RV-10 A/C Installation Notes

[tom kreiner]

I'm new to the Forums, and what I've learned may help new builders, so here goes.

During the last 9 months, several guys in my EAA Chapter - 302 in Conroe, TX - have engaged me to commission their A/C systems. There are three systems I've now been involved with to one degree or another, with additional efforts ongoing.

The first system is from Brazil, and is apparently somewhat of a knock-off, of a system which was said to be originally designed in California (please feel free to correct me.) The second system is from Flightline - div. of Kelly Aerospace - and the third is from Airflow systems. In the rest of these Notes, I'll refer to these as the "Cal" System, or the "Flightline” System.

The first two completed systems are blowing COLD AIR, while #3 is still under construction, but will have wings added this week, and is expected to fly in December.

Well you may ask, how cold is COLD AIR? How about 39.2 degrees F? This is the result of the Flightline system, after an exhaustive troubleshooting process, along with a completely redesigned wiring system. When we flew this plane last week, the OAT temps were in the mid low ‘80's, and the air was too cold! Air coming from the Brazilian system was a few degrees higher - around 42F. In case you haven't studied them, they're very different systems; with slightly differing thermodynamics, and more. My hope is that you'll glean what you need to know if you wish to add A/C to your project plane.

There are two principal designs being used; both of which have an engine driven compressor, along with a tailcone mounted evaporator. Both systems have condensers; the one on the Cal systems is mounted below the baggage compartment, while the Flightline is located to the rear of the tailcone / baggage bulkhead. Only the tailcone mounted condenser from Flightline has an integral fan.

When charging the Cal system, there is insufficient airflow from the propeller from the prop alone to cool the compressed gases in the condenser. With a pressurized charging system, the exact amount of refrigerant may be added to the system. That said, the question becomes, “what is the exact amount of refrigerant for a system?”

It turns out that for any system it is the sum of the volumes of all of the internal components, including compressor, condenser, dryer, evaporator, and lines. Since there is no single setup for lines, it’s hard to determine how much refrigerant will be required, so the empirical approach is used; charge until typical pressures are acquired, along with cooling.

In addition to refrigerant, there is the ABSOLUTE requirement to add oil to all of the components in the system. There are formulae for determining the amount of oil to be added. Too little oil, and the compressor life will be diminished; too much and cooling is reduced. Rules of thumb apply and will be very close, assuming the compressor is filled properly when received, and all other components are new.

Without volumetric information for a new system, charging is done with a gage set. (If you apply significant forethought, along with a precise scale, you may weigh how much refrigerant the fully charged system required, and use that information should it need to be recharged in the future.) In my experience, I've never seen a $5000. charging system on or near an airport; however, I've been using a gage set for over 50 years, and I'm comfortable using it.

The Cal systems will need a strong (i.e. 200 mph) leaf blower propped up under the fuselage blowing air thru the condenser scoop in order to charge the system. The Flightline system will only need to be turned on and run because its integral fan - just like on your car - will move sufficient air thru the condenser to enable charging. Both types of systems will need to be charged with the engine producing between 1600 and 2000 rpm.

If you're unfamiliar with A/C systems, the compressor takes gas in the system, compresses it, and sends the hot gas to the condenser coil, where air flow extracts heat causing the gas to liquify, becoming a hot liquid. From there the refrigerant flows to the evaporator coil, where airflow releases heat as the hot liquid is exposed to a very low pressure, where it boils, thereby absorbing heat.

Just as in a car, during ground operations the continuously running condenser fan in the Flightline system outshines the Cal system. Both of the systems perform a credible job while flying, providing adequate comfort to the airplanes interior.

Because all of the current systems use the overhead console as a duct, the airflow is somewhat lower than many owners expect. There are new designs which, when ready, will address airflow, however.

This is a pretty involved subject, and I've condensed as much of it as possible, while attempting to give the prospective user of the systems some information they may not otherwise have access to. Feel free to comment or ask questions about the systems.

[Mike S]

Tom, welcome aboard the good ship VAF

Thanks for the A/C info.

[No_0ne]

Any insight into a comparison of the two types in regards to installed weight, CG issues, ease of installation? I'm not to the point where I have to make a decision yet, but I'm already looking into the various systems. Thanks for posting your experiences thus far.

[hgerhardt]

Tom, welcome and thanks for your analysis of available systems. I have one of my own design installed in my -6, so I'm familiar with your point about properly charging these systems. Originally, I also used a manifold-gauge set while standing 3 feet from a spinning prop. Not fun, and not accurate for flight conditions either.

Well, recently I discovered Bluetooth refrigeration probes. Figured this might be an awesome way to properly charge and characterize the system while in flight and indeed works amazingly well.

As you know, to properly charge a system, you need the pressures AND temperatures of evaporator and condenser outlets to determine superheat and subcooling. But, it's really hard to fly the plane and calculate those at the same time in a dynamic system. With the BT probes, that math is done for you in real-time as you watch system performance on your phone/iPad. The kit also has psychrometric probes so that you can measure cooling performance, with sensible temps and dewpoints of evap inlet/outlet, and can determine BTU's removed.

Pre-BT probes, I thought I had my system dialed in pretty well, with fairly normal pressures. Turns out I had zero subcooling and too much superheat in flight. Got the charge correct and now I'm amazed at the 50F deltaT I'm getting (37F blowing in my face with 87F return air). The 87F also shows I have a leaky cabin that needs better sealing.
Heinrich

[hgerhardt]

Originally Posted by No_0ne

Any insight into a comparison of the two types in regards to installed weight, CG issues,.....

While I was doing this exact research several years ago, I found a retired engineer in TX who installed an Airflow system in his Glasair II and did a great job documenting the installation. He gave me the data below. I would assume that an RV-10 installation would have about 6' more refrigeration hose and an overhead console added to the 60 lbs.

[digidocs]

Heinrich,

It's really neat to see someone flying a system of their own design. Would you share some more pictures and information about your system?

It looks like you have an accessory case drive (cool!). How did you do your condenser setup?

Edit: Just spied a suction side heat exchanger---serious business. Is that custom?

David

[tom kreiner]

Originally Posted by No_0ne

Any insight into a comparison of the two types in regards to installed weight, CG issues, ease of installation? I'm not to the point where I have to make a decision yet, but I'm already looking into the various systems. Thanks for posting your experiences thus far.

As to the installed weight, it would appear that the two systems are within 5 lbs of each other, although they will need to be weighed carefully. The only weights currently accessible are shipping weights, which include packaging, etc.

Commenting on CG is impossible at the moment as I only have stick time on the plane equipped with the Flightline system. While I won't call that plane "twitchy" in pitch, I must admit that with only a few hours of stick time, I found the plane rather pitch sensitive. Also, I don't know the W&B info, nor proximity to CG...

Installation wise, it's a wash - IMHO. My guess is that when a builder is at the structurally complete lower fuselage point, both units will require roughly the same amount of effort. Should the fuse be complete, however, the addition of the baggage area condenser might take a few hours more, certainly doable in a days effort, but not weeks. Both evaporator sections are virtually identical in assembly & installation requirements.

Since my original post was intended to be informative rather than opinionated, my views on the two systems were absent. Now that you ask, however, I'd lean towards the Flightline system, as it has a condenser fan that creates superb cooling on the ground where it's needed the most.

[DRMA]

I can't provide actual weights for any of the systems, but I installed the Airflow Systems package in my RV-10. For me, the main driver for selection of the Airflow Systems over the Flightline is the lack of a fan and the simple and small penetrations required through the bottom skin.

The lack of the extra fan to supply air to the condenser means a significant reduction in amps required from the alternator. And by having the condenser mounted outside the fuselage, I feel reduces the potential to pull exhaust gas from the engine into the cabin. (Yes, I'm sure you can seal the condenser air flow ducting to keep this air out of the cabin, but it requires a much more complex sealing that the 2 small fittings used to get the refrigerant in/out of the condenser mounted below the fuse. Being an engineer and project manager, I prefer simple when possible, but that's just me.)

I'm sure both systems will work, as they both use the same physics process for cooling. So your call as to which you prefer.

Regards,

[tom kreiner]

Great comments! One of the best parts of forum participation is that you always have a few folks with significant knowledge who can contribute.

I'd thought about adding additional info regarding superheat, but I thought it would be beyond the scope of the original post.

Just in case it's an unknown concept, superheat is referring to a specific state of refrigerant charge where at the end of the last row in the evaporator coil, liquid is turned into gas. Further explaining, if the charge is too low, the liquid entering the evaporator completely "flashes" or boils off very close to the entrance of the evaporator coil. In contrast, in the case of a complete charge using superheat data, the liquid passes almost the entire way thru the coil, and completely flashes just prior to exiting the coil.

Superheat setting procedures enable the charge to be set at a point where the entire evaporator coil is absorbing heat; not just a portion of it.

Setting superheat is traditionally done empirically, although it can be facilitated with instruments like the BT probe, or an excellent IR thermometer.

[tom kreiner]

Dave,

No question about the alternator requirement! The Flightline equipped system with full avionics, position, landing & strobes on will take 55+ amps.

On a 60 A alternator, that's quite a load, and the owner of that plane is considering the swap to a 100 A unit. OUCH!

Nice to hear that your plane's in the air! Congrats!

Tom