Many hundreds of experimental and certified aircraft sport B&C Specialty Product’s various spline-driven alternators as backups, and by all accounts they have an exemplary record for service and reliability over the past 20+ years. With the recent introduction of the more powerful BC462-H, rated at 60 Amps output in the belt driven version, I began thinking about using one as sole alternator for my “electron-dependent” RV-7A.
The idea of eliminating the belt-driven unit up front along with its HEAVY steel support brackets, getting rid of the belt, and even milling the drive pulley off of the flywheel is compelling. I’d be reducing the polar moment of inertia, lowering weight on the nose wheel, improving CG, and adding the crowbar OV protection built into B&C’s external regulator in one fell swoop. The quality and reliability of B&C products is legendary.
My RV has a nominal load of about 15 Amps, occasionally 24 Amps if pitot heat is used. Because the Lycoming spline-drive rotates at just 1.3 x engine RPM, the BC462-H output is physically limited to 40-45 Amps at 14V depending upon cruise RPM. Nathan Bainbridge, son of B&C founder Bill, was very helpful when I asked about the suitability of the 462 for use as primary.
Nathan felt that it should be adequate given my load requirements, but he did point out that output performance of this (and all) alternators is inversely proportional to the operating temperature of the rectifier assembly at the rear of the unit. Heat is generated within this assembly whenever a load is placed on the alternator in amounts that are directly proportional to load and inversely proportional to alternator RPM at any given load. Removal of this heat is directly proportional to cooling air volume and inversely proportional to cooling air input temperature.
The location of the spline drive under the cowlings of most aircraft can get pretty toasty, especially after shutdown on a hot day. It can also be a fairly stagnant region as regards air circulation in cruise. After a great deal of thought, I decided that several factors, all related to heat, would have to be addressed before I could commit to using the BC462-H as primary:
1- Although the alternator has a built-in centrifugal fan designed to circulate air over the rectifiers, it would move far less air at 1.3 x RPM than at the normal belt-driven 5-7 x RPM.
2- The perforated rear cover of the alternator is designed to admit air drawn in by the fan, which is exhausted through openings around the alternator body, but in the absence of significant airflow in that region, the same, ever-hotter air could be continuously re-circulated across the rectifier assembly by the fan!
3- Battery re-charge demand upon the alternator after a hot-start involving a lot of cranking could be extremely high- just when the unit has been thoroughly heat-soaked after shutdown.
4- I like “happy” electrical components, and it can be generally stated that failure rates of all rise exponentially with stresses like heat, vibration, and inadequate or improper wiring.
My solution to all of these concerns was to build a closed shroud around the rectifier assembly, pressure fed with cold air via a 1” SKEET tube connected to the point of highest pressure recovery at the rear engine baffle. This link SHOULD take you to a number of photos with captions that you can scroll through (be sure to leave the album in “roll view”). The captions contain a lot of information, so I’d appreciate your reading them before replying with questions or comments:
I first made the installation leaving the belt-driven alternator in place with an on-off-on alt-field switch installed so that I could select either system. Performance of both alternator and shroud have been spectacular during the 12+ flight hours of testing that I’ve completed, and I’m now confidently removing the belt driven alternator and its drive pulley from my bird. The new system including shroud and regulator added about 7.5 pounds total, and I’ll soon be able to report how much weight the items I’m removing have saved.(edit note- total weight removed at the propellor station= 10.25lb.)
In cruise, the rectifier section exit-air temperature magically stays almost exactly 100°F below oil temperature regardless of altitude or power settings. The indicated bus voltage on my EMS remains locked at 14.0V with an indicated 1-2 Amps positive at all times in normal operation, and I’m happy to report that the low-voltage light supplied with the B&C regulator stops flashing at a fast idle of just 1,000 engine RPM in normal operation.
I conducted the “acid test” during a long cross country, cruising at 9,500’. First, I allowed plenty of time for everything to stabilize, with OAT at 43°F, oil temp at 173°F, and alternator exit air temp at 72°F, 14.0V and +1 Amp indicated on the EMS.
Next, I turned off the alternator field, and flew with an indicated load of about minus 12Amps on battery power for about 20 minutes. My current battery is a near new Odyssey PC-680.
During 20 minute period, with a load of about 12 Amps, indicated bus voltage slowly dropped from 12.2V to 11.7V. Interestingly, the alternator temperature dropped just 3°F to 69°F, probably corresponding to ambient temperature of the pressure cowling, whose differential with the 43°F OAT of 26°F is a combination of pressure recovery elevation and radiant heating from the engine within the pressure cowl itself (which likely explains the fixed alternator temp vs. Oil temp differential). This suggests that quite a lot of air is being “stuffed” through the rectifiers and fan by my shroud. Once I have a chance to fly in hot weather, I may be able to choke down the cooling air supply a bit, but I’m in no hurry to do that.
When I first re-energized the field, the battery absorbed an indicated load of fully 32 Amps, so with the equipment load of 10 Amps the Alternator was putting out 42Amps at 2550 RPM (3315 alternator RPM). The BC462-H was able to maintain 13.6V into this load- pretty impressive. During the next 16 minutes, the alternator temperature slowly climbed from 69°F to a maximum of 79°F and returned to the original 72°F as the battery re-charged and the indicated amperage slowly dropped below +3. Pretty cool!! - Otis Holt