Thorp Air Command - T18.net

Absolutely loving my T18!

I'm taking advantage of the lousy flying weather to do a bit of work on this Thorp, and looking for input to address a forward-biased CG.

The airplane has an IO360 and a constant speed prop. The PC680 battery is aft of the baggage bulkhead.

After installing all new avionics one year ago, the airplane was in a somewhat slightly nose heavy condition- which I'd anticipated; I'd removed a lot of stuff, including a 7 lb DME box behind the baggage area. To address the forward CG, I pulled the old, heavy starter, and installed a lightweight unit. It flew much better with that change. But there was still one piece of old avionics that I needed to remove- a 2.7 lb flux gate was way back in the tail. I pulled that piece out yesterday. To make matters worse, I'm about to install a Monkworkz backup generator on the engine's vacuum pad. It's 2.6 lbs.

I'll calculate the w&b to ensure I stay within the envelope, but clearly, it's going to be very forward with fuel in the header tank.

As I see it, I have a few options...

1. Get a bigger battery. (with the backup alternator, this just seems like added weight, with not a lot of benefit.)

2. Move the existing battery aft. (My napkin calculations show it would have to move about 27 inches just to offset the flux gate removal. And further aft would be required to offset the new generator.) I kind of like this option- but not looking forward to the project!

3. Get a lighter prop. (I want to keep a constant speed, and composite blades are probably beyond my budget! But I do have a pesky AD prop. Ugh.)

4. Add lead to the tail. (I hate adding weight for no purpose other than shifting the CG.) Maybe this option makes sense though?

Any thoughts on this?

Thanks!

Peter

I have ten pounds of lead in the tail of my 360/cs Thorp. Used to have sixteen pounds with old boat anchor starter. Flys great!

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Bob Highley
N711SH
SN 835
KLAL

It does come down to “cost vs benefit”. When I had 218TH’s prop built, I bought GT Carbon Fiber blades as they are designed to be installed in the Hartzell Hub. The biggest plus is that they are 22 pounds lighter that Hartzell Aluminum blades. I’m happy with the performance, looks, and noise level of the blades. From a cost standpoint- to accomplish this was probably equal to or slightly more than just buying a new prop from Cubes. If memory serves-I bought the hub and related parts for 1500. Blades were just over 7 k in 2016. And the prop shop charged ~ 3 K to build it up and balance it.
I had a design goal to keep my airplane at light as possible and I achieved it. Paint, interior and panel brings my empty weight in at 912 lbs.
With the big picture in mind, it’s seems your best way to accomplish what you want is to calculate how much weight and just bolt it in.
Hope this helped

Thanks guys- the input helps.

That lightweight composite prop sure has a lot of appeal! But I don't guess I'll go that route. I will probably just (cringe!) bolt in some additional lead.

I'm certain my airplane has lead in the tail somewhere- but where is it typically bolted in?

Mine is bolted through to the tailwheel bolster.

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Bob Highley
N711SH
SN 835
KLAL

Thanks Bob- that makes perfect sense, and is what I'll plan on doing.

I have done some testing by strapping weight onto the stinger ahead of the rearmost mount. Wish I knew more proper terminology, but basically the fixed part between the two mounts of the stinger. There's around six to eight inches of straight-line, non-moving stinger metal available. With enough Velcro (or other means), one can mount a few pounds of heaviness (lead, etc) securely enough for a flight test once you have weight-and-balanced sufficiently. Be super cautious of course, you do not want it to move at all, even with a juicy tailwheel bounce (not that any of us actually land that way...).

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Philip Mandel
N118BC
S/N 957
Beaverton OR

Thanks for that, Phil.

I removed the inspection plate under the stab, and spent some time looking for any existing lead. I can't find any. Maybe there is no lead ballast in this airplane. With a constant speed, injected IO360, I thought for sure there must be some lead in the tail- somewhere. Maybe the original, heavy avionics installed aft of the baggage was enough to offset the heavy engine / prop combo.

I've given it more study, and it looks like moving up to a slightly bigger battery will work out perfectly. The current Odyssey PC680 (15.4 lbs) is installed 90" aft of the leading edge, and I've just ordered a PC950 (20 lbs.) Sure hate to add weight, but at least a larger battery might provide some potential benefit. (Get that play on words?!)

I really should (and will) get it on scales to generate a true w&b- I have the builder's original- now pretty dated. But with the new lightweight avionics and lightweight starter, it handles great through all the different loading configurations. So, I want to keep the cg fairly close to where it is now.

Peter

So I did in fact change from an Odyssey PC680 to a "real" battery...which I had purchased before I realized there was a 5-lb lead rectangular weight wedged into the battery box next to the Odyssey, meaning I accomplished exactly NOTHING after removing the Odyssey and lead weight, and installing the monster. Checking to see if you have checked to see if there is a lead weight in your battery box as well? Don't spend time and money accomplishing nothing, like I did.

I def plan to get a professionally measured W&B soon. N118BC is nose heavy when I am alone, even with full fuel (single tank, header). Full nose-up trim speed is around 120 mph zero-flap, a bit slower full-flap. I wouldn't mind if it was 100 mph with zero, 80 full. Having to hold back pressure in the pattern, esp on short final, leads to unwanted airspeed and glide path variations thanks to our wonderful airplanes' light stick forces.

On the other hand, with a pax (even a lightweight 140-pounder) and maybe 45 lbs of luggage, she gets super twitchy in pitch as if aft or near-aft CG, even though my current W&B spreadsheet has her well within spec.

Anyway, a fresh W&B will be informative. I may opt to design a weight system that can be clamped and unclamped externally, based on interior loading, as I described above (clamping to the fixed part of the stinger). I would then probably create two W&B spreadsheets, one with and one without the clamped weight, so I can predict what the CG will do during the course of a flight.

PS: N118BC is 180-hp O-360, CS 2-blade prop.

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Philip Mandel
N118BC
S/N 957
Beaverton OR

That's interesting, Phil...

So you surmise your airplane is nose heavy based on the lack of trim at low airspeed?

But the airplane gets twitchy with just a small pax and some baggage?

I wonder if maybe your trim is not set up quite right, based on the above. It sure doesn't sound nose-heavy to me.

My airplane has all the trim I ever need, and is never twitchy. When I first put in the all-new panel, I knew I'd shifted the CG forward. I flew it just once under that condition- very stable in pitch, as you'd expect, with pretty heavy stick force. Landing it wasn't much fun either. But it never ran out of up elevator trim. I then installed the light weight starter, and it flew great again! (Although, I suspect still a bit nose-heavy.)

But now, with the removal of the last of the old avionics bits- the 2.7 lb flux gate (in the tail,) and adding a 2.6 lb perm mag generator on the engine, it will be nose heavy again. Turns out (from my napkin calcs,) the 5 lb heavier battery will solve the issue- (at least maintaining the previous, good-flying cg.) There is no lead to be found anywhere- not at the battery tray. And none visible between the last two bulkheads.

Note that I do take W&B seriously, and would never consider flying an airplane outside of the CG envelope. Especially one that is tail heavy beyond limits. So I'll get my airplane weighed, and generate a current W&B.

Let me know what you find and do to resolve your balance / lack-of-trim/ and "twitchy" characteristic...

Thanks!

Peter

I have been perplexed with my W&B on my plane ever since I've owned it also. I too, did some avionics changes, removed vacuum pump, and a large strike finder antenna aft of the baggage compartment. I have to hold constant back pressure in the pattern when I'm solo, but when I have a passsenger I do not. But, when I have a passenger, mine is very 'twitchy' in pitch as well as if it is near aft-cg.

I've done a new W&B and it shows that I'm usually near the aft cg limit, especially with passenger and cargo. but I have a O-360 and a CS prop....doesn't seem to make sense. Maybe I have some hidden lead somewhere that I don't know about also. I never thought to look in the battery box! I also wonder if my station figures in my W&B are accurate. I did some reverse math on my fuel tank (did W&B with it empty and then when full and used the numbers to calculate an arm) and it didn't match too well with what was on the original W&B.

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Marty G
N50LE
YouTube Channel
martygrosse@yahoo.com

Well, whenever I get another plane in the shop, I always weigh them. You would be surprised what I find. I highly recommend getting them on a scale… not just recalculating the weight.
Then, figure 15-32% MAC and you are good.
Any plane will be harder to control aft CG. But we are pretty short coupled. I try to stay on the forward side of CG.
Drawing 521 will tell you how the trim is supposed to be set up. When you use it, remember to trim before you measure the angles.
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