No Deference for Differential
This article is written by CK Aero Founder Bryan Hebert
I recently received a couple emails from pilots with the same question, “How much differential do you recommend on a pattern plane?”
My Answer? None!
Now this may come as a surprise to some who use it and believe that it works, but if you’re flying with a symmetrical airfoil the biggest reason you may feel you need it is caused from another surface on the airframe. Remember, pattern is precision. Differential is a Band-Aid for a lack of precision elsewhere in the setup. Designers and competitors are keen on making sure they set up and design the airplanes as precise as possible from symmetry to weight distributions. Why would you then intentionally spoil that symmetry with ailerons set up to affect the airplane in a non-symmetrical manner?
"The problem comes from the back!"
So why do you need it then? It`s time to take out the measuring devices and find out. Here is the surprising truth: the problem does not come from the front of the airplane it comes from the back!
With adjustability comes responsibility.
We can tweak and move every surface on our machines with the twist of a hex wrench and expect that the manufacturers have ensured precision with everything else. I`ll let you in on a little secret… for the most part the young men and women putting your airplanes together do not fly model airplanes. They punch a time clock and produce a product that on the outside looks very nice, but the devil is in the details. Our job is to set it up for competition. They are not paid to worry about setup nor should they be. The manufacturers do not advertise as such. We as modelers have become spoiled with pretty airplanes and quick build times, often neglecting the basics. We have to be diligent to measure our airplanes and make sure they are set up straight. We have to fly precise patterns with them so it follows that the machine should be as perfect as we can set it.
"That twist or miss alignment is causing you more grief than you could ever imagine"
When I`m testing a new design, even with my years of experience, it can take months to figure out a problem. Every time I fly for testing I go home, my airplane goes on the setup table, and I measure, inspect, and record what I adjusted until I know there is no way to improve it. This is how you learn. I`m still learning on the Valiant, as evident from my knife edge video. Who would have thought a knife edge performance like this was possible? I was even told I was wasting my time because NASA says it’s not possible!
How much do you think 1mm is in elevator trim? Measure it one day, you’ll find it’s about 15 clicks on the transmitter. On a stab half 1mm is 2 tenths of a degree! Now, can you measure that? (Sure can, I do it all the time) That twist or miss alignment is causing you more grief than you could ever imagine. This needs to be adjusted and fixed. If you need differential it is most likely the reason.
The biplane is most affected by this problem. Your center trim power will be dead because you have stabs or elevator halves opposing each other. This forces you to run the CG 5% farther forward to compensate for the effects of this misalignment (and without this article you would never have suspected it!) This really degrades the inverted elevator feel which affects horizontal rolls, up line roles, landing snaps, precise loops, and opposing knife edge (this is where problems will be evident first). I`ve preached about the dreaded spiral slip stream being blamed for these problem for ten years without divulging the answer. So what are the indicators you have an asymmetrical stab setup?
Here is the list
- Any airplane with more than .5 degrees negative thrust to maintain an up line or that requires more than .5 degrees right thrust.
- Many airplanes that need throttle to rudder mix (the stab miss alignment steers the airplane when the wings are unloaded)
- Any airplane that has a problem doing precise loops (the loop should be almost one input that stays almost constant through the maneuver if the wind is minimal)
- Any airplane that you think has a need for aileron differential
- Spin entries that always want to spin the same direction.
- Any airplane that wants to tip stall or steer off on a slow, high nosed landing
- Any airplane that climbs with throttle
- Any airplane that porpoises inverted when the cg is placed rearward, usually from 32-35%
- Any airplane that displaces on a roll reversal (full or half roll) up line under power
- Any airplane that requires a rudder mix to have an axial roll in an up line
- Any airplane where down line mix makes the 45 degree down lines pitch down
- Any airplane that has excessive pitch to the belly in an up line against the wind
- Any airplane that goes to the belly on one knife edge and to the canopy on the other
There are some more examples but these are the core issues you will find any time you think you have a differential problem. Actually, you probably do but it`s most likely in the stabs or elevator halves. Now imagine how difficult this would be if you have a flying stab!
If your airplane displays two or more of these problems on the list, check you stabs or elevators. Some of these issues are also discussed with the fixes in my trim guide here.
We like to put the blame on fancy aero problems like the spiral slip stream or vertical CG (which is another myth I`ll discuss later) when in fact, if we learn how to accurately measure and adjust our airplane, we can fix almost all the issues if the design is sound.
We have to set a perfect foundation to display perfect flying airplanes. Get the tools I recommend, use the guides we sell here on this site and you will be amazed how good you can make a pattern bird fly… even with the dreaded spiral slip stream 😉