Failing alternator? Or just undersized?

I’ve been practicing cruising at 90 knots at 1,800 feet in my RV-8 with a Lycoming O-360, fixed pitch prop, and 30 amp alternator. The airplane has a VFR GRT Sport EFIS, single radio, ADS-B transponder, autopilot, and a full set of EI gauges.

The best RPM is about 1720, but with the strobes, nav lights, and landing and taxi lights on, I am getting a discharge light and the voltage is showing 12.2 to 12.4 volts.

I’m concerned about discharging the battery on the Ripon approach. Could this mean my alternator is going out?

A new alternator is on the AirVenture shopping list.

Certainly doesn’t look good. If you use higher rpm and turn off the lights, does the buss voltage get up to 14 volts? If no, you have a problem.

Did the problem just start? Some lights pull a lot of power - are yours LEDs or incandescent?

I would resolve that electrical problem before you put yourself in a hectic environment under a heavy mental load. Your head will be rocking like a bobble head doll just doing the approach.

This did not just start, just became more noticeable during these practice flights. The lights are incandescent, maybe halogen for the landing and taxi light. Normal flights with all lights on during the descent keep the battery charged.

I’ve flown the approach in other aircraft six or seven times, so hopefully know what to expect. My plan is to arrive early afternoon the Friday before the show.

Some information is in order . . . to facilitate diagnosis.

Alternator rating.
Amps being drawn.
voltage in normal conditions (is it being regulated normally?)
Has the belt tension been set by the drag torque method?

You will probably draw a conclusion for your self with this in hand. Grossly, if it is producing 14.2-14.5v normally and not slipping, then it is overloaded.

More detailed information could be gained by measurement of the field amps, and voltage under normal and approach loading if the rating is high but amps are lower.

We replaced the alternator belt during the 2020 condition inspection. I work with an experienced A/P-I/A so trusted him to set the tension and then check it during the 2021 inspection completed in May. The alternator and voltage regulator were replaced at 410 tach hours in 2014. The current tach time is about 640.

The voltage usually settles just below 14 volts. The amperage usage seems to fluctuate from 11 to the high teens, low twenties. I usually just fly with the strobes on, as I currently fly day VFR.

werxcv01 said:

The voltage usually settles just below 14 volts. The amperage usage seems to fluctuate from 11 to the high teens, low twenties. I usually just fly with the strobes on, as I currently fly day VFR.

Click to expand...

an alternator should be producing it's max output somewhere in the neighborhood of 2000 RPM, depending upon flywheel type. What did the ammeter read on the practice flight with the low voltage? If it gets to the low 20's without nav and landing ligths, I would not expect the load with both incandescent nav and landing lights on to be under 30 amps, so results not surprising. Using landing lights at OSH makes sense, but I don't see nav lights helping much in daytime recognition. I bet those are drawing at least 5 amps.

Larry

I tested my 70A ND alternator. Produced 26 amps at 1500 rotor rpm, 60 amps at 2800 rotor rpm. Given the typical drive ratio on a Lycoming setup, I'd guess you should have at least 90% of the full output at 1700 engine rpm.

first;;;; you derisive a grade of A+ , for practicing flying, some parts, of flying into OSK , and i back up the idea, from the poster #5 saying ""but I don't see nav lights helping much in daytime recognition. I bet those are drawing at least 5 amps.
good day and good job on your practicing / rick

If you do in fact have only the 30A alternator, yes it's time to upgrade to a bigger one.
Once you are in the Rippon procedure, drop 20 degrees of flap (don't overspeed!) (will require you to run at a higher RPM & hopefully maintain a higher voltage output) After you join the line towards FISK, shut down the landing/taxi/nav lights as at that point they don't add any more safety, till possibly needed to signal FISK. Try to keep the voltage over 13V with RPMs, good enough to maintain your battery short term.

I was considering lowering flaps, but the POH for my aircraft has a Vfe speed of 100 MPH. 90 knots is just over 103 MPH. The instructor in the -8 I flew for transition training allowed 10 degrees of flaps up to 110 MPH. I’m not too concerned for level flight, but if I have to dive for collision avoidance I could overspeed the flaps quickly.

What Vfe speeds do other -8’s use?

Or I could get behind a pair of Kitfoxes like I did in 2009 flying the -6A and end up flying the approach at 75-80 knots with 20 degrees of flaps.

Why fly with a know problem? Period! Flying doesn't fix it.

Thanks RV8JD, that is just what I was looking for.

Someone smarter than me may have to confirm this, but I think you might find that lowering flap and trying to product more drag will only allow you to marginally increase the RPM in a fixed pitch aircraft with the flaps down for the same airspeed. Sure, you will need more throttle to achieve the same RPM, and subsequent airspeed, but this extra fuel burnt will be primarily allocated toward increased torque needed to overcome the extra drag of the flaps, not RPM. All you're really going to do is increase the chance of cooking your oil temp/CHT by flying around with a bunch of extra drag on what is guaranteed to be a stinking hot day in an aircraft not designed to loiter, let alone with flaps down.
Why wait until Oshkosh to buy the new alternator? You live in the United States of Overnight Shipping, and it's not a difficult job to install a new alternator and some correctly sized cables/protection. Better to change it now than be flying around at Fisk whishing you had because your radio just died. You're just delaying expenditure you've got to make anyway.
Then again, easy for me to say that from here on the other side of the planet. I'd have given my right arm for overnight shipping during my build. It would take me at least two weeks just to get the alternator delivered.

Tom.
RV-7

To close out this thread, I did make a successful flight to AirVenture 2021 with no problems with the alternator, discharged battery, etc. I purchased a 60 amp B&C alternator and matching regulator and AeroLEDS lights all around. I have received the B&C order but some of the lights are still on back order. My mechanic and I will start the upgrades soon. Thanks to everyone who posted.