What Difference Does A Bit of Wire Make Anyway?
by Keith Walker
A few weeks ago I finished building my very first sailplane. It is a "Dalmation Lady" which is a very inexpensive and very ordinary 2 meter sailplane. The kit has been in the basement since last November, when I picked it up on special for $15.00. How could I not own a sailplane for that price?
The only modifications I made were to add top spars and sheer webs to the wings, and extend the wing sheeting out to the polyhedral breaks. I naturally had to electrify it and did it by replacing the nose block with a fibreglass fire wall to which is bolted the motor and gear box.. I used a junked "Trinity" car motor that I bought in Toledo for $5.00 and an old "Tekin" low-rate car speed controller which has a brake. I made a cowling from a piece of pop-can tin-plate to cover the gear box. I cut it with scissors to match a paper template. I soldered the two ends together to form a loop and attached it with a couple of screws. It looks great, weighs nothing and took five minutes to make!
The only folding prop I could find was only 10" X 6" so had to select a gear ratio to match the prop to the motor. The motor draws 17 amps on 6 cells, on the ground. I figured that this should give me about six minutes of running time on 6 X 1400 mah in the air at a good steady climb - enough for two good ups!
I have taught five people how to fly using their Goldberg "Electras" and most of the guys I fly with own one or something similar. When I tried my "El-cheapo" I was pleasantly surprised with the results. It flies just as I had planned, with a good steady climb to about 1200 ft in around three minutes. It glides well, too and I am very happy with its performance. The only thing that bothered me about it was why did it perform so much better than similar planes, even those on 7 cells, with better motors and bigger props!
After thinking on this awhile, I realised that there have been a number of occasions when my mediocre equipment has noticeably out-performed other peoples state of the art stuff in identical airframes. I thought about their installations, and what the differences were. Equipment-wise, they should have flown rings around me. I don't use Sermos connectors - I use the standard Tamya ones. I usually use old, low rate speed controllers, even on my Astro 25s. I very rarely allow for any kind of motor or battery cooling. Most of my radio equipment is of "antique" vintage and I do not own a computer controlled transmitter or even one with electronic mixing.
Could the difference be in the length or gauge of wire used between the battery, the speed controller and the motor? I wasn't sure so I checked the AWG tables, pounded on my calculator and sure enough, it doesn't make any difference. Two feet of fourteen gauge wire will only drop 0.1 volt when 20 amps is shoved down it. You can drop this much on a fuse or a pair of good connectors! If you commit a cardinal sin and use 18 AWG hook-up wire, you will only be two and a half times worse. If you shorten it to a total of 10", which is about what I use, it will be the same as the two feet of 14 AWG. You would really have to work at it to screw up here.
Trim and balance can have a drastic effect on the way a plane flies. An old wife's tale says that it's safer to try a new model with the C of G further forward than the recommended position. I have seen nose-heavy planes struggling along in a semi-stall, even though they have lots of power available. I have seen quite a few new planes stall and crash, for the same reason. Why is this? If a plane is nose heavy, the elevator must be raised to try to keep the plane level. Flying with up-elevator causes a lot of drag. This slows the plane down, which reduces the amount of lift. A greater angle of attack is needed on the wings to generate enough lift to keep flying. More up-elevator is needed to make this happen. The extra drag of the elevator and the wings slows the plane down even more. You can see the vicious circle of events, and the ultimate result.
Test your planes with the C of G in the recommended position. At Least you know that the original flew OK like this. If it turns out that it is a little too far back, the elevator and ailerons will be very responsive, making the plane difficult to fly but not impossible. Another symptom of aft C of G is that if you try to climb fairly steeply, you may see the nose slowly bob up and down. If you recognise any of these symptoms soon enough, at least you get a chance to land and correct the condition. Be warned, though, if you continue to fly like this, the plane will snap roll when you least expect it - usually when it is less than one mistake high!
After much pondering, I have concluded that the biggest problem most people have is in selecting the right battery, motor, propeller and gear ratio combination. To do this right, you need at least a tachometer, an ammeter and a bit of common sense. If you don't want to understand how to use these simple tools, at least find someone who does, and get them to do a quick check on your setup. It will immediately tell you what is happening and how your plane could possibly be improved. If you throw up your hands in dismay and say "I don't know nuthin' about that technical mumbo-jumbo!", you missed the point! You don't need to be a whiz-kid. You just need to know a couple of simple facts and how to apply them.
The first fact is that the speed at which a propeller will want to fly is approximately its pitch in inches times its RPM in thousands! i.e.
MPH = pitch X rpm/lO00. Now you know what use a Tach is! You can work the thing out in your head once you have the RPM! In the air, the prop will unload a little, so add about 10% to its static RPM to get its flying RPM. Now you know why a 12 X 6" prop turning at 4,000 RPM will not give a sparkling performance to an aerobatic sport plane that has a stall speed of 25 MPH. You also know that a 12 X 8" folder turning at 6,000 RPM on your 25 MPH sailplane is just eating up batteries and cooking the motor.
The second fact you need to know is that power is amps times volts. At the current levels we use, we can assume that there is about one volt across each nicad cell in the battery pack. Therefore, power = amps x number of cells. This will tell you how much electrical power is being converted in your motor into mechanical energy and heat. Now you know the use of an ammeter! Again, you can do the calculations in your head! In the air, the motor will draw about 20% less current as it unloads, so flying current = 80% of static current. Now you know that if your IOOW can motor is drawing 25 amps on seven cells, you know it is not long for this world! You also know that if your Astro 40 is drawing 10 amps on 16 cells, you need a bigger prop or you might as well use an Astro 05! You must admit, none of the above really strained your brain and you didn't even have to find batteries for your calculator! When you put the two simple facts together, you can find the right sized prop to match your motor and plane. If you can't guess roughly how fast your plane will fly, ask someone with experience. You will know if they have experience because they will ask you what the wing area is and how much the plane weighs. If they can't see your plane, they will also ask you how thick the wings are and what shape the aerofoil is, as well as what kind of performance you want. Then all you need to do is borrow a few props if you don't have any and check the motor current and the RPM for each until you get the results you want. Then you can go out and buy the right sized prop and confidently join me in the clouds.