Seawind Gross Weight Determination
Now, whether your flying, or (especially) during the building process, read this.
Don't let that giant roomy Seawind cockpit fool you. Even though it is a spacious water craft, your Seawind is still an airplane. You can pack a lot more stuff in there than can be safely flown. Your weight and balance planning needs to start early in the building process, not when your friends and/or loved ones are standing there waiting for a ride. How much fuel? Can aux tanks be used (ever)? What constitutes a full load? How far can I go with the allowable fuel on board? You may be surprised. Read on.
Dangerously, some owners are increasing the gross weight of their Seawinds. Builders should understand that this aircraft was tested at 3200 lb. gross. There is no evidence or documentation to support that testing at any heavier weight has ever been done. The mere fact that some may seem to be getting away with it does not make it a safe practice. Most of the builders who are increasing their gross weights more than likely have (or hire) the engineering background, and the piloting skills to do so. Their analysis can only apply to their own airplane.
In about 1990 or 91, the empty weight of the first production Seawind was estimated by SNA to be 2050 to 2100 lb. This estimate was raised to 2300 pounds after they weighed their prototype in 1993. At the time, the unfinished prototype weighed 2450 lb. Logically, they estimated the production aircraft would perhaps weigh less. This empty weight did not include options, radios, paint, interior, etc. It did have the carbon graphite longerons. The currently projected empty weight is still about 2350 lb. This would apply to a completely stripped down, VFR only, bare bones plane, but to date, none have come in that light. Everything you add to your Seawind, including inadvertent or intentional extra reinforcing (perhaps a good idea in some cases) adds to this weight.
The first Seawind built from a production kit weighed 2740 lb, the second: 2710, third: 2720, fourth: 2700, fifth: 2750, etc. (see weights below). All Seawinds exceed 2350 lb empty weight. The tested gross weight remains at 3200 lb.
The current SNA specified gross weight appears to be 3400 lb. for take-off, but not more than 3200 lb. for landing. They have implied that you can use 3400 pounds for an actual gross weight. Donít do it. The tested gross weight is still 3200 lb.
Gross weight of an aircraft is determined by many factors. Some of which are derived from structural engineering calculations. The airframe structure is designed to withstand a given amount of weight, then tested for that weight. Wing tests by SNA were performed using 3200 pounds as a gross weight. The wing was designed for a 3.8 positive and 1.5 negative load. The landing gear legs, axles, trunnions, gear pockets, cross over braces, wheels, brakes, etc. were all designed for this maximum capacity.
It may seem simple for the builder to change the little tag on the airplane to a gross weight of 3600 or 3800 pounds, but it is just not a safe practice without the appropriate design testing.
Useful Load Determination
Useful load is the difference between empty weight and rated Gross weight. There is always some uncertainty about empty weight and what is included. Its actual value depends on what you had in the airplane at the time it was determined. Obviously, anything you add to the airplane after the empty weight was determined must be included in useful load.
If the average empty weight of the Seawind is about 2720 lb., how much can you carry? First, if you are flying alone and weigh less than 200 lb., 50 lb. of nose ballast may be required. This can include an anchor and tie downs as long as they are stowed in the nose compartment. Never overlook the need for ballast, this airplane cannot be flown if you are out of your aft CG limit.
Determining how much you can then put in the plane (for short flights) is as follows: 3200 Ė 2720 = 480 (useful load) 480 - 50 (for ballast) = 430 lb. If you weigh about 180 lb, this leaves 430 pounds for everything else including fuel. You can take 70 gallons. (Note, this is NOT even full main tanks.) You certainly could not put in another person nor any auxiliary fuel unless you plan to fly long enough to get back down to your specified gross weight for landing.
Lets run one hypothetical example: Two people. Lets say you weigh180 lb and your wife weighs 125. An anchor, tie downs, and charts are onboard, about 20 lb. You can remove the ballast in front since you are carrying a passenger. You're going away for the weekend and carry 50 pounds of luggage for the both of you. (I donít know about you, but I canít get my wife to pack that light.) Let's plan on flying off enough fuel so that we will be within our gross weight for landing. Your airplane weighs about average, 2720 lb. Using our guidelines above, let's use 3400 lb for take-off and no more than 3200 lb for landing.
How much fuel can you carry? About 305 pounds or 50 gallons. How far can you fly with 50 gallons? You need a 45 minute reserve of 12 gallons left in the tanks upon landing, so lets put that aside. We now have 38 gallons of usable fuel. Burn for the first hour including taxi, take off and climb would be 20 gallons. Your ground speed would be an average of 125 knots per hour. (considering you climb to 5000 feet) Subsequent hours are at least 15 gallons per hour if leaned correctly. With a ground speed of 147 knots, you can travel about 300 miles. This is probably not what you thought, but it's real. Live with it and DO NOT raise the gross weight. Then, do not over fly your reserve.***
I will leave it to you to manipulate the numbers for your personal situation. I urge you to set up a spreadsheet and run lots of numbers and combinations. There are a number of weight and balance spreadsheets available from ISPA members. Start your W&B planning now.
This exercise may affect your choices about some of the options you want in your Seawind. For example, because of this information and a couple of the entries in the Seawind incident log, I am questioning whether it makes any sense for me to include the added fuel system complexity required for auxiliary tanks in my Seawind. Iím sure this may make good sense for many of you, but for me, it looks doubtful. I leave it to you to draw your own conclusions from this data. Be a smart builder / pilot, not a dead one.
Note, the lightest known Seawind, N23G was built by Ken Wheeler (not from an SNA kit). It contained large amounts of carbon fiber and other lightweight materials. It doesnít make sense to include its weight of 2449 lb here. It's just interesting.
The ISPA makes no claim as to the accuracy of any of this data. Use at your own risk.