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Apr 02, 2013, 12:08 PM
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Mini-HowTo

Creating a plumb line pitch gauge


In my previous blog, I discussed a few simple techniques for setting up a collective pitch helicopter, which focused on leveling the swash plate and setting the blades to 0-degrees pitch-angle when the throttle is set to 50%. Once those tasks are finished, the next step is to set up the helicopter's pitch curve. For those new to collective pitch helicopters, the term "pitch curve" merely refers to a sequence of settings in the transmitter that governs how much the main blades tilt, or pitch, as the throttle stick is moved from 0% to 100% throttle. If the main blades have too little pitch, the helicopter will have very little lift and may not get off the ground even when the motor is at full throttle. Too much pitch can lead to motor bogging and overheating. Or, if your motor is reasonably powerful, too much pitch can make the helicopter jumpy and difficult to hover. Either extreme can lead to hours, if not days of frustration for those just starting out with a collective pitch helicopter. Clearly, measuring the pitch-angle of the main blades at various throttle values is an essential part of setting up any collective pitch helicopter.

It is difficult to measure the pitch-angle of the main blades just by eyeballing the blades. Thus, it is essential to actually measure the pitch-angle with a suitable tool, and there are a variety of tools available for just this purpose. Probably the most common and cheapest tool for measuring blade pitch is a simple protractor pitch gauge that mounts on the helicopter's blades, such as the QMAX Q750081 Pitch Gauge. The protractor pitch gauge enables one to determine the angle between the main blades and a flybar, or other similar horizontal rod attached to the top of the helicopter's main shaft. While the protractor pitch gauge does the job, I wasn't very happy with it because I didn't feel like I was achieving a very high level of accuracy and also the gauge cannot be used on very small helicopters.

Another tool for measuring blade pitch is the RC Logger Digital Pitch Gauge 2 (40002RC). The Digital Pitch Gauge 2 can be mounted on the main blades and then the gauge simply displays the blade pitch. I rather like the features the Digital Pitch Gauge 2 has to offer, and it appears to be very accurate, too. I have not purchased one, however, because it is very expensive and I am unconvinced that it can be used effectively on very small helicopters.

I then set out to build a simple, inexpensive pitch gauge that I could use accurately on any helicopter. After all, measuring blade pitch with everyday, around the house tools shouldn't be too difficult, right? I tried a variety of bubble levels, bull's eye levels, various protractors, and techniques for measuring the edges of the blades with rulers and vernier calipers. All of these approaches led to gross inaccuracies, were unduly difficult to set up, or were not useful for all sizes of helicopters. Eventually, I settled on a simple plumb line pitch gauge as shown in Figure 1. The plumb line pitch gauge is simply a folded post card taped to the end of the helicopter's main blade, a protractor pivotally attached to the post card, and a length of thread with a small weight suspended from the protractor. Not only is the plumb line pitch gauge very accurate, it is also dirt cheap to build, and it is versatile enough to be used on all sizes of helicopters. Moreover, the plumb line pitch gauge is easy to use—just tape the post card to the end of one of the main blades, put the throttle to 50%, and then rotate the protractor so that 0-degrees lines up with the length of thread. Next, move the throttle stick while observing the angle on the protractor that aligns with the thread.

Figure 2 shows the protractor and post card before having been assembled. The post card is a typical 3" x 5" white card with about 20mm of one end of the card folded into a 90-degree angle. The 20mm portion has a piece of Scotch tape folded over the edge to protect the card from tearing when it is later taped to and removed from the helicopter's main blade. The protractor in Figure 2 is just a piece of paper with angles printed on it and then trimmed to match the size of the post card. The angles can be either hand drawn with a plastic protractor and ruler or drawn with CAD software and printed. For convenience, the paper protractor shown in Figure 2 is included below as a PDF.

An essential feature of the plumb line pitch gauge shown in Figure 1 is that the protractor can be rotated even when the post card is attached to the helicopter's main blade. This allows the pitch gauge to be calibrated to 0-degrees without having to tilt the helicopter or search for a perfectly level surface. A short 2.0mm bolt with two washers and two nuts, shown in Figure 3, is used to loosely attach the protractor to the post card. A critical step in assembling the plumb line pitch gauge is to make certain that the hole for the bolt is centered as precisely as possible where the angle lines intersect on the paper protractor. A needle can be used to poke a hole where the lines intersect on the protractor and then a pencil can be used to carefully enlarge the hole enough to receive the bolt. The post card and protractor are held gently together between the washers and nuts, while a length of the bolt is left extending beyond the surface of the protractor. As shown in Figure 4, a loop of thin thread is hung on the portion of the bolt extending beyond the protractor while a small weight is suspended from the other end of the thread just below the bottom of the protractor. A wide variety of spare helicopter parts can be used as the weight, so long as it isn't too heavy. The weight shown in Figure 4 is an old tail rotor hub from a Trex 450 Pro.

Figure 4 shows the plumb line pitch gauge being used on a 250-sized helicopter. A small tab of Scotch tape holds the post card on the helicopter's main blade. The helicopter is sitting on an old helicopter parts box to allow some extra clearance between the weight on the thread and the surface of the table. The helicopter's motor is disconnected and the throttle is set to precisely 50%, at which point the main blades have a pitch-angle of 0-degrees. Since the table and box are nowhere near level, the protractor initially displayed a positive pitch-angle of 2-degrees. It was easy to carefully rotate the protractor until the thread became aligned with the 0-degrees mark as shown in Figure 4.

The User's Manual for the particular helicopter shown in Figures 4-6 recommends a maximum positive and negative pitch of 12-degrees when the transmitter is in Idle mode. Moving the throttle stick to the highest position rotates the main blades and the protractor as shown in Figure 5. It is easy to see that the positive blade pitch-angle is very nearly 12-degrees. Next, moving the throttle stick to the lowest position rotates the main blades and protractor to the position shown in Figure 6. Again, it is straightforward to see that the negative blade pitch-angle is precisely 12-degrees. Although not shown here, an identical procedure can be used to measure the cyclic throws, as well. Moreover, the plumb line pitch gauge can even be used to set the tail rotor to 8-degrees in Rate mode, if one feels so motivated.

Once you are satisfied that the blade pitch-angles are correctly set for all throttle values, the plumb line pitch gauge can be removed from the helicopter by merely pulling the tab of tape loose from the main blade. The tab of tape can be removed from the post card and then the pitch gauge is ready to be stored for future use. The plumb line pitch gauge is inexpensive, simple to make, and easy to use on any helicopter.
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Jun 18, 2013, 04:08 PM
Registered User

Great Idea


Thanks for posting, I am looking to purchase a Walkera V120D02S and could not find an instrument to measure pitch on a helicopter this small. I have created printout in png format for anyone wanting to use your idea.
Jun 18, 2013, 04:14 PM
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Quote:
Originally Posted by mrthrun
Thanks for posting, I am looking to purchase a Walkera V120D02S and could not find an instrument to measure pitch on a helicopter this small. I have created printout in png format for anyone wanting to use your idea.
I am glad you found the post helpful--thanks for posting a printout, too.


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