25 August 2007

Some updates from too shy to post Simon Nelson

Young Ryan Nelson was given a chance to make reserve for Junior team for the forthcoming F3J worlds by attending the recent F3J league event and achieving a score of 80%, which he did by a whisker on 80.6%! Well done to the Nelsons and long time since the KZN lads have been so close to the world arena.

The event info here:

Recent success by cheapie air instead of tedious road travel prompted Dave to influence Simon to give this a go, which meant lightweight box making, air travel with a visit to the cockpit and the adventure of teaming up with the Goodies for the competition and accommodation. Team Goodies tend to keep the line makers in business, thus the surplus line pic at the bottom..... ;-) The Nelson's are indebted to Craig and Michelle for their kind help and hospitality.

Over to Simon -

Stuff for the Blog.

The line we threw away, "reel, reel, give me a reel, my kingdom for a reel.....!!!!

Ryan in the Captains seat, 737 - 200, 24 years old plane. Could not fine the tow hook!

The ' Box' 5 mm correx, 1.6 mm angle, aluminum, about 350 rivets, and a aching arm, about 3 hours to do. With 4 planes inside it weighs 24 kg, max is 25kg. Size is, (big end) 400 x 400, small end is 400 x 300, big end is for the fins and a few tools etc.


Simon was stunned by the tension and significant launch flap angle (20mm at root) that Craig Goodie used on launch and also sent this interest article on the subject - I will remove if any copyright dramas:

Analysing Martin Weberschock’s Launch setup
Marcus Stent

This is basically my opinion on how Martin Weberschock’s sets up the launch mode of his F3B glider. This was deduced by listening to his lecture in 2000 and applying my learning from the XFoil program (Profili 2). Please feel free to make comment on my observations…

Step 1. The tow hook for Launch Mode is not dependent on where the C.G. is, but rather where the Aerodynamic Centre of the model is. This is because during launch we add an external force to the model (the winch) which adds probably 50Kg of force to the Model. This force needs to balanced with the Lift force of the Wing, so we get a smooth launch, and this Lift force is generated at the Aerodynamic Centre of the model. The fact that there is a 2Kg model stuck in between these two huge forces is just about irrelevant, and weather the 2Kg mass is 5mm forward or back is going to have even less effect.. The mass of the model is just along for the ride. As you can see the relationship between the line tension (set by the tow hook) and the lift of the model (located at the Aerodynamic Centre) are the most important factors.

Step 2. What the C.G. does do to the launch is to change the elevator trim position that you fly with. e.g. Lets assume that you are flying in Thermal Mode and you have two identical models with a fixed tow hook and flap settings. Then you set up Model 1 in Thermal Mode to have a forward CG of 30% and therefore some up trim (e.g. +2%) is required in the elevator to compensate for the forward CG. Model 2 is set up with a rearward CG of 40% and therefore you have some down trim (e.g. -2%) to compensate for the rearward CG. Both models fly around in Thermal Mode very nicely ‘balanced’.

So then what happens during Launch mode is this: Model 1 with its up trim (+2% from the Thermal Mode) during launch (because we use the same elevator settings for our Thermal Mode as we do for the Launch Mode, don’t we!) so the plane pulls hard and wants to tip stall and every one yells "move the tow hook forward!!". Model 2 is exactly the opposite and the down trim in the elevator (-2% from Thermal mode) causes the plane to launch very flat and every one shouts "move the tow hook back!"

What this shows is that two identical planes can end up with two different tow hook positions depending on where you set the CG, but what we learnt from the Step 1 is that the tow hook should be set relative to the Aerodynamic Centre of the model and not the CG. So how do we fix this? I am glad you asked...

Step 3. What we need is an independent elevator setting that gives us the best balance for Launch irrespective of where your elevator trim is during Thermal mode.
We start by looking at the theoretical polar of an airfoil with a high flap deflection (15 degrees) and you will see there is only one optimal Lift position and this corresponds to a specific angle of attack. Too much angle of attack or too little angle of attack and the Airfoil generates lots and lots of drag. So if we can find the optimal angle of attack (elevator pre set) for this flap setting, then we will have the optimal launch.

To do this, fly around on the slope with your model in Launch Mode (Launch flap set at say 15 degrees) and adjust your Launch Mode elevator preset (or angle of attack) until the model flies around without stalling or diving. By doing this you have found the best angle of attack for the airfoil with 15 degrees of Flap. This new elevator setting you have discovered is making the wing work at its optimal angle of attack.
What you are doing is matching the practical behaviour of the model to the theoretical polar. eg Because you have found the best angle of attack (elevator preset) by flying around, you must have found the best angle of attack on the theoretical polar. Too much angle of attack and the plane gets draggy and stalls, too low an angle of attack and the plane dives and loses height, just like the theoretical polar.

You can then play with different flap settings and find the optimal elevator preset for each one. Then you will have a table of Flap settings with their corresponding optimum elevator presets.

Step 4. Now you can take a model that you know has the best setup for that particular flap setting (Flap and elevator setting from Step 3) and apply the 50 Kg of Launch loads to the system. Now, it is a matter of matching the tow hook position to the Aerodynamic centre of the model (remember Step 1) by moving the tow hook back. As you move the tow hook back you will eventually find the spot that lies under the Aerodynamic Centre and the model will be balanced and you will get a smooth launch. What you also know is that at the same time the combination of the Flap and the elevator preset that you discovered in Step 3 is giving the most efficient aerodynamic Lift and this should result in the best launch.

In Conclusion: We are using the same principle here as we do when we change into camber mode when we are thermal flying. e.g. You are flying around with a nicely trimmed model and then you 'flick' into Camber mode which gives you 2mm of thermal flap. What happens? The model pitches up, stalls and comes down quicker, until we add a little down elevator pre-set and find the new 'optimum point' between the flaps and the elevator.

In a nutshell: "Finding the optimal Launch setup is like finding the optimal thermal setting, the only difference is there is 50Kg of concrete hanging off the bottom of the plane in Launch mode" In both cases you want the best combination of flap and elevator setting to keep you up the longest.

All the best with your launch setup.

Last but not least, the recent F3F talk prompted Simon to remind us that the Griffin has been the beastie to beat, locally, in past years and is living testimony to the adage that there ain't no substitute for ballast......

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