WEIGHT and BALANCE

WEIGHT & BALANCE QUESTION...
From: Alan Dicker (jdicker@sprynet.com)
Subject: Re: Swift Weight and Balance
Reading the Swift notes I get the impression that Swifts "behave" better with a rearward center of gravity. Is there an "ideal" empty c of g? How much weight was added to the tail in a stock airplane? 80733 has 145hp engine and McCauley fixed pitch prop. The battery has been moved to behind the baggage compartment. With this configuration, how much weight is needed in the tail?

Alan,
I wouldn't say they "behave" better, but they definitely go faster - they probably go fastest at a CG BEYOND the aft limit - where they might be unsafe to fly. They also land easier with the CG near the aft limit. I have heard experienced Swift pilots say they can't 3 point land, and they use 85 or 90 mph for approach. I can almost guarantee their airplanes' CG is forward on the envelope.  N80733 is a converted GC-1A. Most Swifts had between 9.5 lbs. and 15 lbs. of lead in the tail. The earlier airplanes had lighter skin on the aft fuselage, so they needed more ballast. The late Temcos had 9.5 lbs.

A couple of anecdotes. I installed a C-145 in an early Swift (N80740) It originally was a GC-1A, but I had removed a C-125. It had NO lead in the tail. I installed the engine, and went out and flew it. With a strong wind down the runway when the power came up, so did the tail, and I ALMOST bit a chunk out of the runway! (Definitely a forward CG) I had N2334B weighed carefully. It had a big Scott tailwheel, 9.5 lbs of ballast, a Sensenich prop (3 lbs. lighter than McCauley) and for a while, no oil cooler. With baggage, it was possible to load it out of limits. It sure landed nice! For a 3 point landing, a 65 mph approach speed worked out just right. (an aft CG)

Another anecdote: I had a Luscombe 8A. 65 hp, no electric. It wouldn't hardly aileron roll, but snap rolled really good. Just approach a stall, pull some "G", and kick full rudder, and BANG, around it went. I converted it to an 8F, and installed a C-90-12 with starter, generator and battery. Afterwards, you could approach a stall, pull "G", kick full rudder, and it would just yaw and continue to fly straight ahead. Which was better? I suppose it was actually safer with the forward CG, but not nearly as much fun! And it landed lousy with the forward CG. What is right for N80733? You'll have to weigh it to be sure. Even with the battery moved back it probably should have some weight in the tail. The limits are 29.6 to 34.7 for a GC-1B, interestingly, the GC-1A limits are 29.3 to 35.5 --- Jim

WEIGHTY ISSUE FOR OUR “ANSWER GUYS”...
New Arizona based Swifter Greg Schmillen (DaSchmil@aol.com) wrote:
“I'm having trouble finding a combination of Pilot/Passenger and Fuel that will keep me under gross weight and still give me enough fuel to do something more than a couple touch n go's. My empty weight is 1249 and the book says 1710 is the gross. 461 pounds of useful load ??? Do I have the correct gross weight ? If I do, what, if anything, can be done to increase the useful load ?”

Steve Wilson replies:
“Diet maybe... Two (2) 170# people + 20 gal fuel? That's about right unfortunately, for an average Swift.  Jim Montague has a fine article in the past two months issues of Vintage Airplane, the official rag for EAA's Antique/Classic Division. He covers the issue of keeping the airplane light about as good as anyone has ever done. Get copies! If you follow his advice closely, you can save several pounds. As I recall Jim's 145 weighs about 1225# which is light for that airplane. Just think if you lowered your empty wt 24#, you could go with nearly full fuel...

There are no easy or cheap answers. Merlyn has a gross wt increase kit available (well maybe). If your horsepower is 125 to 150 you can increase it to 1835#; 180 and above to 1970#. The kits are expensive and very hard to obtain. My 145 is just about the same wt as Jim's and took a lot of work to get there. I have the 9 gal belly tanks, so with the 1835# gross wt kit it works fine for me. I could lose a little wt myself though {;-), and that would help out the situation... Good luck!”

Jim Montague replies:
“Yes, the normal gross weight is 1710 lbs. You can get Merlyns GW increase to 1835 if you want to spend some money, and if they feel like selling it this week. Actually, unless you operate from high altitude or short runways the Swift will cope quite well with 1800 lb loads. You might lower the empty weight a little, but getting a Swift below 1200 lb empty is pretty difficult. If you are concerned about performance, use a prop pitched for a 125, and live with the higher RPM in cruise. (legal with the Merlyn STC)

If its legality for insurance concerns, again buy the Merlyn STC. If you are concerned about structural strenghth, don't worry about it. The CG you control yourself - use good sense in loading. Are you married? If not get a 100 lb girl friend, that should allow full fuel and 35 lb baggage!”

WEIGHT AND BALANCE...
Bob Runge (ejectr@javanet.com) writes...
I am confused about weight and balance though. The Swift spec states that the "allowable" CG range is 29.6 - 34.7. On an old weight and balance sheet in my records I see "actual" CG range 31.2 - 33.8. There are also a couple of loading examples on it labeled "Most Rearward and "Most Forward", Condition 1 and Condition 2. Condition 1 shows full fuel, 2 standard passengers and 34 pounds baggage at gross, reaching max actual CG aft. Condition 2 shows less of everything meeting max actual forward CG limit. It also shows the weight of the left, right and tail on scales during a weighing and a formula

D - TxL/W =CG where they came out with a CG of 28.77 as weighed (with oil).

How did they come to the conclusions of actual forward and actual rearward CG figures? They aren't like the allowable from the specs and looking over all the figures, I can't see how they came up with the figures they did.

Bob,
We just got home from CA & I have about 30 Emails to reply to so I will be a little short. The GC-1B aft limit is indeed 34.7 I would have to see the actual figures, but I don't see anything too unusual in what you have there, whats the problem? Of course, I just got up, maybe my brain is not fully engaged!
Jim

...later, after Jim’s brain engaged...

Bob,
I re-read your mail. I guess you don't understand the FAA method of wt & balance. Read AC43.13-1A chap 13. The extremes must be calculated. The simple way to say it, is, for the forward extreme, you want max. loading for forward items and min. loading for aft items, and vice versa for the aft extreme. So for a Swift, it would be for fwd -

oil - 2gal - 15 lbs

fuel - 63 lb (min fuel for 125 hp; formula 1/2 lb per hp)

pilot - 170 lb

aft can vary, use several combinations for examples, you can limit baggage for GW, like evidently they did on yours, i.e. 34 lb for 1710 GW.  --  Jim

Subject: Re: Wt & Bal
From: Dennis Mee <jmmee@nh.ultranet.com>
This question concerns the flight characteristics with ref to aft CG, everyone seems to favor an aft CG but how far aft, and how critical is a difference of .75" in cg? I've weighed the airplane with and without the 9.5 lb ballast. Now I'm calculating the extreme cg conditions per AC 43.13 and making a loading schedule, I have the Machen 1970 lb GW increase kit with the Machen wing aux tanks, (13 gal each), Machen cont 210 conversion with the battery moved to aft of the cargo compartment. The difference between having ballast in or out is an avg of about .75" through the cg range. The extreme fwd cg per 43.13 it is 30.8 without ballast, 31.6 with, no problem. I will have to limit the load to stay within the aft cg limit and max GW, again no problem to satisfy the feds, the question has to do with reality. The cg will at times more forward than that required by 43.13 because the fuel on board will be less than 17.5 gal, (210hp/12) even with only 6 gal on board, no pax or cargo, the cg would be 30.1 without ballast and 30.9 with ballast, either way it falls within the 29.6" limit. No problem staying within the forward limit, now the aft limit.

Please note that I am not proposing that I nor anyone else should ever takeoff overloaded; having said that look at this scenario: Leaving on a trip to Athens without the 9.5 lb ballast and with full fuel (52 gal), 2 FAA size people (170 lbs ea.), and no baggage we would be 77 lbs over MGW with a cg of 33.4", (within the aft limit of 33.5"). Under the same condition with ballast we would be 87 lbs over MGW with a cg of 34.0, (.5" aft of limit). This all works out to giving up 13 gal of fuel to stay below 1970 lbs GW and within cg limits without ballast, or 15 gal of fuel to stay within the 33.5' cg limit and below MGW with ballast. Note that without a passenger on board and full fuel with up 63 lbs of baggage without ballast or 18 lbs with ballast the limits are not a problem. Most of my flying will be done without fuel in the aux tanks, the wt & bal limits present no problem there. So the question; is it better to have a cg of 32.3 or 33.0 with an average GW of 1900 lbs.? How much difference is there in performance or how it handles by moving the cg .75 in aft? I plan to leave the ballast out what are your thoughts?

I know I could go fly it under several different configurations but I was hoping that your experience and expertise might save me that time and trouble. Thanks Monty, Dennis Mee

Dennis,
It kind of depends on FAA legal or safe to fly. As you know the higher GW has a restricted CG aft limit of 33.5 It is my opinion that they did this to avoid some spin testing for the STC. The GC-1A has an aft limit of 35.5 I think a Swift is safe to fly at this aft CG if competently flown - no acro, and no slow speed maneuvering in the pattern and don't level off for landing until 2 feet above the runway! The idea of the 1970 GW in my mind is similar - fly gently! No acro, and avoid high speeds in rough air. It sounds like you will be FAA legal with or without the ballast. Yes, below minimum fuel for a 210 you will be out of the forward limit. Realistically, you will be landing with less than 17.5 gal of fuel on board, so if landing with a forward CG presents a problem, I guess I would install the ballast. I am amazed, many old time Swifters don't know, or appreciate the requirement for minimum fuel.

Above all use common sense. Know what the regs. are, but also know why. It sounds to me like you have a pretty good handle on this. I hope that we Swifters can by using common sense avoid a situation like the Cessna 150 with the 150 and 180 hp conversions. By AD note, they are pretty much restricted to being a single place airplane. I hope I didn't gloss over something, if I can elaborate, write back. -- Jim

PS - I'm adding this in case Denis uses it in his newsletter. The FAA formula for minimum fuel is 1/2 pound per horsepower. So for 210 hp its 105 lb.or 17.5 gal. (actually, it's METO hp [maximum except take off], so a IO-360A which is only rated 210 hp for 5 min. has a slight advantage here!)

PSS - I reread your letter. I see you have determined you will be within the fwd. CG limit when below min. fuel. I really doubt if you'll see much difference with or without ballast. Many 210 Swifts are out of fwd CG limits below min fuel. Also, I glossed over the question of 32" or 33" being preferable. I guess I would like 33. Did I leave anything else out? -- Jim

TWISTED SWIFTS CONTINUED...(7299)
From: SteveWlson@aol.com
Subject: Twisted Swifts
First let's establish up front... I am talking about nearly original airplanes; not big engines, etc.

One of the most interesting conversations I had at Athens this year was with Mike Jester. He and Mike Williams ("The Two Mikes"), have probably weighed more Swifts than anyone else. Jester is full of information, but unless you ask, he doesn't say much! Jester said there is a tremendous difference in the location of the datum relative to the center of the landing gear axles, like 3" or more!!! He has a theory about the speed of specific airplanes relative to the inches aft of datum where the wheels are located. I can't say that I follow that, but I would like to pursue this to find out if there is validity in his theory. Anyway, is it true and if so, why, what is ideal, what accounts for the difference, and more? Either one of you guys run into this difference? Maybe unless you were calculating the CG (weighing airplane) you might not notice. Hmmmm... Comments??? ...Steve

Steve,
Well, the datum is a fixed point. (the firewall) The differences are in the gearbox bulkheads which allow some difference in the location of the axles and the wheels. (the weighing point) Perhaps if the airplane was weighed on the jack points..... Nah.... someone will find differences there too! I knew an old mechanic who measured every individual airplane with a tape measure. He said no two were alike! (not just Swifts!)

I just use the original figure of +23.5 for the main gear weight. If the measurement is taken, most are 23 & 3/4". Also, the moment change due to retraction of the gear is +157 in. lbs. This I also ignore. If an airplane is out of CG limit the measurement it can be looked at a little more closely, perhaps this will make it OK on paper. I've never seen one 3 inches off. Maybe 1/2" You know my view on using an aft CG, if an airplane is particularly slow, the CG is definitely one thing to check. -- Jim

From: Don Bartholomew <spectro@nanosecond.com>
Subject: weight and balance
Thoughts on CG:
I have learned through experience that if I am going to do a weight and balance, I had better measure the arms. I learned about this the first time I weighed my plane, many years ago. I had a copy of the original weight calculations so I made some assumptions and used what was there. The mains were close, but the tail arm was listed on the documents as around 145". I ASSUMED this was at the center of the tailwheel. It wasn't. The center of the tailwheel was about 159". This made a BIG difference in the calculations!

The arm on the main gear has varied up to .5" on the planes I have measured. This is probably due to 1) manufacturing differences, 2) repairs to or around the firewall, and 3) repairs to the landing gear bulkheads. I have found variations in the tailwheel arm to about .75". The same causes as above with the addition of different tailwheel and A-frames are different lengths.

Now the part that I haven't figured out, yet. Most of the planes I work on here are modified planes (big engines, etc.). Typically I find they have an aft cg and if they have aux tanks, can, if not loaded carefully, exceed the aft cg limit imposed by the gross weight increase. Two particular planes were quite interesting to me. They were both 210 Continental, sticks, polished with stripe, and battery relocated, 150 seats, aux tanks, and similar equipment (weight) in the instrument panel. One had a glass (heavy) cowl and the other a stock (light) cowl. The difference in the empty cg was 2.5" with no lead in the tail of either. The one with the aft cg had the heavy cowl had the aft cg and the light cowl had the forward cg. I spent a lot of time looking at the two airplanes to find the differences, and never found it.

Jim, have you ever weighed control surfaces to see if the installed counter weight varied? It is something I intend to do, just haven't had a chance yet. I suspect some differences at least in ailerons because most I have installed were very overbalanced but some were close to neutral. I haven't noticed it with elevators or rudders, but I haven't looked for it yet either. Take care, Don and Helo

From: Monty747@aol.com
Subject: Re: weight and balance
Don:
All the ailerons I have seen are balanced 100%. In other words, if suspended from the hinge points they will hang level. After s/n 26 there are only two types, the one with the steel rod in the leading edge and the 10/32 screws (which develop the cracks mentioned in customer service report #10.) And the late Temco's which have the weight riveted in. I have a several late ailerons here, but quite frankly, I've never looked inside one to see what the weight looked like. It is important to check the balance upside down so the aileron hangs level. If you try it the other way the aileron appears to be overbalanced. The elevators are somewhat tail heavy, but I can't say how much. And of course the rudder and flaps are not balanced. On the wt and balance, I usually use the factory figures of 23.5 for the mains and 182 for the tail. -- Jim

On to page two of Swift weight & balance questions...