Excellent video with visual example of airflow over and airfoil during a stall.
Airflow Over Stalled Surface
Being a Good Test Pilot: Part 4, Trim Pre Flight – Flight Plan
Our series about becoming a solid test pilot continues…..
Make sure every surface is perfectly nuetral. No slight rudder, or aileron inputs. Check the elevator for “slight up”. I would define this as the elevator being set dead nuetral then putting in about 1/32″ up or about 3 clicks of up trim. It’s always easier to control a model that is climbing too much than to fight on that is diving for the earth.
If we’ve done our job correctly, the model should fly well without any big surprises. We need a flight plan. First, how do we deal with an out of trim condition? If the model is badly out of trim, the two most important things to you are altitude and hang time to make the adjustments. You gain altitude by powering up to 100 feet at a minimum before you start trimming. I like to deal with the elevator first, then the ailerons. I am thinking of giving myself the best possible time to deal with the model. If it’s trying to pitch into the earth, constantly, you want to deal with this first. Be ready for it to climb too much as we started the flight out of trim to the upside on purpose. If it’s only a few clicks off, put the clicks in. If it’s a bunch more than that, slow down a bit, this will give you more time to input the elevator trim. Likewise, if the model is rolling to the right or left, start clicking in trim until it’s neutral. If by chance it’s rolling aggressively, to one side and you fell under pressure to get the adjustment in quickly before the model rolls over on you, try slowing down a bit, putting the model in a slight climb and rolling to about 45 degrees opposite the direction it’s out of trim. By doing so, you give your self a little hang time to get the adjustment in. I’ve seen models crashed because the pilot was a bit over his head with the model badly out of adjustment and they just kept fighting it without ever thinking to slow the model down.
Once we get the model in trim, the first act of business is to climb higher, perhaps to 200 or 300 feet. Do a couple of transitions into a stall, in effect a simulated landing 2 or 3 mistakes high. We want to get a fell for what speed the model is going to stall at and if it has any nasty snap tendency to one side or the other. Do this high just in case. Once you feel comfortable you can land it, fly a few figure 8’s to get a feel for control balance and rates.
By now we should be at around the 2 minute point in the flight. Land it and inspect for excessive motor/esc/battery heat, fuel leaks, jittering servos, loose control horns and the works. A general inspection is in order prior to flying again. If all is well go for it again.
Special notes for electric models: I like to spend 3 to 5 flights working my way up to a full power full aggression full length flight. If something is going to overheat and be damaged. Slowly working up my flight duration and aggressiveness gives me a chance to pinpoint any heat problems in the power system before damaging a major component. Maybe something needs more airflow or perhaps a propeller adjustment is in order.
Pay attention to how many mah your charger is putting back in the pack after each flight. This way you can get a good idea of your maximum safe flight time without ever hitting BEC cut off.
Take a minute to explain any improvements or adjustments you think the model needs. Perhaps if you have too much aileron trim in, you can look at taking a warp out of the wing if it exists or put in some sub trim so you can neutralize your radio trims.
It’s always a good idea if the owner makes these changes to put it up and reset it to good trims for them.
For my own models, I don’t really consider them ready to sport fly until I’ve put on a number of flights and gotten the throws just right. It’s always a good idea to look at any model as a work in progress for a while. Any model can be made to fly and handle better. Your worth the effort.
Being A Good Test Pilot: Part 3, On the Wheels
A continuing series on how to be a solid test flight, we continue……
Check the RX pack, that it’s state of charge is high. For an electric model. Verify with the owner that the pack is full. When was it last charged? Did you do any setup on the bench after the last charge? If everything is not fresh, freshen it up now.
Ok, we’ve been a second set of eyes and looked over the model, both mechanics and setup (throws etc..). Now it’s time to run the engine or the motor if it’s electric. For fuel engines, we are looking for a good transition from idle to wide open throttle. Also, a good prolonged idle with instant acceleration to wide open throttle is a sign of a well tuned engine. For an electric model, we are looking for the ESC to properly arm and for the motor to reliably start. Take the motor from off to full throttle several times with different rates of increase on the stick. Make sure it’s not going to have a timing issue and fail to increase from some point of the throttle.
It’s never a bad idea to hold a model and feel the wide open throttle power for just a few seconds. This can be a heads up if a model is under powered and will take a longer take-off distance and shallower climb angle or perhaps so overpowered that it wouldn’t be advisable to take off at full throttle. I have encountered many models at both ends of the spectrum.
Standing behind the model, ALWAYS one stick at a time, move the controls and make sure everything is moving in the right direction. Check it twice.
Do a taxi test to make sure the model tracks well with neutral rudder trim. If it needs more than a little adjustment, fix it before you fly. Avoid taxing so fast that you accidentally end up in the air. This test can be as simple as driving in a figure of 8 and a couple of straight lines to see if it rolls straight. When doing your figure 8’s, are you getting adequate right and left turns? How tight are they? My clubs paved strip is about 40′ wide. A good amount of steering for a tricycle gear ship on the ground will allow me to do a figure 8 just barely in the width of the runway (turns around a 10′ radius). If it’s much tighter than that, it’s going to become very sensitive as the ground speed increases. If it’s a tail dragger, it can as tight as 1.5 to 2 figure 8’s in a 40′ long area (turns around 5-7.5′ radius). Tail draggers usually lift the tail and the wheel becomes non-effectual around 10 or 15mph, so there is no wheel on the ground at higher speeds to be a problem.
Before we fly……………To be continued…
Being A Good Test Pilot: Part 2, Setup
Part 2 in an ongoing series about being a good test pilot. These articals are less about step by step hand holding instructions and more about how to think. We continue….
I always conduct an interview of sorts to fill in the blanks of things I may not be aware of such as level of experience, how long the project took, what the MFG says about CG location, suggested weight, what does it actually weigh and etc…. What your fishing for anything that could be a surprise. After the crash, you don’t want to hear the builder say “I thought 1/2″ behind reccomended CG was close enough?!?!”. Your looking for any shortcuts or oversights in the model. Did they use the reccommended servo’s or at least close? Is the battery large enough? How old is it? If the RX is new, has it been run a while? If the RX has come from another model, was it flying fine or did it’s last flight end in a crash? Has the TX been performing well? Are there any used or harvested components in the systems?
Always check the CG against common sense and what the builder says it should be. NEVER trust that it is right. Hold the model up in the air and check it yourself. Measure if you must. There is NO excuse for crashing over a missed CG check.
Control surface throws: I’ve run into many first time builders that started with a few foamy’s, have built their first sport or scale model and they put throws in the model that look like the 3d foamy they’ve most recently been flying. Not realizing the throws apropriate for a 25-30mph ship will be grossly excessive in a 70mph aircraft. Besides asking about suggested throws, apply your own stink test. If it doesn’t look right, it probably isn’t right. I like throws for a test flight just a tad on the high side of suggested first setups. However, you can take this way too far and end up with an over sensitive model. A model that is way out of trim and also way too sensitive is a real handful. Don’t walk into any such traps. Also, if a throw such as the elevator is too low and the model turns out to have an incidence or nose heavy problem, you might not be able to land at a reasonable speed and hold the nose up. Too little therefor can be just as bad a mistake. Try to be reasonable about what’s correct for “this” model. Generally, the faster it flys, the lower the throws should be. Something else to aid here is look for similar level of throws. If the ailerons are deflecting 30 degrees and the elevator only 15 degree’s perhaps the throws will be “out of balance”. Out of balance controls would be one control being sensitive and antoher being soft. This is more difficult to process in your brain when your under pressure trimming out a difficult model. Consider it before proceding.
To Be Continued……..
Being A Good Test Pilot: Part 1, Mechanical Soundness
Testing a new model out is an important task regardless if it is yours or a friends. Many things go into a successful test flight. This will be the first in a series of articals on this subject. I consider it a high honor to be asked to test a new model and you should as well. Lets honor those requests by doing the job well. My writing will be mostly from the perspective of testing anothers model to simplify the language.
When a modeler brings you model to test fly, not only is he choosing you for your flying skills and experience, but also for your wisdom and judgment. Obvioulsy your trusted. Don’t shy away from telling the builder what they need to hear. If the model is not really ready to fly, say so. Your were choosen because you have the judgement and skills to make that deturmination as well as the skills to fly the model if it’s ready.
The first thing to do is size up the modeler and the model. If the modeler is known to you, is the complexity of this model in line with this persons experience and flying skills? For example, a pilot on his 3rd model presenting you with a multi engine, air-retract model could be reaching out a bit far. You’ll need to inspect every detail of this model very carefully as mechanical errors are more likely. Check every connection in the retract system. Look the servos over, are the appropriate for the model? Is the linkage up to snuff for the expected speed and performance? Is other hardware like wheels and landing gear mounting substantial enough to handle landing loads? Is the wiring done well? Are connectors properly safety tied where needed? Is wiring mounted or flopping around inside? Is the battery and RX secure? One of these loose in the model could lead to plugs opening up or severe CG changes. How is the antenna routed? Is it shadowed by wiring or other objects? Look at all the basics. If there is anything at all your not comfortable with, now is the time to discuss and address any problems.
To be continued…….
The Texas Sharpshooter Fallacy
I wrote previously on the subject of “Confirmation Bias”. Sometimes it is difficult to discover the answer to a technical problem because the person bringing you the problem has a hitch, assumption, or faulty logic step in their diagnostic process. Often people draw conclusions from spotty evidence. For instance a customer shows me a receiver and say’s “This receiver is bad.” I ask: “Why do you think it is bad? The answer almost always comes back something like “I plugged it in and it does not work.” The person is saying from that one test or measurement they have drawn a conclusion. It seems reasonable doesn’t it? But, really it’s pretty silly when you think about all the things that can cause an RX not to respond to a TX. You see, the real and only conclusion you can draw from the customers test is this. “In one trial, the RX produced no apparent response.” That is quite a bit different from “This receiver is bad.” Understanding the difference in those two conclusions is why some people are good at diagnostics and others are not. To be good at figuring out a problem, you are greatly advantaged by not making any assumptions or broad conclusions.
In the case of a receiver, lets go over many measurements and tests that you might perform to decide if it is in fact “Bad”.
1. Has it ever functioned successfully?
2. Does it really match the Transmitter? (is it talking the right language DSMII vs DSMX or PCM vs FM vs AM and etc…)
3. Is it on the same channel? (in the case of non-2.4ghz gear)
4. Besides looking at the stickers, did you actually look at the tags on the TX and RX xtal?
5. Is the shift the same? For example, a positive shift JR TX is never going to drive a negative shift Hitec or Futaba RX.
6. Have you driven the servo you used on the tested RX with a servo tester to make sure it actually wiggles?
7. Have you load tested the battery your driving the RX with to see that it is high enough to actually turn on an RX?
8. Are you using a switch between the battery and RX? Plug the battery in directly so your not actually testing if the switch is good.
9. Can you demonstrate the TX driving another RX to establish that your testing with a working TX? One might complain, “I flew it a week ago!” However, we’re not testing the troubled RX a week ago, we’re testing it now. 😉
10. Have you plugged a voltmeter into an empty servo port to see if there really is voltage finding it’s way to the RX?
11. Is the crystal really fitting tightly in the socket or is it loose and wobbly?
12. Does the TX have the capability of being on for programming without broadcasting?
13. Is the meter on the TX a voltmeter or RF Output indicator? What does it say?
I’m sure a sharp thinker can come up with some more things to consider. Many of the things above we’ve found at one time or another to be the cause of a non-responsive RX. Assume nothing.
Recently we had an A123 RX pack returned by a customer. He said it tested poorly, only a few hundred mah. The customer appeared to be correct, it was testing bad after several charge/discharge cycles on our bench. And, the charger would increase in voltage rapidly when we applied charge current. Strange. However, even after several trials, a good mechanic still hasn’t drawn any conclusion. He may be moving towards condemning the battery but all tests were not complete. He cut the shrink off the pack. The tabs all looked fine. He re soldered the tabs anyway just in case there was an unseen cold joint. Note: He had originally built the pack, but without emotion, he redid his original work anyway. Many people fail at this step because “they couldn’t possibly have done anything wrong.” (yea right!). The pack was cycled again with the same poor result. Now, finding a bad battery pack is rare, exceedingly rare. We know this to be true from many years of experience. So, we keep looking. I examined the pack under magnification (even though it had been re soldered by a respected pro) and all looked good. I looked at the plug under magnification and found a thin transparent film on the plastic shell. The more I looked, the more I saw this film all over the shell. Is this paint? We decided to solder a second lead onto the pack and test again. The pack tested good. What was the problem you wonder? We can only conclude the film on the plug was thin CA the customer had somehow accidentally allowed to come into contact with the plug. It had a high resistance because one or more connector pins was evidently coated in glue. After replacing this plug, the apparently bad battery pack was proven that it was always as good as new.
So, if a battery pack fails a discharge test or an RX fails to respond, is it bad?
To read about The Texas Sharp Shooter Fallacy, check out this wiki link. Reading it is what inspired me to write today’s article. It describes in somewhat technical language a common way to foul up a test.
“Texas Sharpshooter Fallicy” Wiki Link.