Build Tip: "Ungluing" Hot Glue

While looking at FPV videos (my next RC venture)  I came across this video:
http://flitetest.com/articles/hot-glue-tricks
About the only place I use hot glues in my builds is to spot glue tack welds when shaping the nose and for gluing on my servos.  I've read all kind good and bad advice on the forums about taking servos off without destroying the plane or just cleaning them up to recycle them.  In this video they show how denatured alcohol quickly separates the hot glue.  Holy mackerel, its ridiculous how easy it is!!!! Go get some, now!!  My favorite tip of all time.

Another 2212/6 2700 Kv motor from Deal Extreme

Hi everyone -

I was poking around some more on the Deal Extreme website and found another 2212/6 2700 Kv motor http://www.dx.com/p/a2212-2700kv-brushless-motor-set-for-r-c-toy-golden-silver-361849#.VJWrwsCA



This one appears to be identical to the Focal Price 2700 Kv motor that is still currently out of stock.  I have ordered one to test it out, I will call it the DX Gold motor as I am already running another DX 2700 Kv motor which is black, so it will be the DX Black http://www.dx.com/p/a2212s-2700kv-outrunner-brushless-motor-45134#.VJWw5sCA to hopefully prevent confusion between the two.





The good news about this recent find if it is the same as the Focal Price, Ebay and DX Black motors, it is much cheaper at $9.71 USD compared to $12.39 for the DX Black, $12.29 for the Focal Price motor (if they ever get them back in stock) , $10.99 for the YUSHOP66 (EBay) 2700 Kv motor.

So if nothing else, if the performance matches up to the specs, it will be another source of what seems to be a very popular size and Kv motor these days, especially within the park jet community.  If you buy 3 of the DX Gold motors, the price drops to $8.61 per motor and shipping is still free (although quite slow, expect 4-6 weeks if you are in North America).

I have put about 50 flights now on my DX Black motor and it is running very smooth and strong, a bit peppier than my other 2212/6 2700 Kv motors drawing on average about 37 amps and producing about 444 watts with a 6x3 EMP prop.

More to follow on this DX Gold motor once it arrives and I get a chance to test it on the bench and at the field.

Cheers,

Scott

The Continuing KF4 Conundrum

My RC Holy Grail used to be straight line knife edge flight.  Well, time to move on.  I have always been fascinated with the science and voodoo surrounding KFs.  I have scoured the internet and read everything that is evidenced based as well as the because I said so self believers.  There is a whole lot more voodoo than science, the more I get into RC the less I believe what I read.  Here at NAMC, Scott enormous number of test flights have helped separate the wheat from the chaff.

When I test flew my last prototype, I dialed it in without KFs and then with 40% KF4s.  The designers of the KF4s specified 50% but the community seems to have adopted 40% from the KF2.  When I added the KF4s I had to move the CG forward.  After reading extensively, the center of pressure (COP) therefore moved forward with the addition of the 40% KF4s.  The CG I was using was calculated based on mean area chord and theoretically the optimal CG.  I honestly preferred the feel of the balance better at the MAC CG rather than the forward CG with the 40% KF4s.

For the present build I am going to make 50% KF4s which should move the COP back and into alignment with the MAC CG.  Theoretically at least, I'll soon see and report back.

Low altitude high alpha with a park jet

Hi everyone -

I know the title might sound like a bit of an oxymoron, but in recent testing with a couple of planes working on my high alpha skills, I have noticed a few things with respect to low level high alpha that got me thinking about what is going on.  I could just be speculating here, but perhaps it will make for interesting discussion... :)

When I say low level, I'm thinking mostly here about where the tail and most of the back end of the plane are within about two feet of the ground or in the "ground effect".  So I had to do a little more research to better understand what is normally going on with a plane in ground effect.  A fairly simple explanation (at least one I can understand) is here http://en.wikipedia.org/wiki/Ground_effect_%28aerodynamics%29

So I'm not sure that being at a fairly extreme AOA (angle of attack) in the ground effect is taking advantage of what would normally be increased lift and reduced drag, perhaps I have lost those benefits.  Without benefit of a wind tunnel, I can only go with what I see, hear and feel is going on with my plane as I push it close to the ground in an already challenging flight profile.

So you might say why put myself in such a situation doing this type of flying so close to the ground?  Well, no reason really I guess other than it is challenging and pretty cool, getting that tail down super low and sometimes even touching it to the ground is somewhat akin to a 3D pilot dipping their tail in the water while hovering their plane.  Not the most scale of flying as you wouldn't see a real F18 or Mig pilot try a "tail touch", but fun to play around with.  Very high on the cool factor, but as often is the case, a bit higher on the danger factor.  Like with any extreme flying, the margin for error is pretty small, but fortunately the plane doesn't have far to fall if it does decide to reunite itself with terra firma.

So what I was noticing yesterday as I pushed the limits with the plane I was flying was that as I got closer to the ground, within about two feet, some very interesting things started to happen with my plane.  The closer I got to the ground, I found the controls starting to get "mushy" and sluggish to respond and any minor changes in upward pitch would set off wing rock at a much lower AOA than normal.  I could also hear the prop wash hitting the ground and the sound of the prop changing as perhaps it was chopping through dirty, turbulent air being bounced back up off the ground.

I have drawn this very crude diagram to help explain what I think might be happening.

So what I started wondering with how the plane sounded and handled as it got closer to the ground is whether a bubble of dirty/turbulent air was being trapped behind the prop and under the back deck of the plane, especially as the elevons got closer to the ground, blocking some of the flow of air moving backwards.  This now coupled with the prop being close enough to the ground that perhaps the prop wash was now also getting bounced back up into the plane and maybe even back into the prop, causing the difference in the sound of the prop.  

I could be speculating, but perhaps as the tail of the plane was now starting to "rest" on a bubble of turbulent air as it moved forward so close to the ground, that could help explain why the back end and the controls elevons and rudders were feeling "mushy" and slow to respond or in the pitch far more sensitive.  I know that as soon as I lost my nerve or needed to recover out of being that low, once I got up to about 3-4 ft off the ground, the controls and plane felt much more responsive again.

Anyway, it is interesting to speculate, I definitely think that the prop wash being so close to the ground with the unique mid motor mount/prop in slot park jet style of plane has in impact on high alpha performance.  With a normal puller type plane where the prop would be much higher off the ground, perhaps this would not be as much of a problem, I suspect what I an seeing, hearing and feeling is unique to the park jets I fly.

So how do I recover out of it if (or should I say when... :/) I get into trouble?  Well, again, high on the cool factor is to hammer the throttle and power out of it, but this requires a bit of nerve, especially if the plane is starting to wing rock or do other crazy things.  Hammering the power when the plane is already a bit mushy, slow to respond and with spoilerons still deployed can end up in a fairly aggressive maneuver very close to the ground resulting in less than positive results...been there, done that... :/  It is also crucial that I have a motor that has enough power and instantaneous acceleration to get me out of trouble, not enough power and things can go bad as normally the plane is heading towards something it shouldn't be with sloppy controls and not enough power... :/.  

As crazy as it might sound, normally the safest thing for me to do  is to just chop the power and let the plane "flop" to the ground.  This normally ends up in less catastrophic results, allows me to launch the plane and continue on.

So as I mentioned, I could be speculating what is happening here, but I think what I have said makes sense based on what I have observed is going on.  Anyway, low altitude high alpha might not be for everyone, but as I mention, it is high on the cool factor, allows me to push my skills and focus on really being smooth with the controls... :)

Cheers,

Scott

Merry Christmas!

Every now and then we need a break from our thoughts and time being consumed with Migs.
My wife and I started putting together a Christmas CD years ago that we gave to our friends.  The CDs always start a little crazy for the kids and then wind down to a soothing classical piece.
This year I've gone 21st century and posted it on the web.
Merry Christmas
Stephan
Christmas Music

"Differential spoilerons" for high alpha handling

Hi everyone -

I was out dialing in a plane today trying to optimize it's balance and handling for best high alpha performance.

Since Stephan wrote this great article http://migsrus.blogspot.ca/2014/12/trimming-rc-airplane-determining-cg.html , I have been trimming all my planes using this method and have noticed a much more responsive and balanced plane as a result as well as my CG and components being moved back significantly.  I had obviously just grown used to flying my planes far too nose heavy and didn't really notice until Stephan educated me on these new methods of properly finding CG and trimming the plane for a more balanced and maneuverable setup.  Big thank you to Stephan once again... :)

The planes all rotate much better in all axes which makes general handling and aerobatics much more fun and responsive.

However, as the weight has been moved back and the rotation made easier, I have also noticed the affect of prop wash in high alpha has become more pronounced, especially on the Mig I was flying today.

Stephan and I have both written articles that might help understand better what we mean about the affects of prop wash in high alpha

http://migsrus.blogspot.ca/2014/08/mig29-v4m3e3-hi-alpha-testing.html

http://migsrus.blogspot.ca/2014/08/the-high-alpha-right-turn.html

So today as I was dialing in this plane, I found that right turns in high alpha were a real challenge and that if I took my thumb off the rudder stick, it would do a left turn on it's own, completing a full circle in about 25 ft.  So I suppose I could just turn left all day, but that gets pretty boring after awhile and doesn't challenge my skills much.  So I fought with it, had the tail slide around pretty aggressively a few times as the rudder bit too aggressively in right turns and at one point over controlled/dumb thumbed my way into the ground.  So a bit of a nose "gonk", my pride hurt more than my plane, it will be easily fixable.

But as I did my short, solitary "walk of shame", I started wondering if there was some way I could put a setting in my radio to help offset the effects of prop wash to make high alpha a bit more manageable and right turns not so difficult.

I use a Turnigy 9x radio with er9x firmware, so it is far more capable than I will probably ever utilize, but it is pretty versatile for the price.

So at first I thought about having a switch where when I started high alpha I could select preset rudder trim to the right.  This would help, but would then reduce my rudder travel to the right if I needed it (which I knew I would).  It is also another switch to select in addition to spoilerons which I like to use for high alpha to help keep the plane more stable and the nose up a little higher without too much elevator/elevon deflection.  Not a good option as too many switches spells trouble for my limited skills.

Then I thought, why not have the spoilerons deflect differently?  In other words, have the right spoileron deflect a bit more, helping to keep the plane from pulling left all the time.  I'm sure others have thought of this and use it, but honestly I never have heard or read about anyone using it.  I have always been locked into the paradigm that both spoilerons have to be deflected the same amount in high alpha, drilled into me by whoever had my attention at the time when I was still learning high alpha.

So as I looked at the blemished nose on my plane I thought, what the heck, I've had one crash already, what is the worst that could happen?

So since I can adjust the "weight" or percentage of the spoileron deflection individually in each of the "aileron/spoileron" servos without affecting how the aileron works when spoilerons are not employed, I started off with 10% more weight on the right spoileron than the left.  I should add that this plane requires very little spoileron deflection for good high alpha angle of attack (AOA) and stability, actually only about 1/4" deflection.  So this 10% difference did help, but I felt more could be done.  So through several flights, I increased it by 5% at a time until I had 30% more "weight" on the right spoileron than the left.

Although not eliminating the effects of prop wash completely, it makes high alpha handling much smoother and more relaxing even.  The plane still drifts slightly to the left, but would take about 100 ft to do a full circle on it's own if I let it go and for about 80% of the time flies pretty straight with no right rudder input, requiring just the odd minor correction now and then instead of constant right rudder input like I needed without "differential spoilerons".

This setup made right turns much easier, it still occasionally got hung up/delayed, but about 80% of the time, it only needed gentle right rudder input to come around to the right and it responded almost right away instead of having considerable delay like often happened without the "differential spoilerons".

However, as my sage friend from Alabama (Stephan) has often said, aerodynamics is all about compromise, change one thing, it can often affect another.  So there is a caution I will pass along if you try this that I learned today (didn't result in a crash, just a little more excitement than I wanted at the time... :/).  If I got too slow or didn't have the power setting up high enough when I deployed these differential spoilerons, it would pull to the right and drop the right wing.

I have been able to alleviate some of this again with my radio.  In the er9x firmware I can set the speed with which the function deploys, so I slowed the right spoileron down about 30%, meaning it would take a couple seconds longer to fully deploy than the left one, making for a more balanced transition into high alpha.  This didn't mean once established I could get lazy, as it would still drop a wing or pull right if I got too slow with too little power, but once established, the improvement in handling is well worth the risk in my experience.

So now basically I have 1/4" deflection on my left spoileron and about 3/8" deflection on my right spoileron, doesn't seem like much, but it is a 50% increase in deflection to help with high alpha prop wash.

So if you have a radio that allows for this type of adjustment and have noticed this constant "drift to the left/difficulty turning right" with your planes in high alpha, I encourage you to try it or think about it.  Start off high obviously to allow for recovery if it goes wonky on you and I would suggest starting off with a small difference working up in very small increments until you find what you like.  Just because 30% worked for me on this plane, doesn't mean it will work on all planes or that it will work for you, but even with the slight risk, I mention above, it was well worth the reward with much smoother overall handling and performance in high alpha.

Cheers,

Scott

Trimming a RC Airplane & Determining CG from Mean Area Chord in Airplane Designing

In the background, Scott and I have been working on a Mig-35 version of the RCP Mig-29v4.  We have referred to this plane in many ways including using NAMC.  I have had more time to think than fly and build lately.  As they say, "a little bit of knowledge is a dangerous thing."

In the test flight of my last prototype build, I set the CG using the "arc" method that is basically a powered glide test.  I personally like the arc over the glide method for several reasons.  I prefer to be climbing at altitude when I am doing surface control adjustments; it gives me more room for recovery.  More importantly, the arc method is more dynamic visually.  Another method I found in Googling away my time was to fly inverted and the attitude of the plane and amount of elevator required indicate the CG.  Next time out, I will do all three maneuvers.  Here is a great summary of trimming a RC plane (arc method was found in a different post):

Trimming a RC Plane

How do you determine the CG on a wing?  Off to Google and my textbooks and the best method is to determine the mean area chord (MAC).  MAC is the line that divides the wing into equal areas.  Depending on the wing type, the CG is 25-33% of the MAC from the leading edge for straight and delta wings respectively.  Using my program, I calculated the MAC in two different ways.  First I used the "true" wing lateral to the side plates (blue in picture) and second I used the "full" wing to the centerline of the plane (green).  The picture has a lot of overlays:  the original Mig-29v4 wing, Mig-35 prototype wing, and the MAC drawings.  The black and red CG are what I determined after test flights with Mig-35.  The Mig-29 v4 CG is marked in simple text.  The red CG was where I determined the plane to be very slightly nose heavy and just ahead of the calculated MAC CG for a full wing. Clearly, the "true" wing in blue is not the way to calculate the MAC.

From a design standpoint, now that I know the wing CG,  what is the relationship between the  fuselage and the wing?  It's simple, balanced with varying polar moments (see my prior posts about polar moment of inertia). The CG of a fuselage including the tail but without the wing would intersect the CG of the wing. In RC we have battery and ESC as the movable components to adjust the CG and polar moment.  On my present build that is foam only stage of construction without KFs or components, the CG is presently 2 inches aft.

My next question to ponder is how do the KFs affect the wings center of lift (COL)?  Will they change the COL and hence the calculated CG?  For straight wings from wind tunnel testing, we know the ideal KF is 40%.  There is no similar data on the best KF ratio for swept or delta wings.  In some prior flight testing on the Mig-29v3, I found that 40% was better than the stock KF that was varied from root to tip.  With this present build I am going to trim the plane with and without 40% KFs to see if the wing COL changes the measured CG.

The epoxy has set, back to building the latest prototype.

Stephan