January 24, 2014

Trimming Your Plane And Your Radio

It's been a few months now of working my way through the new maze of nitro planes versus electric! Wow! There's a lot to learn, and it has been both challenging and rewarding! Sometimes I do miss the advantage of just plugging in a battery and heading for the runway!

One thing I've noticed is the amount of down time spent solving mechanical issues due to engines, fuel lines, adjusting the carburetor (both high and low ends, etc). It can get frustrating to be all ready to fly, only to discover that there are problems getting the plane to start, and then you see bubbles in the fuel line. In my case, it took a visit to my friends garage to finally figure out that the bearings in the engine were worn out (creating an air leak), and we had to replace them.

Once that was done, we had a  week or so of getting things dialed in at the field when I discovered that the lines from the fuel tank were old and cracked, leaking fuel, and the fuel tank needed to be replaced. Due to where it was located in the fuselage, that meant cutting out the top, replacing the tank and lines, and then patching the newly created "hatch" back.

All of this has been because I operate on a shoestring budget and don't have the luxury of owning brand new stuff, but it has taught me a lot, and in the end I believe I will be a better pilot for it! I'm pleased to report that last weekend, after spending a week of doing the things I'm going to discuss in this session, my experience was that the plane I was flying was as if it was brand new! It felt "tight", and was spot on in balance and handling!

So, having said all that, Now to this topic. Once I had done all the mechanical repairs I knew needed attention, I was encouraged by my buddies to do some work re-trimming the plane to its maximum. The weather of late has been especially poor (constant freezing fog), so it was the optimal opportunity for some shop time.

I had known enough to know how to adjust the high and low rates for the throws of the various control surfaces. I had gone into my Spektrum Dx6i and played with the percentages in the high/low category of the Expo page. But here's a more thorough way to do it.

Start by going into the Sub Trim settings, and dial everything down to zero. Then, go to each respective servo and unscrew the horn and replace it so that the horn is at 90 degrees to the servo. If you find that the grooves in the servo won't allow you to get what you believe is a true 90 degrees, try switching the horns from one servo and trade it with the other (if there is in fact a pair, i.e. the ailerons). As a last resort, if you can't get to 90 degrees, then use the sub trim to move the horn.

Next, adjust the clevis to the horn on the control surface so that it is flush with the wing or stabilizer. Use a straight edge to check that the control surface is aligned to the fixed surface. You now have confidence that your servo is at neutral, with equal movement in each direction, and your control surface is locked in so you can begin to measure movement from neutral accurately.

If the manufacturer has specs on recommended high and low rates, that can be a way to start, although I found that on this particular plane the settings were way too "hot". So if you are at all in doubt, create your own conservative settings, ones that will allow you to start at low, have total control, and then bump to high to see if that setting is more to your liking. If so, you can slowly adjust the settings up or down between flights to your preference, until you are confident with where things are. You may want a lower setting on the elevator for landings, for instance, than what you want in flight for more delicate maneuvers.

The next area of you radio that you will want to adjust is the Travel Adjust section. Before you start playing in this section, go to your Dual Rates/Expo section and set both high and low rates to 100%. This is a baseline. Now go back to Travel Adjust, with your high rates switch selected, and adjust the throw of the control surface to the desired high rate distance. Remember that the adjustment must be done with the stick in its full direction, so if you are adjusting the elevator, for instance, the stick must be pulling full up elevator, and you are dialing the travel adjust to the proper amount, and then reversing the stick and adjusting the travel to the same amount in the opposite direction.

Ideally you do not want to exceed 100%. If you are, you might readjust the position of the clevis on the control horn of the surface your working on to increase the travel there (moving the clevis one hole closer to the fuselage), and then come back to Travel Adjust and see if you can keep it to at or below 100%. Your Travel Adjust will go to 125% or more, but that is not ideal for the care and life of your servo. If you are at a lower percentage in Travel Adjust and getting all the throw you need, you might consider moving the clevis on the servo horn in your cockpit one hole closer to the servo. This will put less stress on the servo. Again, check to see if you are now exceeding 100%, and if not, your plane is happy!

Once you have set the high rates using Travel Adjust, close out of that section and do not go back. In the Dual Rates section your high rates are now 100% and set to give you the proper amount of throw. Switch to low rates in the Dual Rates section, and then adjust the percentage until you have the desired amount of throw. Again, remember to do an adjustment while holding the stick in the extreme position for both up, then down, or right, then left, and do an adjustment for each while giving the same amount of throw.

I will tell you that having your radio and your plane in sync with each other is an amazing feel! What these radios are capable of is impressive, but if we aren't aware of all the settings, it is easy to simply play with expo, and the high and low, and think all is good.

Also, with foamies, there is a limitation here. The aileron servos are glued into the recesses of the wing, so you can't just take off the horn and reset it after bringing your sub trim to zero. My Corsair is a mess...I have separate aileron controls...one through the gear channel, and the throws just wouldn't match up. I suppose I could take them out and go through this process, but there are decals over them that I don't want to screw up. So I have both wings with different percentage settings in high/low, and still couldn't get a match. So one wing is going to one hole on the control horn of the aileron, and another wing going to a different hole on the other, just to try and get the same amount of throw from each wing and servo independently! Just remember that on the servo horn, the outermost hole from the servo produces the most travel, while at the control surface, the hole furthest from the control surface produces the least amount of travel.

I hope this is not too confusing! But I can tell you that I have learned that a well trimmed plane is worth the time and trouble it takes in the hanger to get all these settings right! And with nitro, the good news is that the servos, horns, and clevises are all larger and easier to access than on the foam park flyers!

Happy flying!

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