Since many of you builders out there are getting to the point where you are setting up controls - the "Light at the End of the Tunnel", I have prepared this page to help you along.

Shown above is a nice photo of the Gyrobee head assembly made during our first flying season. I have labeled the critical parts to make it easier to make sense of the discussion which follows.

We installed a pitch trim spring since everybody had them. With our original 25 foot Rotordynes, so little spring tension was required, that we finally removed it entirely. With no spring, "hands-off trim speed falls between 45 and 50 mph - perfect! The Brock blades also flew well with no spring. In contrast, the original version of the Dragon Wings (no trailing edge reflex) require a lot of spring pressure and we even had to move the spring attachment point out to the control-bar end of the pitch bar to make maximum use of available leverage. The current Dragon Wings blades have a reflexed trailing edge, which should reduce the downward pitching-moment, but I have yet to fly a set.

 On the prototype Gyrobee, the range of head movement in roll is 10^o right and 11^o left. Some heads have less and I would strive to try to get the 20 degree total range. In the photo above, note how the head mounting blocks are offset upward just a bit to provide clearance for the bottom edge of the roll block. If they were down a bit between the cheek plates, the roll block lower edge would hit the upper edge of the cheek plates, limiting the range of movement in roll. Little details like this can add precious degrees to your total range!

During the early stages of blade spin-up, it is desirable to be able to hold the blades angled back as far as possible. The steeper the angle of the blades to the rear, the easier the initial stages of spin-up and the shorter will be the taxi distance required for takeoff. This is particularly critical for the Gyrobee, since it does not rock very far back on the tail. The limits to rearward travel of the blades are the possibility of the blade tips striking the ground or the blades hitting the vertical fin. Given the tall mast of the Gyrobee, hitting the ground is not an issue, even with a 25 foot rotor disc. Hitting the tail is another story, since the tail-boom of the Gyrobee is longer than most, to simultaneous make the rudder more effective at low airspeed (or engine-out) yet less sensitive in normal flight. The practical rear limit of head movement can be determined by bringing the head all the way back and pulling the blades down to their teeter-limit stop on the head. At this point, you should have about 6 inches of clearance between the blades and the point of closest approach to the tail. This occurs on the Gyrobee (with the Brock fin/rudder) at 20 degrees back with the mast vertical.

Note: If you are designing your own tail group, making it too tall will restrict your rearward limit.

In practical terms, you need enough forward stick to maintain approach speed in the event of an engine-out landing. On the other hand, I did not want so much forward travel that it was easy for the rotor disk to "go negative". On the Gyrobee, the head is set up so that, with the mast vertical, the pitch bar is dead level at the forward limit of travel. The only time we ever have the stick at its forward limit when we are moving on the ground and want to maximize vertical blade clearance in all directions. Both real and simulated engine-out landings have demonstrated ample stick range well short of fully forward, so it is possible that the forward-limit of head travel could stop at the +4-5 degree point without a problem.

All the head travel measurements quoted so far were made using a magnetic protractor, available at most Ace Hardware stores. It is a good idea to check your head, prior to mounting, to see the range of travel available. In the case of our Rotordyne head, we had a total of 20 degrees range in both pitch and roll. Other heads seem to have less range (16-18 degrees). Often you can widen the range a bit in the pitch axis by carefully filing or milling the top of the mounting blocks, since these often serve as the limiting factor in pitch travel. Any alterations should be slight (you don’t have to take off a lot of metal to make a significant change, and the surface should be angled such that the pitch bar hits the limits parallel to the finished surface. This will minimize stress on the pitch bar, as opposed to coming to a stop against a sharp angle.

The cheek plates serve to hold the head at a specific angle to the mast, as well as a specific position, fore and aft, as determined in your hang test. On a Bensen-type machine, the mast is angled back 9-10 degrees and the head is aligned with the mast, resulting in the head axis being angled back approximately 10 degrees. This angle approximates the angle of the rotor thrust vector in flight, but there is a fair amount of latitude in this angle that we can use to our advantage in setting up the Gyrobee rotor system. You can work everything out in whatever way that suits you, but the following sequence of steps should get you into the ballpark with as little wasted time as possible.

(1) Dummy Cheek Plates. It would be a good idea to have materials in hand to make several sets of dummy cheek plates out of half-inch plywood. At different stages, these plates can be attached to the mast and the head with hardware store bolts and washers (stainless hardware is excellent), as we will be simply doing set-up. The final aluminum flight-ready plates will be attached with aircraft hardware at a later point.

(2) Calculate the Head Angle. The first step is to calculate a target angle (HA) for the head based on the range of head travel in pitch (HT) and the desired rear limit (RL) for head travel. The formula looks like this:

If I run these numbers for the prototype Gyrobee, the desired rear limit of travel (RL) = 20^o and the total head travel (HT) on the Rotordyne head = 20^o: Substituting, we get:

Thus, I would want to set the head at 10 degrees (aft) with respect to the mast (which is vertical). Let's say you want the same rear limit (20^o) but are using a Rotor Hawk head that only provides 16 degrees of total travel (HT). In that case we would get:

In the case of the Rotor Hawk head, we would need to set the head at 12 degrees (aft) with respect to the mast to get the same rear limit. You can plug in your own numbers to see what you will require.

(3) Calculate the Forward Limit of Travel. Having worked out the head angle for the rear travel limit (the most important parameter) we can now check out what that will give you in terms of the forward limit (FL):

Remember, with the Gyrobee prototype, HT was 20^o and HA was 10^o. Substituting, we get:

Not surprisingly, this is what we actually get! In the case of the Rotor Hawk blades (HA = 12^o and HT = 16^o), the numbers would be a bit different:

 As indicated earlier, we definitely don't want a negative angle for FL. In this case, 4 degrees positive would probably work just fine. Again, you can work out the numbers for your components and aircraft. If FL is greater than 5 degrees, you might want to work on the head to get a greater range of Travel (HT) and then repeat the last two steps.

(4) Check Out the Numbers. Now you have a set of target values, make a dummy set of cheek plates with the head mounted at angle HA and then see if the head travel limits (mast vertical) match your calculations.

(5) Do the Hang Test. If all is well, do the hang test with your dummy plates - remember, half a tank of fuel or water and you in the seat. The desired "hang angle" is 10 degrees nose down as measured at the keel. If you are willing to settle for a degree or so of error, it should be off on the nose high side, not nose low. If you have to make additional dummy plates, use the same value for HA, but simply more the head forward or backward with respect to the mast until you get the desired hang angle.

(6) Real Cheek Plates. Once you have the head positioned for the proper hang angle, make your "real" plates from 6061-T6 (1/8 inch), following the plywood dummy layout. Install the plates and head with the specified aircraft hardware.

(7) Control Rods. Once the head is properly angled and positioned and permanently installed, proceed to fabricate the control rods and do a check of actual head range with the stick. At this point, any fine-tuning should involve the stick and linkages, since everything else is known to be set up properly.

Proper blade rigging requires that the blades and hub bar system meet four criteria:

• The rotor must be properly balanced with respect to blade chord
• The rotor must be balanced span-wise
• Both blades must be set to equal pitch
• The blades must be "in-string"

I will cover each of these criteria in the sections below.

Chord-wise, your blades should balance at about the 25% point. With most modern blades, this is something that is taken care of by the manufacturer. If you are planning to use the Fleck extruded blades, you have to install the supplied rod stock inside the extruded blade section to achieve proper chord-wise balance.

If you purchase your blades and hub bar as a set, the manufacturer will have set them up for proper span-wise balance. The one caution is that the blades will be individually labeled with a letter or number (1 and 2 or A and B) with corresponding labels at the end of the hub bar. Just make sure that you match up these labels when mounting the blades.

Although there are several ways of expressing the pitch of a rotor blade, when it comes to rigging, pitch is expressed in positive degrees (pitched up) with respect to the inboard section of the rotor hub bar. How much the blades are pitched is a trade-off between two attributes - lift and ease of spin-up. Increasing blade pitch, to a point, will increase lift and limit forward speed. Unfortunately, greater blade pitch settings make manual starting and spin-up more difficult. When flying our Rotordynes and testing the Brock blades, we pitched the blades at +0.75 degrees. Other blades may perform better at different pitch settings.

No matter what value you use for blade pitch, it is very important that both blades be set to the same value. If they are not, they will be out-of-track and stick-shake will be the result.

You do not need to worry about setting the pitch for Dragon Wings or Sky Wheels blades. Dragon wings use blade twist instead of pitch. The twist is built into the blades and there is no provision for adjustable pitch. Sky Wheels blades plug into the composite center-section and the pitch is thus preset. For other blades, you will have to set the desired pitch using the adjustable pitch blocks.

Pitch adjustment cannot be done accurately using the scribe marks on the hub bar. We use a pair of rods, clamped on either side of the pitch blocks (pointed forward relative to the blade). The rods are made of 1/2 inch aluminum angle stock (1/8 inch thick), so they don't bend easily and the chance of error is minimized. Small C-clamps are used to secure the rods against the surface of the hub bar. If the rods are set up to measure 57.3 inches from the center of the hub bar to the far end of the rods, 1 inch is displacement is equal to 1 degree of pitch. If the distance is reduced to 28.6 inches, 1 degree of pitch will equal a displacement of 0.5 inches. The ends of the angle-stock rods can be angled toward each other slightly so they almost touch at the far end, making it easuer toi measure the displacement.

We use the longer rods, so that one-inch is displacement is equal to 1 degree of pitch. To set the blades at +0.75 degrees, for example:

1. Loosen the pitch block bolts slightly.
2. With the rods in place, rotate the outer (blade end) section of one pitch block until the outer (blade end) rod is 0.75 inches higher than the inboard rod.
3. Carefully tighten the bolts on the pitch block to retain the offset you used.
4. Repeat the process at the other end, trying to achieve exactly the offset you put into the first blade.

Setting the pitch this way is easy and accurate - far more so than any other way you can do the job.

Some blades, by virtue of how they mount, are automatically aligned. These include the Sky Wheels and Brock Blades. Some other blades are made to such close tolerances, such as Ernie Boyette's Dragon Wings, that they be aligned just fine from the start. It never hurts to check, since near perfect blade alignment is a requirement id you want to avoid stick shake.

Your blades are properly aligned with each other and the hub bar when a line projected from any point on one blade to the same point on the opposite blade, passes over the geometric center of the teeter block at the center of the hub bar - as shown in he simplified diagram above. Since we need something more practical than an imaginary line to check this, a "string" is usually strung from the reference point on one blade to the same point on the opposite blade. The blades are said to be "in-string" when the line passes directly over the center of the teeter block. The bolt holes on most blade straps are just slightly oversize, so when the straps are bolted to the bar (finger-tight) you can adjust blade alignment until they are in-string. At that point, you carefully tighten the blade strap retention bolts to keep them properly aligned as the nuts are torqued. While simple in principle, there are a few practical tips to make it easier to do.

1. If the top-center of the teeter block is not already marked, use a sharp pencil and straight edge and draw lines from opposite corners. Use a center-punch to permanently mark where the lines cross. Do this job carefully or you will waste you time each time you have to string the blades!
2. Use a length of heavy (20 lb. Test) mono-filament fishing line as the "string".
3. Attach the blades to the hub bar with the bolts finger-tight.
4. Block up the blade tips and hub-bar center so when the line is strung between the blade tips it passes just over the teeter block, without touching the block.
5. String the line, taught, between the same two points on each blade tip. If you use the tip if the trailing edge, you can clamp the line with small, spring-loaded paper clamps.
6. Adjust the blades so the mono-filament line crosses the marked center point on the top of the teeter block.
7. Carefully tighten one set of blade-strap bolts/nuts, striving to hold the alignment.
8. Readjust the remaining blade slightly, if required, and tighten the bolts/nuts on the remaining blade straps.

At this point you should have a properly set-up rotor head and your blades are equally pitched and in-string. If everything else is ready (including your training!), it's time to fly!