|Oct 06, 2012, 02:36 AM|
Thomas....What a wizz!
That little one is more than fast enough for an oldie like me.. and it also has a very good speed range... what is the power setup on it?
and have you done a plan of the lines?
3 or 6mm steps...
From purely subjective tests I feel that the "step effect" is fairly proportional to both the height of the step and the speed of the model..
And as the speed of the model is also related to drag... the thickness of its wing plays a big part in how fast it will go..
On wizzy model like your little one, less deep steps, KFm4 style should keep it sweet when at speed and although the KF effect is less when going slow I doubt you will notice any difference .
|Oct 07, 2012, 06:05 AM|
I need to pick the collective brain.
I've just finished my latest build, the NSW. http://www.rcgroups.com/forums/showthread.php?t=1738983. It's a low wing war bird using a KFm4 with 50% top step and 40% bottom step.
She flies well but I'm getting a bit of ballooning (I think that's what it is anyway). CoG is if anything a little forward of the predicted CoG using this calc - http://www.geistware.com/rcmodeling/cg_super_calc.htm.
Now, so far on my builds I've used Flat plate wings or KFm2 and got away with 0/0 angle of thrust on the motor. It seems that the KFm4, based on what I'm seeing, has noticeably more lift and I'm thinking that a bit of down thrust is going to be required.
Does this tie in with what everyone is finding? Does it make sense? Am I missing something?
Wing and H stab are at 0 incidence.
I should also say that I'm loving the KFm4 from a construction point of view. The three laminates allowed me to score and bend the layers of foam over a very simple jig and the glue held the dihedral perfectly. No more cutting and gluing tricky angles for me!!
|Oct 07, 2012, 07:46 AM|
Forward CG can cause a nose-up tendency under power.
Up elevator is needed to counter the diving due to the forward CG, and speed under high power and the slipstream over the elevator can cause the pitch-up.
|Oct 07, 2012, 08:07 AM|
Does that mean you doubt thrust angle is having an effect?
|Oct 07, 2012, 08:30 AM|
Numerous times it has been found that the Blu Baby 33's needed some down and right thrust to keep the nose from pitch up when throttle up. And it also contributed to better turns to the left.
I used 2-3 degrees down and about half of that to the right in most cases and did it by adding flat washers between the firewall (built plumb and square to the fuselage) and the legs of the "X" mounts.
If your elevator trim is flat now in level cruise flight your CG is about right. Have you ever read the CG Location article here:
That article and the CG−position diagnosis via Dive Test image you can download from there are what really got me sorted out on CG locations and diagnosing or appraising what was going on some of my planes.
The article is written for gliders but it works for powered flight too. If you start off by reducing power to just enough to stay a little above a stall and trim until you get level flight, you can induce the small dive and watch the recovery and decide all the things mentioned there. It happens a little different under power than it does with a glider but it works the same. And it is best done in still air if you can.
Then if you land and find that you have some up or down trim on the elevator you have your telling signal. If there is up trim (holding the tail down) or down trim (holding the tail up) you can move the CG a little, re-set the elevator to flat, and repeat the test.
In the end you can get the "desirable behavior" recovery and will also have a plane that has a CG location that is just positive enough to keep the plane in forward flight.
And in any case, the goal is less to make the plane match any standards for CG location, it is to get the plane flying and recovering the way you prefer it to it.
Once you get the CG location set, as you launch into winds of various speeds, you'll find that a couple of clicks or so of down elevator will correct for that day's wind. If you're flying for the world championship or something like that, you may want to adjust the CG location on a day to day basis to keep the elevator flat in that day's winds and have the absolute minimum drag.
|Oct 07, 2012, 09:30 AM|
There is no doubt some models benefit from down-thrust when the airspeed under power results in excessive lift.
Because everything is inter related I'd sort out the CG before I fine-tuned the thrust-line.
My KFm4 40% top and bottom step planes rigged at zero everything, except for a tad of right rudder, have no 'pitching under power' issues.
|Oct 07, 2012, 10:01 AM|
The thrust line adjustment will stop the nose pitching up. But the speed will increase and that will also increase the lift and the plane should ascend on the increased lift while still flying level.
A lot of folks that started on Slow Sticks thought that it was natural for the nose to pitch up when power was increased and that was because the stick mounted motors did not have any thrust angle nor was it easy to change that.
Then there are gliders...
Gliders will always pitch up and always need some down elevator to counter it. I don't think you could put enough down angle on a glide to counter it as it would take a *lot* of it and it would be very ugly and also increase drag. So on gliders you just put them on at zero/zero and learn to deal with it.
When I built and flew my first glider a couple of years ago, the Blu Sail II, I knew nothing about flying gliders. I spent a couple of very frustrating flying sessions trying to get it trimmed out to fly the same both under power and on the glide before I got smart (found some basic how to fly a glider the info on the forums here).
So now with the Blue Sail II, as I throttle I add elevator down, and as soon as it is power off that is reversed and everything is right.
The addiction brings a learning curve with that never seems to end...
|Oct 07, 2012, 05:33 PM|
Here's a great concept.
Here is a very large flying wing that is modular. Looks like it has a KFm2 airfoil.
|Oct 07, 2012, 06:12 PM|
Dick that guy builds some intriguing stuff.
This looks to me like a KFm2 ornithopter (with a very thin wing section!).
An accident of construction rather than intention I suspect, but interesting.
|Oct 08, 2012, 02:39 AM|
Trimming, COG and Thrust line setups all depend on how you like your model to fly...
First do the COG... do unpowered 45degree dive tests and also inverted flight...
The spot on COG is when the dive stays on the angle you started it at..and inverted needs little or no corrections
When you have the COG as you like it ..then sort out thrust lines by observing what happens when you add power.. Ideal is when the model just gets faster without any significant pulling off line..
note...you can only get this perfect when the cog is spot on..
More usually , and what I prefer, is a bit of self leveling or recovery.... this is a slightly forward COG where the trim needed for level powered flight will pull the model out of the dive ..
And when I add power I get a bit of a climb..
A more forward COG is more relaxing to fly, and less affected by windy conditions
Its all a personal choice and it also depends on what you are trying to achieve in the models flight
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