The International Cessna 170 Association

Could anyone comment on how the various engine conversions affect the CG on a 170b? I'm most interested in the Del Aire or Avcon O360 with Hartnell CS, but any info helps.

Without getting into specific W&B calls, I've read that you can end up outside the envelope pretty easily with no rear passengers and low fuel. I've also heard of putting a 20# lead weight in the tail, moving the battery behind the bulkhead, sand bags etc.

If this CG issue is a commonality with the weight of the CS prop out there, what are owners doing to keep the plane safe/legal, while not needlessly sacfificing useful load?

I had this C of G issues when I did my conversion ,but decided to keep the battery on the fire wall and couldn’t see the sense of hauling around 20 lbs in the tail that didn’t do anything constructive .so looked at all the propellers on the market and found the sensenich suited my requirements. MT was considered but availability and service was a issue to me. The fixed pitch sensenich [60in] was selected and suits my missions perfectly . It was the lightest on the market at that time but Hartzel now has a composite prop on the market with good reviews. I was paranoid that 60 in was too coarse. Sensenich reassured me that the prop,s pitch could be changed if required . It gets me off soft sand or mud with no issues and climbs like a home sick angle if your climb speed is set at 80 kts. It’s a good all round prop.

Doyn 0-320 conversion with Harrzell. I moved the battery aft but the penalty is the 00 cable weight, forget exactly but it’s around 15-20 pounds. At max load CG just forward of limit. My reality , I’m always carrying something in the back. Managing this is really a non issue in my mind.

My Avcon is at forward limit unless I have rear seat passengers or cargo. Flies fine, plenty of elevator authority, no need to “fix” it.

Slight deviation from CG, but would you consider an EW of 1485 lbs including a 20# weight in the tail to be uncharacteristically heavy?

This is for a 170A with 172 wings, an o360 and Hartzel CS prop. The aircraft also has some bush mods like 180 gear, sportsmans Stol, etc.

My airplane with an O-360, Hartzell prop, and lady legs was weighed at 1410. After accounting for the 20 pound weight, your airplane is 55 pounds heavier than mine. Seems a little heavy, but I don't know how much the STOL kit and 180 gear legs weigh.

Within the realm of possibilities weight wise. The gear would be aft of the datum so the additional weight of the 180 gear ( depending on the model of 180 gear) should help the CG. Vertical, I think you’ll find many examples in the forum and discussions over mistakes made when weighing our aircraft.
Sounds like you’re considering a Franken - 170 , might be a fun machine but expect a lot of unknowns, dead end papertrails and Cert issues but possibly great performance and fun.

Jim,
Far be it for me to correct anyone, especially one of your integrity. However, the main landing gear on a 170 is forward of the CG. If it were not so, we would need nose wheels on our 170s. Any weight added forward of the CG will move the CG forward and vise versa. We do not know the exact horizontal CG location, that is why we do the calculation. But, if the aircraft is stationary and all three wheels are on the ground, we are certain it is located somewhere between the wheels. Grin! The vertical location takes much more analysis. Fortunately, we are not very concerned about the Z axis.

Happy Thanksgiving, my friend,
Charlie

Charlie-
Jim said “aft of datum”, not “aft of CG”.

Richard,
Correct! My point is the main gear(where the weight increase is proposed) is forward of the CG. Adding weight forward of the CG moves the CG forward and visa versa). We are talking about a 170 where the main gear is forward of the CG? Add weight at the main gear of a 172 and the CG does indeed move aft.
Kind regards,
Charlie

I was reading about the weights of each aircraft mentioned and was very interested in what each aircraft weighted. I always thought mine was a heavy slug but it sounds in I am [my aircraft actually] is in the middle of the road. I have added to the equipment list without due regard to the final weight. I usually fly by myself and full fuel. However I think my real weight is the paint in/on my aircraft . The interior fuselage was coated with Zinc Chromate and they must have shoveled it on . Plus a float plane kit has been fitted to the left hand side of the fuselage only .why????.my aircraft weights 1431 lbs.

Well you might have to stand in line if you want to have a crack at trying to correct me Charlie. And you’re welcome anytime. Aircraft power off and acting like a lawn dart, may not be desirable.

float plane kit has been fitted to the left hand side of the fuselage only .why????

They may have ran out of parts or ambition but most likely money

There are two CGs to consider with regard to landing gear position...either of which can be misleading if using the datum alone. An excellent example is some of the Beechcraft products that use a datum which is several inches AHEAD of the radome/nose of the aircraft. Everything added to that airplane is aft of the datum!

There are airborne CGs and ground CGs that should be considered. As Charlie points out, the ground CG must lie between the main gear and the "little" wheel. (And in the case of both the 170 and the 172 it is so...and it is aft of the main gear in the 170...and forward of the main gear in the 172.

In the air, the CG needs to remain somewhere between the aft Center-of-Lift and the forward C/L.

For this reason, adding heavier gear legs (approx 11 addt'l lbs for 180 gear) moves the ground CG forward...and the airborne CG... also forward. (Consider that the most fwd allowable CG of any model 170 is more than 36" aft of datum and the main wheels of a 170 are at 22" aft of datum, so a cursory glance indicates that addt'l wt near that postion will move CG fwd.)

Jim also makes an excellent point regarding shifting the battery.... It's not "free" CG-movement because it adds considerable weight in the form of longer battery cables. (And, BTW, where exactly is the CG of that battery cable which stretches from forward of the datum to waay-behind the datum? HInt: it may not be the average of the length of that cable because a portion of that cable is likely looped-around somewhere vertically forward of the datum, I.E., more cableweight-per-inch forward than aft of datum.)

This is why we should occasionally actually weigh our airplanes.

Ghostflyer, your airplane is not as heavy as many in the fleet, a lot of A and B models when actually weighed are found in the mid-to-high 1400# range, but many that undergo avionics upgrades since the 1960s will lose some of that with lighter radios (not all of which will be realized without re-weighing.)

Sorry everyone, I thought this thread was a about that big C/S prop weight and arm forward of the datum causing an exceedance issue with the forward limits, I just thought maybe putting weight behind the datum to counter it would help.