonex and had a blast with it. I had to sell it to help get ready for retirement. Now, 12 years later I'm back, I'm retired, and I'm ready to have some more fun. I halfway expected the cold air induction issue would be resolved and a "standard installation" documented for reference. I've been poking around the forum and haven't seen a thing.

I'ts almost funny. Could anyone imagine any of the Vans aircraft without the induction scoop on the front. They could just leave it off and keep carb heat on all the time. Sounds silly doesn't it. I think so.

Anyway, I'm getting myself all worked up. Sorry for the tone. It just seems like an obvious short coming that either the factory or the community should have addressed long ago.

Blue Skies,

Wes

[lpaaruule]

I wonder what the induction air temperature is when the VW engines are rated for horsepower? I seem to remember it being an estimate based on RPM and propeller specs, but I don't know if it was in a dyno situation, or an in aircraft number.

I have cold air induction in my Sonex, but it has a Jabiru 3300. I was going to use a NACA vent, but decided that just a hole in the bottom on the cowl as far back and to the side (out of exhaust path) as possible was a decent solution. During climb, I might get a little more air pressure, but it's not enough to negatively effect fuel flow, and perhaps gives me a little more power.

From the hole on the bottom of the cowl, I have a 2.5" scat tube that goes straight up to an air box with a K&N filter. From there it goes into a carb heat box that's directly connected directly to the air filter box. (I pull hot air from about an inch behind a rear exhaust pipe). I then use a tapered silicone tube that gradually bends 90 degrees into my AeroInjector.

I only use carb heat as needed, like a Piper Cherokee 140, and so far I haven't needed it.

[radfordc]

This brings me right back to my original question: "Does it seem strange to anyone else that a lot of folks just toss 8 HP without giving it a second thought?"

While in theory you may be losing 8 hp (~10%) I doubt that it is that much in the real world. Both Bob Mika and I used cool air induction and could not measure any difference in climb or speed performance from before we installed it. We both thought the engine "sounded" better.

[WesRagle]

I wonder what the induction air temperature is when the VW engines are rated for horsepower? I seem to remember it being an estimate based on RPM and propeller specs, but I don't know if it was in a dyno situation, or an in aircraft number.

Hi Paul,

It's seems standard for manufactures to rate there engines based on "standard day" conditions. That is at sea level, Altimeter:29.92 in/Hg at 15 °C (59 °F). But I certainly can't speak for Sonex LLC.

Yea, I used 2.5" scat from the plenum to the air box. It was a tight fit, had to remove part of the flange on the flange (if that makes sense). As far as the NACA is concerned I don't believe that it added any ram effect. It did provide adequate air for oil cooling so there was a net pressure difference between the inlet and the low pressure area of the cowl, just don't know how much.

Questions:
Did you ever fly without cold air induction?
If so, what was the performance difference?
Do you have any pictures of your setup you can share?

Thanks