Radio Control: Helicopters
Paul Tradelius, 4620 Barracuda Dr., Bradenton, FL 34208
During the flying season I received letters and calls about pitch curves. What are they? Why are they needed? How do I make initial adjustments? Can't my computer radio do all the setup that I need? Since these are valid questions and concerns, this is a good time to tackle the subject.
The term "pitch curve" may seem complicated, but it's nothing more than adjusting the collective pitch of your helicopter to make it perform as you desire. One of the nice things about helicopters is that you can make them perform to your specifications (within reason) by making fairly simple changes to the setup of the rotor system.
Not so many years ago, helicopter radios had only a high and low pot adjustment for the "pitch curve." The radios to which I'm referring had a bunch of knobs under a protective cover, usually in the back of the radio, which enabled the user to adjust the high and low endpoints of a particular pitch curve, and the radio made a straight-line change between those set points. However, these radios had some serious drawbacks that the more modern computer radios have attempted to fix.
The endpoints of the pitch curve were difficult to set because there was no readout of the pot position. A pitch gauge was mounted on the rotor blade, and small changes were made to one of the pots until a desired reading was obtained. This was not that difficult, but there was no way to tell what the settings were by looking at the radio, and changes at the field were hit-and-miss as the pots were "tweaked" to improve helicopter performance.
The pitch curve was not really a curve at all, but the setting of the endpoints, with a straight-line change in between. I'm not sure why it was called a curve, except that the hovering pitch knob did make a change on the hover pitch; therefore, a curve of sorts was made.
The point of all of this is to talk about new computer radios, point out some of their features, and see how they make the helicopter setup so much easier than it was in the past. With the computer radios that so many people are using, pilots have the ability to make the curve just as they want, resulting in improvements in the helicopter's flying performance.
For example, the collective pitch range needed for hovering and normal forward flight is not what is best for aerobatics, and different aerobatic maneuvers are better when flown with their own pitch curves.
The Futaba 8UHF helicopter radio, for instance, offers the standard normal pitch curve, plus additional aerobatic curves for Idle Up 1 and Idle Up 2, and another for autorotations using throttle hold. These curves really are curves, not just endpoint adjustments, because three points on the curve can be adjusted independently of the two endpoints, which is a total of five points that can be adjusted for the pitch curve of your choice.
Another nice feature of the 8UHF is that the pitch curve is displayed as a digital readout of servo-throw percentage, which means that the factory default curve is shown with digital percent values for the 0, 25, 50, 75, and 100% positions. If you were to draw this standard pitch curve, it would be a straight line. However, you now have the opportunity to change the shape of the curve simply by changing the digital values of the curve.
Although changing the shape of the pitch curve will change (and hopefully improve) the flying characteristics of your helicopter, it's important to make sure that you are starting with a straight line, and then make changes. It sounds easy enough, since the factory settings mentioned above are a straight line, but the problem develops when you don't want to start with the endpoints of 0 and 100%.
Suppose that you put a pitch gauge on your rotor blade and determine that the low-end angle of attack that you want is obtained at 10%, and the high-end point is at 90%. If you were to leave the three other points at their factory settings of 25, 50, and 75%, the problem becomes: how can you adjust the endpoints to the values you need, then set the other points along the curve to the correct settings to give you a straight-line curve as a starting point?
There is an easy way to do this with simple mathematics and the help of almost any inexpensive electronic calculator. Use the following steps once the high and low endpoints are determined; in this case, use the 10 and 90% points.
- Subtract the low point from the high point to get the pitch range (PR).
- PR = 90 − 10 = 80
- The number of pitch curve deviations (D) is equal to the number of points on the curve that you can set, minus one. Since five points can be adjusted in the example:
- D = 5 − 1 = 4
- You can find the straight-line percentage change (PC) for each deviation by dividing the PR by the D.
- PC = 80 ÷ 4 = 20
- Start with the first (low) point, and keep adding the PC to get all of the straight-line points.
- 10 (first point) + 20 = 30 (second point)
- 30 + 20 = 50 (middle point)
- 50 + 20 = 70 (fourth point)
- 70 + 20 = 90 (high-end point)
All of the numbers were easy to calculate in this example, but it's just as easy with a calculator to find the PR, then the PC, then keep adding the PC to each successive point.
Engine Update
As mentioned previously, I'm working with Dub Jett of Jett Engineering (6110 Milwee, Suite J, Houston, TX 77092) to see how his high-quality performance engines will work in helicopters. I have his Sport .40 in my Ergo, and it continues to perform flawlessly, with more available power than any larger .46 engine that I have ever used; the Ergo reached a rotor speed of more than 2,000 rpm in forward flight. That's really moving!
I have been flying this combination for several months, with the engine showing a smooth idle, smooth transition to and from idle, and since I set the needle valve I haven't had to make an adjustment. I keep trying to find fault with this engine, but I haven't done so yet.
I hope to work with Dub to check out his other engines for helicopter use. I will let you know how they perform!
— PT
Transcribed from original scans by AI. Minor OCR errors may remain.
