Genesee Valley Aero Modelers
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21 Highview Trail Pittsford, NY 14534 Prez Sez FUEL ORDER HAS ARRIVED SEE PAGE THREE The Prez is missing in action. The e-mail electrons went astray. We will try again next month. At The Field March 12th was a busy day at the field. With the weather warm, the sun out and the wind down there was quite a crowd. Nick spent so much time at the field he was concerned his wife was going to send the R/C police to bring him home. We had about 12 or 13 members enjoying the afternoon. We enjoyed the last indoor flying for the year on March 18th. There was a good turn out. John had his single blade helicopter and Walt’s mighty midget had a few more flights. Eric brought an electric that I think he got at the WRAMS show and did some taxi tests. Max has been coaching a couple of students in a program called "The Wright Stuff". The objective of the competition is to build a rubber powered free flight model. The model with the longest flight time being declared the winner. The two students he coached came in second place. One of the students (Amy) was at the gym with her airplane and enjoyed the afternoon with us. Hopefully by the next Newsletter we will be flying outdoors and enjoying our flying site. Be careful driving in. The driveway takes quite a while to dry out and even if it looks and feels dry you may get a nasty surprise. Speaking of surprises there is only one more month until the "Titusville Terror" returns for the summer. April is our last formal meeting until October. As is customary bring in your winter project to share with the other club members. If we get locked out again, maybe Mark can give us another parking lot demo of his electric helicopter. Remember the trees are coming out of hibernation and they are hungry.  Calendar of Events June 3, 4 Batavia RC Club Fun-Fly July Olean June 24, 25 Ray Edmunds Memorial Fun Fly July 22, 23 RAMS Open House Fun-Fly July 29, 30 Sky Rovers fly-in Aug.13 (Tentative) GVAM Picnic Aug. 19, 20 Great Electric Fun Fly (RCCR) Aug. 25-27 Helicopter Jamboree Sept. 2 Sky Rovers War Bird Fly-in Oct. GVAM Dinner
Hal Stewart, editor When you are looking around for something a bit smaller to build and fly in a gym or schoolyard, don't overlook converting a rubber-powered Free Flight model into electric-powered model. Make a mount for the electric motor of your choice. Be sure the motor is set forward enough to allow the propeller to clear the nose of the model Since the model will be heavier with electric power, use a basic wire landing strut (not the all-balsa stick landing gear struts on many rubber-powered models). Use the stick-type struts as fairings for the wire landing gear. You will also need to add provisions for the battery pack, arming switch and any radio gear—including the speed controller. Unfortunately many rubber-powered model kit plans never locate the center of gravity. To find it, start by balancing the model at 1/3 the wing chord back from the leading edge and adjust during your test flights to trim the model out. You have the choice of buying a kit or building from plans. The model can be a scale model of some airplane you’ve always liked, or it can be simple rubber-powered contest model. Remember, really does not matter what you use for power, as long as the model balances properly and wing loading doesn't get excessive. Rubber-powered models were usually designed to be lighter than their gas powered counterparts. Give it a try and have fun.
Basics of Electric Flight by Pat Tritle I really enjoy getting together with clubs and speaking to the group about the basics of electric power. However, because there is so much information that needs to be passed along, it would be difficult, if not impossible, for those attending to remember much of the pertinent information. For that reason, it's better to write up the basic guidelines so that those who are interested in getting into electrics would have the information available for reference at a later date. Here goes. I'll keep the numbers as simple as possible to avoid unnecessary confusion. OK, here's how it all shakes out. The basic power required to fly an electric model is as follows: Direct Drive Systems 60 watts/pound Gear Drive Systems 50 watts/pound Mild aerobatic performance 70-80 watts/pound For all-out aerobatics 100-110 watts/pound 3-D performance 150 watts/pound or more The above numbers are based on models with wing loadings from 8-16 oz./square foot. As with gas models, higher wing loadings require more power since they must fly faster to support the added weight. By the same token, a lightly-loaded model with a wing loading in the 3-5 oz./square foot range will fly very well at 25 -30 watts/pound. What's a 'watt'; and where can I get some? Wattage is the term used in electric flight to relate the level of power that an electric drive system will produce. To relate it to terms we're familiar with, 746 watts = 1 horsepower. To calculate the wattage delivered by a given system looks like this: amps x volts = watts. So where do these numbers come from and how do I know how many volts and amps are needed to fly a given model? Okay, let's say you want a mildly aerobatic sport model with a 14 oz/square foot wing loading that will weigh in at 2 pounds. We already know that the power requirement for a model like this is about 70 watts/pound, so we're going to need to generate about 140 watts. Let's assume that you are going to use an eight-cell Ni-Cd battery. At 1.2 volts per cell, eight cells will deliver 9.6 volts. To arrive at the necessary current draw to achieve 140 watts, simply divide 140 (watts) by 9.6 (volts) and you arrive at 14.58 amps. Now, let's assume that you have a three-cell Li-Poly battery for the model, which is rated at 11.1 volts. The formula is the same; 140 (watts) divided by 11.1 (volts) = 12.6 amps. As you can see, as the available voltage increases, the lower the current draw needs to be to deliver the necessary wattage. Now here's something to consider when selecting your system: the higher the current draw, the shorter the flight duration on any given battery. Therefore, the ideal setup would be to use a higher-voltage battery with lower current draw for maximum duration. On the downside, when using Ni-Cd and NiMH batteries, as the cell count goes up, the weight will increase significantly as well. It works that way with Lithium too, but Lithium batteries are dramatically lighter then the old "round" cells. Okay, let's say we're going to use an 11.1 volt Li-Poly battery. All we need to do now is select a motor that will swing enough propeller at 12.6 amps to fly the model at a top speed of around 40-45 mph and we're in business. Now that you know the parameters, visit your local hobby shop and select a motor that fits that description. Gear Drive vs. Direct Drive: Why is one better than the other? Well, it all depends on the kind of performance you're looking for. If you're looking to go fast, go with direct drive. Going fast requires a high-pitch propeller turning high rpm. The formula to calculate propeller pitch speed is an easy one; it looks like this: rpm x pitch (in inches)/1056 = mph. Let's say that you are turning a 7-6 propeller at 14,000 rpm. 14,000 x 6 = 84,000/1056 = 79.55 mph Now, let's assume you are setting up a slow, relaxing park flyer with about a 5 oz/square foot wing loading. If we swing a 9-7 propeller at about 3,500 rpm, we'd be looking at a top speed of roughly 23 mph. To swing that much propeller with a small, light drive system, we would use a gear drive unit at a very low current draw and a small, light battery. Again, to make a known comparison, we can relate all this to riding a 10-speed bicycle. A gear drive swinging a big propeller is like riding your bike in low gear. You pedal like mad with little effort, you don't go very fast, but you can climb steep hills with ease. The direct drive system could be compared to riding the bike in high gear. It'll really go fast, and even though you're pedaling slower, it requires considerably more effort. What all this boils down to is "propeller disc loading." We all know what wing loading is: it's the amount of the model's weight that each square foot of wing must carry. Prop disc-loading works the same way. A large propeller will be more lightly loaded, thus delivering more torque then a smaller propeller turning high rpm. The tradeoff, of course, will be speed. One more thing to cover and we'll give you a rest. Batteries are rated in "voltage" and "amperage." Voltage dictates the amount of power the battery will deliver. The amperage rating dictates for how long the battery will deliver that power. To relate that to glow fuel, consider the voltage as nitro content. High voltage (nitro) means more power. The amperage is related to the quantity of fuel, or simply the "size of the tank." To figure the size of battery needed, let's go back to our 140-watt sport airplane. If we're pulling 14 amps from a 1400 mAh (1.4 amp hour) battery, we will have full power duration of five to six minutes. In the real world, with proper throttle management, you'll see flight times of approximately eight minutes. These are common flight times, even with liquid-fueled models. To arrive at that number, divide the battery amp rating by the current draw: 1.4 (amp hours)/14 (amps) = 0.1. Then take 60 (minutes per amp hour) x 0.1 = 6 minutes. Now, to double the duration, you must either cut the current draw in half (to 7 amps), or double the battery size (to 2800 mAh or 2.8 amp hours)—again we see tradeoffs. To reduce the current draw, we can use a larger, higher-pitch propeller turning slower with very little weight penalty. If we double the size of the battery capacity, the weight penalty is quite high unless we go over to the new Lithium batteries in which we will discover we have benefited from a tremendous weight reduction, but at a higher price then conventional batteries. Okay, I promise I'll quit before we all end up in "system overload." Once again, there's a tremendous amount of information here for a newcomer to electrics to digest, so let's do this: if you have specific questions about setting up an electric model, please feel free to drop me a line and I'll do what I can to steer you in the right direction. For now, I’ll offer up one last piece of advice. To get started, work with a known good design, and use the recommended equipment that has been proven to work. Talk to the people who are successful and copy what they're doing. The one thing I do know about modelers is that they are always willing to share their knowledge with those interested in what they are doing. Contact Pat at patscustommodels@aol.com 
G.V.A.M. Newsletter
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