From “RGSCHMI”:
Now that cold weather is here, we are again trying to ‘winterize’ our Sundowner. One major air draft source seems to be the around the Gascolator opening. Has anyone found a good way to seal the fuselage/Gascolator gap? Any other hints on stopping air leaks?
Our Sundowner does not have the rear heat vents. Does anyone know if they can be installed in a 1973 Sundowner? It gets MIGHTY cold back there in Winter. The cold-soaked spar sure doesn’t help matters either. The heater works well for the front occupants, but the rear is always cold.
Search strings: cabin heater performance, cabin heating, rear seat heat
Technical Editor:
Personal experience/personal opinion is that the heater should be good down to about zero to five degrees F. “Good” means that the cabin will be tolerable down to that temperature. At higher OATs the cabin temperature will naturally be warmer; to the point that the heater can “run you out of the front seat” at OATs of freezing or above.
This assumes six things:
1. The door seals and windlace strips are in good condition are are working properly (minimal air leakage through the perimeter of the doors).
2. The internal muffler baffles are in good shape, so that the shell can transfer enough heat to the air flowing over it. If the baffles are all burned out, it raises the minimum “functional OAT” by at least fifteen degrees.
3. The cabin air ducts are in good shape; not broken, crushed, collapsed, leaking, etc. They should be checked anyway, due to corrosion concerns from the old fabric jacketing and plain steel piano wire internal coil.
4. The duct connections at the engine, for alternate air/carb heat, and the cabin heater, are not all mixed up.
5. The air control and defrost cables and valves are working right.
6. The engine is being operated at leaned power settings close to 75% in cold weather. Heat output declines significantly at lower power settings, and in an un-leaned engine.
Two Notes:
1. There are significant differences in both ventilation and heating, across all the years of manufacture. The 1977 and up airframes have the best ventilation, by a wide margin. Many even have the optional powered air blower for the overhead vents.
2. For the heat to work right, in the various models, the CABIN AIR must be IN (on); and the CABIN HEAT must be OUT (on). If the DEFROST is also pulled OUT (on), it will greatly reduce the heat going to the lower cabin area (just like in most older cars). All of these controls must work properly, for the heat to work properly.
My opinion on cabin air leakage: Try to make sure that the doors are reasonably well sealed. This is as much a function of nice, flexible windlace strip that should overlap the joint on a closed door, as it is a function of good door seals. Some people think that the windlace is just decorative trim, but that’s not why it has its name. Door leaks are the primary source of cold drafts, and a lot of noise, unless you have holes in your air ducts or bad air valves.
As far as trying to seal fuselage openings like the Gascolator cut-out, I suspect it is a waste of time. You have probably seen the oft-repeated comment that “Musketeers don’t float”. Fact is, no similar aluminum light plane will float (though some composite homebuilts WILL float). The reason our certified spam-cans won’t float is because they are riddled with water drain holes. These things are everywhere, or they had better be. Otherwise one can only imagine the damage that would be done by freezing water, or the corrosion from trapped water. The plane is a flying sieve; trying to hold in the heat is basically a waste of time. The best approaches are to stop nearby drafts (including any coming from under the instrument panel), and to get good heat distribution.
As of M1586, and M1600 onward (late 1974), the Sundowner, Sierra, and Sport came with a true front and rear heat distribution system via low-mounted outlets (as opposed to just a firewall hot air outlet for the cabin). The front cheek outlets and any overhead outlets, are only for outside air. In my opinion, planes from 1970 onward (same fuselage, etc.) could have the later design heat distribution system installed as a minor change, with a logbook entry. It is basically trim, with no effect on any primary systems.
If you round up all the needed parts from salvage aircraft of the later years, but similarly configured except for the heat and air vent system, then worst-case you should be able to convert your airframe to the later heat design via a Form 337, using the Beech parts book as “approved data”. You will need access to the parts book to identify the needed parts, and to understand the placement and routing. You will probably also need to look at an existing installation. The parts you have to find are the duct attach fittings (like the front heat splitter), air outlets, and trim. You also have the option of “owner produced parts”, to the extent you can copy the original parts. The air ducts themselves can be standard duct. While the CAT is acceptable, and the CEET is better, I strongly recommend the SCEET (lined red silicone duct). The SCEET costs much more, but it has stainless steel wire; and the silicone jacket won’t absorb and hold moisture regardless of its age. The original CAT and CEET ducting in these planes has caused considerable damage to the airframes, brake, and fuel system lines over the years. All ducts must be positioned free of contact with aluminum fluid lines, must have “caterpillar” or other chafe protection where they pass through openings, and should not be positioned tightly against aluminum structure. Where it lies loose in the fuselage, it should be sprayed with something like Corrosion X to prevent chafing corrosion from vibration.
One last suggestion. When repair stations redo interiors, they typically offer many alternatives on heat, ventilation, and lighting placement and styles; it is a part of redoing the interior. In our planes, the original rear seating heat system for the later planes used heat outlets in the bottom forward side of the ABS plastic spar cover for the rear seat, and in the forward panel of the footwell for the baggage area seat (if so equipped). If you are retrofitting an earlier plane with the later heat distribution system, I suggest that you locate the heat outlets in the rear seat side upholstery panels rather than in the forward side of the spar cover (“installation with minor deviation for placement”). The stock placement either fries the rear seat passengers’ ankles while their torso freezes, or blows the hot air under the front seats to the pilot/co-pilot positions, or both.
Using 1.25″ duct for the last portion of the routing into the side panels will make outlet relocation possible. The 1.25″ duct will screw nicely up inside the 1.5″ duct used for the main portions of the run; with a couple of feet of overlap, there is almost no leakage, and you can tape it if you prefer. Just make sure that when you select the rear outlet placement, that you have no interference with shoulder belts, headphone jacks, longerons, etc.
Having mentioned this placement alternative, I have two last observations. If you don’t mind a slight reduction in total airflow, somewhat offset by the use of the “slicker” SCEET duct, using the 1.25″ duct for the full length of the front-to-back run will make it vastly easier to install the duct. The area it occupies at the front outboard sides of the cabin, where it turns up on to the sidewalls, is very difficult to access with the 1.5″ duct. The other observation is that installing the rear duct outlets in the spar cover (stock location) will definitely be easier, and requires only a simple air outlet ring (rather than a Wemac-style adjustable swivel vent). If you use a Wemac vent for a floor outlet, it needs to be the large-opening style as used on the Bonanzas, or you won’t get enough heat out of it.