Warbird Information Exchange

Yes, the Jasco alternator is a frequent modification on a Merlin. The original generator is reliable, but it needs perhaps 1600 rpm to "cut in" whereas the alternator will charge well from about 1200 rpm. It is also smaller and lighter than the original part and bolts right on. I wanted to send mine back for overhaul once and when I phoned the factory they said they never made one for a 12 volt system! Of course they did and they overhauled mine promptly.

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Bill Greenwood
Spitfire N308WK

Ding ding ding ding! Jasco alternator it is.

My Stearman actually had some oddball tubeless 150 MPH landing speed main gear tires on it from a commuter plane or something (with tubes fo course). I never could wear them out...

Don't know what the gen drive ratio is for a Merlin but Jasco does have quite a curve to reach their max output.
http://www.skytronicsinc.com/drive_gear_ratio.htm
Too many folks install these thinking "hey, I'm gonna install a coffee maker now....I've got 50 amps!" Not so unless your gen drive ratio can get the alt to 4000+rpm. Not an easy thing to do when the original engine was designed around slower turning gens. Ranger L440's in PT's are typical. Unless you blow the heads off the thing you can only hope to do 40 amps...max at max throttle. At cruise you're lucky to get 35amp. You can do better with a dear ol' Eclipse. Dollar to a donut that a whole gob of Jasco equipped ships out there have incorrectly calculated electrical load tests and therefore do not carry the mandated 20% surplus charging capacity simply because the installing mechanic failed to observe the above little chart and calculated with corrected amperage output and not what's on the box label.

The nose gear tire from a convair is 27". They will fit, some t-6 guys run them. They are a bit more square in cross-section compared to the stock tire.

Of course you generally aren't running the original tube type radios anymore either. That ought to cut your power needs waaaay down, right?

That might be the one! I wonder if the wear characteristics are that much better?

Correct, but furture installations may assume the previous alterations were correctly entered and add equipment and load calculations based upon the previous installers data. Who knows what the next owner asks to be installed. Rather dangerous thing to assume, but how many alterations are done affecting weight and balance and no actual re-weigh is accomplished? Merely calculated based upon the most current W&B.
It still does not rectify incorrect alterations. One would be wise to err to the safe side. Entering 50amp charging capacity when 35 is actual is on the wrong side of ledger, just as having 100lbs lighter empty weight than actual in the W&B.

Weight and balance calculations are an accepted method when equipement is either removed or installed. You don't re-weigh the aircraft when you add fuel, take a passenger or put a suitcase in the baggage compartment, do you?

There is an STC for adding the Jasco alternator. I understand your point that the 50 amp rating is achieved at a specific RPM, but that does not make the installation an "incorrect alteration."

Very very few and none of the aforementioned aircraft. Skytronics holds but 19 for airframes and but one of those may apply to a W670 Conti and it is actually for a Cessna 190 which I believe is a W670-23 (240HP). And I see none for specific engines (they all appear to be airframe STC's)
STC List
A quick search thru the FAA online STC list for the Continental W670 shows no current STC for Jasco installation. Perhaps there is but it's obscure.
This means that any other installation in standard catagory aircraft (like stock Stearmans) are unapproved installations and thus require block 3 signoffs by the administrator on a 337 form (field approval). As such, one would/should run an electrical load test (calculated) and be submitted to the administrator as data while seeking their field approval. While submitting a load test is not required, it is nearly necessary with the Administrator's increasing reluctance to grant block 3 signoffs. If the alteration in question (installing a Jasco alternator) is made w/o an STC or a blk 3 signoff on a standard catagory aircraft, certainly, it is incorrect.


Those examples are not permanent alterations to an aircraft's W&B report. A major structural alteration usually is. Yes, calculation is acceptable but it is based upon the assumption that the groundwork laid previously by unknown parties is accurate.

I show a Jasco/Skytronics Approval from '82 based on A75N1, N75321

The Emair MA-1B Paymaster has an R-1340 and there were a couple of R-985 powered aircraft on the list and there have been plenty of field approvals.

I'm afraid I don't understand your point on this. I just need to charge the battery and run a couple of radios and occasionally run a landing light and a strobe. Are you suggesting that the Jasco alternator is somehow unsafe?

Does anyone have data to suggest that warbird operators have had problems with the Jasco alternator? I was very satisfied with the one in my Stearman. Maybe if I had a T-28 with a bunch of inverters this would be a problem, but that had a larger generator anyhow I suspect.