Daryl Perkins once posted something about towhook position. I’ll search around to see if I can find it. If I remember rightly what the great DP said, you mark a spot under the wing at the c.g., tilt the plane back about 45 degrees, and draw a line straight down. The point where the ring meets the hook should be on that line or a bit in front of it. Note that this puts the hook behind the c.g. To make it work, you need a lotta tension, a hard throw, a quick hand on the stick, and a properly set-up launch mode.
Additional, words of wisdom follow:
These two write ups from Joe Wurts and Daryl Perkins are not new and some of you may have read them before but is always good to compare their thoughts to yours on launching techniques.
Maximizing Your Launch Potential
by Joe Wurts
There are a few items that make for a good launch. We can break it down into three different areas, which I cover here. The first is the aircraft set-up, which includes the launch pre-sets, mixes used during the launch, and the towhook location. The second is the actual launch up to the zoom segment. This includes pre-tensioning, the throw, and the climb while on the line. The third is the zoom itself, which encompasses the dive to capture the line tension built-up during the climb, the pullout, and the free climb to convert kinetic energy (speed) into altitude.
For the aircraft set-up that I use for launch, I have full-span camber (ailerons and flaps down equally) of about 20-25 degrees, along with the maximum aileron-rudder mix allowable. Also with this is the maximum safe up-elevator pre-set that I can get away with, which is dependent on the towhook position. A more forward towhook position needs more up-preset than an aft one. The towhook should be very near the CG, or more accurately, 10-15 degrees forward of the aircraft neutral point for the technically inclined. This is an aggressive set-up which should be used by advanced fliers only (sometimes this is aft of the CG being used!). For newer fliers or those wanting a more docile but lower launch, place the towhook 10-15 degrees ahead of the CG. The idea is to get the maximum useable Cl (lift) out of the wing during the launch.
To develop the up elevator preset, I do a series of launches, each having a little more up preset, until the plane becomes unstable on tow, then back off just a little from this. What I look for is an airplane that goes up the line pulling strongly hands-off. The method used for optimizing the actual launch depends on several variables, wind speed, type of line, and whether there is a retriever hooked up. For all cases a good strong throw is very important. How much line tension you build up before the throw should be dependent on the amount of wind at the time. In general, have more tension for less wind. If there is no wind or a tailwind (Visalia comes to mind…), throw strongly with as much tension as you feel comfortable with. During the initial climb portion try to keep as much line tension as the model can stand in no-wind situations. Conversely, if you are launching into a good breeze, let the wind do the work and use only a moderate tension during the climb (line on the drum = altitude that you don’t get).
As you get to the upper part of the launch, build up as much tension as you feel comfortable with (you and the model both!) in preparation for the zoom. The optimum zoom point will vary with the wind speed. In no-wind situations, the best zoom point is almost over the turnaround, but in moderate to strong winds you want to start your zoom much earlier. In anything over about 12mph of headwind, you want to start your zoom at about 60 degrees up from the turnaround for best results, which is surprisingly early. The amount of dive should be quite short, and most people dive for too long. Along with this, the dive should be fairly steep as you are trying to get the energy out of the line as quickly as possible (drag is high at high speeds).After this quick dive you want to pull out smartly to quickly convert speed into altitude. The optimum pull-out will be at the Cl for L/D max, which turns out to be a rather abrupt maneuver. The best climb is rather steep, 60 degrees or more.
Give this a try. You will be pleased with the results!
by Daryl Perkins
“He who launches highest, WINS.” Every form of RC Soaring competition is driven by the launch. From Hand Launch to F3B to Thermal Duration to F3J, if it is possible to start just a few feet higher than the next guy, then you’ll have a huge advantage. Assuming an L/D of 20/1, a 20-foot launch advantage equates to 400 ft. of forward flight in search of a thermal, before realizing the same altitude. It’s also about an extra 15 seconds of flight time assuming a no-lift situation. Have you ever noticed that top flyers typically outlaunch the competition? How do they do it? It’s a combination of sailplane set-up, and launch technique.
My planes will rotate instantly out of my hand, and will pull tremendously hard all the way up tow. This is accomplished in three ways: towhook position, launch camber, and elevator pre-set.
Towhook position. This is what creates the instant rotation of the model. In order to determine the optimum position for the towhook, we need to determine where the model TRULY needs to be CG’d. I do this by slope flying the model prior to installation of the hook. But then of course, I don’t assume manufacturers will get it right. After I have determined a CG that works for me, I’ll then set my towhook so the TE of the hook is about even with the CG of the model. This may sound a bit extreme for some of you, but we’re talking about optimizing the plane for launch. I could probably recommend about 3/32″ in front of the CG. No more, or you’ll lose performance. Your plane will now rotate, hard!
Launch Camber. We need to create a situation where the wing will pull as high a Cl as is possible. We do this with camber. I don’t worry about drag for launch. Why? We have to overcome the amount of line drag, which is huge. On an F3B launch, we can generate up to 50-60 lb. of line drag in the wind, so I don’t personally worry about drag induced from excess camber. I use camber to produce as much lift as possible, and then I do something different from most: I use the camber to stabilize the plane. What we’re really looking for is a wing that will pull as hard as possible without stalling. This doesn’t just mean tipstall, as some planes will actually stall at the root first (depending on planform). So, for best results, run full span camber, meaning, use the same amount of aileron deflection as flap deflection. If the control surfaces are different sizes, then run equal degrees of deflection: if you run 15 degrees of flap deflection, then use 15 degrees of aileron. It has been said in the past, and is still believed by most thermal pilots today, that less aileron deflection creates “washout” and should make the model more stable on tow. This simply is not the case in practice. You will need to learn to use your rudder on tow, as your ailerons become less effective with more droop. I probably run between 15 and 20 degrees of full span camber. Thinner, lower camber sections can’t carry as much as thicker, higher camber sections.
How do you set your elevator pre-set? That’s the easy part. If your towhook is in the right place, it won’t take any up at all. I’ve actually had planes that were stable with the hook behind the CG, and required down throw for the pre-set. I don’t recommend you do this. If your hook is in front of the CG, you’ll need to add some up. This isn’t something that I can define. It’s totally dependent on the model, the hook position, and your personal trim settings. But I can tell you what to look for. This is where everything comes together. The model should come out of your hand, and give the appearance of going straight vertical. If this doesn’t happen, keep adding up until it does. Keep adding up until it stalls. Be ready for this, and be careful, but you have to go past optimum setup to find out where optimum is. If the plane stalls under a lot of line tension, you have too much up.
You’ll need to work your camber against your elevator pre-set to determine optimum settings for both. You’ll do this by trial and error. If I don’t have a plane that’s pulling as hard as I think it ought to, I keep changing the setup until it’s right. Once it’s right, it’s right for all conditions, from windy weather to downwind launches.
As far as technique goes, practice, practice, practice. Your technique will change in varying conditions, so it would be tough for me to go through it. But once your plane is right, it makes the rest pretty easy. If you can master this, then I guess I’ll be seeing you at the Worlds!