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aeajr
477 Posts |
 Posted - 07/30/2010 : 08:11:03 AM
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“Ooh! Did you see that I’m in lift!”
by Gordy Stahl
So many times I’ve heard newer pilots exclaim that phrase, only to watch them circle to the Earth.
Here’s the scenario: The model is flying along then suddenly shoots its nose upward. Must be lift, after all the nose went “up”! Really? Did the nose go up, or was it lifted? It’s not semantics, but it is a clue. 
I’m not sure why it is that most guys don’t understand what does what on their sailplanes, I suppose they chalk things up to the mystical world of aerodynamics,. But if they would take a minute to really think about what the parts of the model are for, identifying lift would be a whole lot easier…and flights would be long…on purpose.
Of course the context of this information is task flying. Task flying is different than flying around on the weekend. That’s just flying around, if you have a set time to make, its completely different. Guys say, “ I put in an hour flight this Saturday”, yet if someone were to put a stop watch on them and ask them to get 6mins, likely it wouldn’t happen in 6 attempts….because they don’t actually know how to identify lift, and of course don’t know how to stay with it since they’ve never found it on purpose.
I’ll start with the question I ask all on my travels…..how could a thermal lift the nose of your model? I mean out of all the pieces of your model, other than the tail boom, the nose has the least amount of flat surface for a thermal to push up on!
So I’ll ask you,,,. How can a thermal push the nose of your model up. And don’t bother with some lame answer that the thermal lifted the wing so the nose went up, because it wasn’t the wing that went up, it was the nose.
Okay since its not possible for any thermal to push up on the nose, then possibly something pushed down on the tail.
And this is where I ask you, what directs the nose of the model in flight? Not ailerons because they don’t change the direction of the nose. The nose stays in the same direction while the wings rotate around it.
And rudder doesn’t make the nose go up and down. So guess what, here’s where you get to engage those little grey cells. The tail directs the nose. In the case of the nose suddenly shooting upwards, the tail got pushed downwards….by cold air dropping. The forward movement of the model followed the nose. The plane DID go up, but it wasn’t lifted.
Okay when the nose of the model leaves level flight on its own and shoots upwards, did the model find lift or sink? I know you didn’t answer lift!
Lets talk about the word UP….in the context of lift. Would you prefer when the CD called for a 15min task, that your whole model rose vertically (up) or the nose shot up?
You see one is lift, one is direction. Whether you can see the tail go up or down, you can certainly always see the nose go up. DON’T turn! Now if the nose went up really dramatically, chances are there is lift near by, because the sink was pretty strong.
Now we get to talk about the rudder. You know that left stick, you never move. The only surface on a competition sailplane that will add a minute or more to your task time…yep that’s real. Sounds like a pretty important surface for you not to be working on learning more than anything else you do on a Sunday.
The rudder is a tiny half surface (no left and right half like aileron or elevator) The Rudder is located wayyyyy in the back of the model, stacked behind just about everything..
Time for you all to learn a chant….”Airspeed empowers tail feathers”….chant that while you are shaving and driving till you get it. You see if you hold your model, or teeter it on the bench, you can move your rudder and elevator and nothing will happen…the nose won’t move. In order for those surfaces to direct the nose, there has to be airspeed, air moving over those surfaces. In the case of that half surface (the rudder), it needs a LOT of air in order for it to have any power…to move the nose. Remember its pretty small and located way in the back, stacked behind stuff.
You can quit reading now if you don’t want to actually learn what the rest of us guys who manage to get our times figured out….likely at this point most of you have glazed over and are thinking about anything other than the rudder and the forces that make it the most important surface to making your task times.
The rest of you, here’s the deal…the big secret, the way most of us guys know we just passed into lift, not through it. Airspeed empowers tail feathers, and since the rudder needs more air/energy than the other surfaces, the only way you can get the rudder to make the plane move is for it to have a lot of airspeed! We have only one way to get extra airspeed, and that’s to turn on our motor….lift…. rising energy that causes the models nose to point down, allowing gravity to pull the model forward (airspeed increase), allowing the wing to do its job.
We have already determined that when the model enters sink during level flight, the tail gets pushed down, (cold air dropping on it), and the nose points up from what was directed level flight (directed because the elevator trim was holding the model level) the model will slow down.
What happens to the model’s airspeed when the tail is lifted? I can’t believe you are hesitating with your answer!!! Its not a trick question….gravity pulls on the model’s nose and the model speeds up. SPEED - both lift and sink change the model’s speed.
Sink causes the tail to be held down from level flight (your local expert will tell you that you have a tail heavy model and should add some nose weight…argh!!!!)
Rising thermal energy causes the tail to be lifted from level flight (IF you don’t have the nose full of lead on the advice of the local expert, causing your model to have to be flying around all the time with up elevator trim making it nearly impossible for rising energy to push up on the tail!)….soooo in a way he’s right! The model IS tail heavy….not from weight though, but from that up trim making the tail “heavy” with down force.
Clue here! Lift and sink are indicated by airspeed changes, but here’s the catch….so is a crooked model indicated by airspeed changes! A poorly balanced sailplane with too much nose weight will have to be flown with up trim (incidence), as soon as the air speed of the model rises, the elevator up-trim will direct the nose upwards….pretending to have indicated sink. As soon as the model slows, that up-trim will have less ability to resist the pull of gravity on the models nose, and the model will drop its nose, picking up speed….pretending to be in lift.
Okay we know that a plane that has nosed up just hit sink, we know that a model with its tail hanging down is not tail heavy, its flying in sink. We know that nose up is not what we need to get our task time; we need the model going up. We know a model hitting sink slows, and that a model speeding up….has entered lift…the rising part of a thermal.
Airspeed empowers tail feathers. In order for your model’s rudder to make the model’s nose to move it needs lots of airspeed. Not a little.
Another clue: The rudder like the other surfaces causes drag. If the model hits sink, its tail droops, its airspeed drops way down, the wing’s angle of attack is high….all the ingredients for a stall. Moving the rudder while in sink will cause the model to stall, roll to one side and circle downwards.
Here comes that oh so important rudder! IF the model is in sink, moving the rudder right or left, will cause extra drag. The loss of airspeed will cause the nose to circle downward. Get it? You think the model is in lift, so you test your guess by moving the rudder and the model begins to circle downward….DON”T finish that turn!
If the model is in neutral air, moving rudder will have no effect, no stall, no yaw….no circle. Don’t try to make the model turn!
IF the model is in lift, the rudder will be supercharged, the model’s nose will snap around, and the model will circle UPWARDS! Keep turning!
There was a clue about the importance of having a properly balanced task sailplane back a bit. You balance a task sailplane so that it communicates lift and sink, not airspeed changes…. You don’t balance a task ship to make it feel like the last piece of junk you just got rid of, not because it feels right to you, not for some lame stability factor.
You balance a task sailplane so that it tells you the truth about lift and sink. Not to indicate the smallest fart of lift - not for some aerodynamic advantage….you balance a task sailplane so that it tells you the truth about lift and sink. Communication - not performance or stability. Task sailplanes are set up to help the pilot read air, not to make the model “right”. The pilot is supposed to be doing the “flying”.
So how can you identify lift and sink? Watch your model for airspeed changes, if it speeds up…move your rudder immediately, if it circles upwards, keep turning. If you see the model’s airspeed change, try the rudder FIRST. (by the way, rudder/aileron mix has nothing to do with the rudder for identifying thermals….its used for keeping directional control during launch for the most part.)
Got it? Get your Task model balanced for task work first. (by the way, the easiest and most accurate way to check balance is to throw your model flat and level with some energy. It should NOT nose up, the whole plane should rise while staying level, airspeed empowers tail feathers!).
Once balanced for task work, then learn its normal hands off level flight speed. Then watch your model for any airspeed changes from its normal level flight speed. IF you see the model speed up, try the rudder, if the model circles upwards, TURN! If it does anything other than circle up immediately, don’t turn! As soon as you see the speed come up…hit the rudder, test for a thermal!
So let’s say you put your model up on the winch or high start and as soon as it comes off, you put it into level flight, and it looks like its flying pretty fast…try the rudder! If it comes off, and you get it level and it seems like its flying slow…try the rudder!
I am now going to give you permission! You can try the rudder up to 100 times per 10 minute flight.. Trying it any amount of times below 20 times will doom you to always missing your times.
Here’s a tip about how to find lift off the launch….thermals pull cold air toward them. IF there is wind in your face during the launch, what is the chance that there is lift in front of you? Pretty slim, you think? J. IF the wind is pulling to the right of its average direction – head right!
Balance your full flying stab models for task work, become the pilot that model needs to control it, pay attention to its airspeed throughout the flight, test for lift by moving the rudder stick if you see its speed increase.
Your model is not in lift if the nose goes up. Your model is not tail heavy if its flying with its tail down. (If the model was balanced so that the tail ‘tipped’ backwards due to weight, it would only fly backwards!) If the tail is down, your friend, your contest partner is yelling down to you to get it out of that stinky air….move!!! Left or right doesn’t matter, just move.
You can put your nose up to this information or you can be lifted up the score board by working to understand your model and the effects airspeed, incidence, lift and sink on its components. |
Best regards,
Ed Anderson
Long Island Silent Flyers
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Edited by - aeajr on 07/30/2010 08:26:43 AM |
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Ooh! Did you see that I'm in lift!
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