By: Anthony McGee
One question that prospective new Challenger owners usually ask is “If I weigh 200 pounds and my friend weighs 180, can we fly a Challenger?” Well, the actual question is, “What is the real-world useful load?” I believe the reason this is asked is that many airplane companies stretch the numbers a little to make their plane look more appealing. Quad City Ultralight Aircraft advertises the useful load on a long wing Challenger II as 500 pounds.
So, is this a realistic useful load? This question has as many answers as people who ask the question. Let's take a look at how Maximum Gross Weight is determined in order to attempt to answer this question. We'll also look at a real world example to give you some possible alternatives to having low useful loads.
Some Information to Consider First
Quad City says the Challenger II has an empty weight of 300 lbs. and a maximum gross weight of 800 lbs. They also say the G rating is +6 and -3. When the Challenger was first designed, this was a reasonable empty weight and maximum gross weight. Since that time, over 20 years ago, many optional upgrades have been made available. Some of these upgrades actually help the performance of the aircraft, but others only help make it more eye appealing to some people or make the pilot and passenger more comfortable. No matter what they help improve, they all have one down side to them: they ALL increase the weight of the plane. As a result, most people are building planes with an empty weight ranging from 400 lbs. to over 500 lbs.
I have even heard of some with weights over 600 lbs. The result of the increasing empty weight is a decrease in the useful load. This decrease would result in a very small useful load for some of these aircraft. We need to keep in mind that useful load includes pilot, passenger, fuel, mixing oil, and any other thing that is added to the aircraft from the time the initial weight and balance calculation to the time the flight takes place.
How Does the Designer Determine Maximum Gross Weight?
The maximum gross weight of an aircraft is determined by considering the structural strength of the aircraft and the desired G rating. What is a “G rating?” We all experience the effects of gravity every day. Under most circumstances we are in a 1G condition, or 1 times the force of gravity. This is what gives us our weight when we step on a scale. We also experience G loads over 1 or less than 1 in certain circumstances.
When we step in an elevator and it starts up, we get a feeling of being heavier than we are. This is an increased G load of maybe 2 times the force of gravity. When the elevator stops, we experience a weightless or less than normal weight feeling. This is a decreased G load of, say, 0 times the force of gravity. We can also experience G forces in all other directions as well. They are not limited to up and down.
Aircraft are constantly being exposed to increased and decreased loads during flight. Every time the plane climbs, it experiences a momentary increase in G's, and when it stops climbing or begins descending it experiences a momentary decrease in G's. The aircraft can also experience changes in G's during a turn or when the nose is moved back and forth using the rudder.
The aircraft is also exposed to G load changes during turbulence. This is why we feel all of those “bumps” in the air. With this information in mind, the designers of aircraft place a maximum positive and negative G rating on aircraft. If this rating is exceeded, then structural damage may occur even if it's not visible to the naked eye.
To determine maximum gross weight and G rating, the designer tests the structure to determine what load the structure can undergo without damage. With the Challenger II long wing this load happens to be 4800 pounds positive and 2400 pounds negative. The designer then takes this maximum load number and desired G ratings and determines maximum gross weight. If you take the 4800 lbs. positive load and divide it by the positive G rating of 6G's, then you get 800 lbs. If you take 2400 lbs. negative load and divide it by the negative 3G rating, you get 800 lbs.
A real world example
When I built my Challenger II, I faced the same concerns that many other builders face. I knew from information I had gathered from the FlyChallenger list that my plane would probably weigh around 450 to 475 lbs. This weight presented a slight problem with useful load. When it was all said and done, my plane weighed 472 lbs.
If I took the 472 lbs. for the plane, 60 lbs. for 10 gallons of fuel, .8 lbs. for the mixing oil, and 155 lbs. for me as a pilot, that gave me a flight weight of 682.8 lbs. This only left 117.2 lbs. for a passenger or anything else that I wanted to carry with me. Looking at these numbers, I knew of only 1 or 2 people that could fly with me if I was going to keep the gross weight below 800. We all know that flying over gross weight is both illegal (FAR 91.9) and not wise. This started a search for a solution.
Aside from building a light plane, there is another option to achieve a reasonable useful load. Under the FAA's Amateur Built Experimental aircraft guidelines, the manufacturer (the builder) sets the gross weight of the aircraft. With this flexibility you can increase the maximum gross weight to more than 800lbs. Although the builder can assign any weight he feels appropriate, when doing so he should keep in mind the original design load and the G load factor used to determine the designed gross weight.
As you increase your gross weight, you must decrease the G rating of the aircraft to keep from going over the design load of the aircraft. With this in mind, let's go back to the information provided earlier about G rating, gross weight and design load.
If you are willing to reduce your G ratings and use more precautions in rough conditions, then you can increase your gross weight. Let's look back to the example airplane. The gross weight was 800 lbs. and the empty weight was 472-lbs. and left room after pilot fuel, and oil for 117.2 lbs. of passenger. If I were to increase the gross weight to 900 pounds that would give me room for a 217.2-pound passenger, which would give more flexibility in passengers.
I knew that if I lowered the maximum G ratings for my plane, that would increase my gross weight. If you take the positive design load of 4800 lbs. and divide it by the new maximum G rating of +5, that would give a gross weight of 960 lbs. If you take the negative design load of 2400 lbs. and divide it by 960lbs, you get a G rating of -2.5G's.
This change leaves a fairly large margin of safety in turbulence and also gives some increased utility since there is now a higher gross weight. This change would now give me room for a passenger who weighed 277.2 pounds and would allow me to carry anyone that I know who would want to fly. Increasing the gross weight does not come without its drawbacks though.
The Down Side to Increasing Gross Weight
Like anything else in aviation, a gain in one place means a loss in another place. There are some things to consider when raising the gross weight. Among those things are decreased performance, increased fuel consumption, possible center of gravity problems, and, of course, decreased G ratings.
Because of the higher gross weight, the take-off distance, landing distance and stall speed will be increased and the climb rate will decrease. The pilot will have to be aware of the G ratings when flying in turbulence and when doing “yank and bank” flying. The pilot will have to remember when flying at the increased weight to slow down more in turbulence and be easier on the controls. He will also need to remember that stall speed will be higher and it will take more distance to take off and land.
He will also have to pay more attention to the center of gravity. The Challenger has a wide center of gravity envelope, but all of the weight that is placed in the cockpit moves the CG forward. With higher payloads, you may go past the forward limit for center of gravity. The good news is that if you are flying solo, then you are most likely below 800 pound gross weight flying a plane with a designed +6 and -3G range.
Ultimate G rating and working G rating
Something not mentioned thus far is the difference in ultimate G rating and working G rating. When you compare the G numbers of a Challenger to a production aircraft (Cessna, piper, etc), you are not doing actual apples-to-apples comparison. Production aircraft are rated in working G rating while the Challenger numbers are ultimate G rating. Working G rating has a safety margin of 50% figured into it, where the ultimate G rating is where damage will occur. To get an apples-to-apples comparison you must multiply the Challenger G numbers by .67. Whatever maximum gross weight you choose needs to be chosen by using common sense and carefully evaluating the risks associated with it.
One Last Thing
One last thing to think about is the information that you put on the Aircraft Data Plate. With the information provided above, you could safely fly the Challenger above the factory recommended maximum gross weight if you take proper precautions. So, what maximum gross weight will you put on your weight and balance papers, and what maximum gross weight will you put on the Aircraft Data Plate?
If you are ever planning on flying over 800 pounds, the increased weight needs to be put on the Aircraft Data Plate and weight and balance papers. According to 14 CFR 91.9 Civil aircraft flight manual, marking, and placard requirements.
I mention this to say that if you have 800 lbs. on the paperwork and Aircraft Data Plate and are flying at 801 lbs., then you are actually flying illegally. The chances of ever being ramp checked are small, but I would rather be safe than sorry.
I hope I have provided some useful information for you to consider when deciding if the Challenger is the right aircraft for you, or for you to consider when preparing your airworthiness application papers. This is not meant to be a suggestion, but is provided to help explain one possible option to the useful load issue. Always keep in mind that the best solution to the problem is to build the plane lighter. The lighter weight has several advantages, but some of us are willing to trade some performance for some bells and whistles and better looks.