Thorp Air Command - T18.net

Hello everyone!

I'm excited to say I just bought a T18! And it looks like a good one! (I've been searching for months!) After a CFI checkout, I flew it home last Friday. It has performance to spare!!

N41RS was completed and first flown in 1978 by a Northrup aeronautical engineer. It has an IO360, constant speed prop, both header and wing fuel tanks.

It was set up for IFR flight (back in the day) with what looks to be a heavy Narco DME, Brittain autopilot, etc etc. I will probably do some updates....

But more pressing, I need to improve the engine crank speed. I don't know what the compression ratio is for this engine, but suspect the battery cable is undersized. I measured it at .325" (Is that a #6 cable?) The battery is an Oddysey PC680, mounted about two feet aft of the baggage area. I think hot starts are out of the question until I can improve the crank speed. I'll be giving this some study...

In any case, I wanted to say hi, and I look forward to learning, flying to events, meeting people, and having a ball!

Peter in KSAC (Sacramento, CA)

Welcome to the T18 world. You will have a ball!
Tony
ZK-VMS
T18C

Thanks Tony!

I'm sure you're right- I will have a ball!

Peter

Beautiful plane, enjoy!

Thanks Ryan!

Peter

Hi Peter, Welcome to the place for all things Thorp. There's a wealth of information here and lots of guys with great build experience.

I put an PC 680 in my previous Thorp. I mounted it on the firewall and used the appropriate sized copper cables. That was on an o-320 and had no issues turning the engine. Before changing cables you may want to check your starter as well as the cable ends. Any corrosion on the cable ends will add resistance.
I know you'll have a great time with your new to you bird.

Terry
N218TH

Thanks Terry!

I wish my battery was on the firewall! But I don't think the W&B will permit it. Especially after my planned removal of this heavy DME equipment!

I'll be checking the cables / terminals today. Looking forward to dialing it all in....!

Peter

Unfortunately, with your configuration - the battery is where it is. I would suspect that considering when yours was built there was a lead acid battery installed. Those typically weigh about 24 pounds. The PC680 comes in at 11- net effect moves your CG forward. My Father's Thorp (N45881) was similarily configured - had the battery mounted aft of the baggage compartment and still had 13 pounds of lead in the tail. With you removing the heavy radios - that'll help move CG aft slightly.

Having had the experience with my Dad's airplane - When I built mine I was on a quest for light. Lightweight starter, light battery (EarthX - 4.6 lbs) Hartzell Hub with GT carbon fiber blades (saves 11 pounds per blade) After paint , she now comes in at 938 lbs empty. That's with my IO-320 and 9:1 CR pistons turning about 178 hp.
You can't beat the torque of the o-360, but I was looking for light and economical. You're going to have a blast as you really settle in to what a performance monster these airplanes are.

Blue Skies,

Terry

Right! I found on my first solo flight the airplane was airborne before I had the throttle fully in! Quickly up to 8500', and then had to pull the power back a bit to keep it comfortably in the yellow! Wow! Didn't calculate the TAS, but 200 mph over the ground in a headwind... Just love it! And a solid ride with the wing loading about matching my old Debonair. Just great!!

I'm not really a tailwheel pilot- so I have quite a bit to learn before I'm really comfortable. But it is sure a blast!


Peter

Is that measurement of the battery cable inclusive of the insulation? Another consideration may be to replace the starter with a lightweight/ high torque starter. And you can always put a meter on the starter to see what value voltage you get when you engage the starter.

For comparison , My battery is mounted under the right seat and we've run #2 from there - through the firewall to the starter.

Keep the group posted on what you discover
Blue Skies,

Terry

That .325 measurement is including the insulation.

As I was checking the wiring connections, I noticed a pretty heavy looking starter. And, ironically, I jumped back on this forum to see what people were doing for lightweight starters- and saw your post!

I'll need to figure out the cg arrangement and see if I can substitute a Flyweight starter and then move the battery under the seat. That seems like a good plan.

Will post back when I learn more....

Thanks!

Peter

Welcome Peter! No question is too much or too little. This is a very open and honest forum.
I strongly suggest a light weight, hi torque starter and the Earth X battery. The cold cranking amps on the Earth X is a lot more than the PC680.
If you feel like it, zing me your cell phone and I’ll hook up with you on my next SMF overnight.
Cubes
702 232-4261

If you need any parts for the Brittain let me know - I've got a complete B-5 that was removed from my Bonanza for an upgrade.


Chad

Thanks for the welcome, Cubes! My cell is 916 627-9625.

Chad, thank you for the Brittain offer. I've not made any decisions, but will probably install a Garmin system. (I have some spares from another project.)

I flew it a bit over the weekend- did a flight of two with a buddy flying his Grumman Cheetah- KSAC to KHES (Healdsburg, CA) He took off first. He captured the video of me passing him at probably 50 knots. I didn't mean to- but didn't realize how fast I was gaining, and failed to slow it early enough. This airplane puts a smile on my face...! Now my Cheetah buddy wants a T-18!

Peter

Hey Peter,
Welcome. I would recommend a lithium battery as well. Not just for the weight savings but resting voltage is 13.2 volts compared to 12.5-12.6 volts in lead acid or AMG. the cranking amps are higher too with translates to faster cranking. I use a 4 lbs 15 A/hr Anti gravity battery 480 CCA and it works great on my O-320.
Mike Durocher
N2WB