Article 1.001 - Why are OF multi blade systems better than others?

First of all, we need to remember that the characteristics that determine how well a multi rotor model helicopter flys really can broken into general ratio of 30% for the rotor head design and setup and 70% from the rotor blades. It is infact the OF rotor blades that contribute the most to the stable hovering and forward flight characteristics.

In the following description, we are discussing in general terms about the pitch used in scale models. Your particular pitch setup will be different and you should not consider our pitch values as any kind of standard, because each scale model is unique and will have a different pitch range. Talking strictly about the rotor head, what separates the OF multiple blade rotor head from other manufacturers is the use of a flexible beam design to limit the maximum pitch that a single rotor blade can achieve. Consider by comparison, a traditional dual-bearing blade grip with a thrust bearing which attaches to the main rotor hub. Disconnect the pushrod control link from the blade grip and commonly the blade grip can rotate freely 360° on its shaft. On an OF rotor head, the blade grip will move freely from 0 pitch until around 15 degrees of positive or negative pitch, which is broken down into 9 degrees of collective lift + upto 6 degree of cyclic.

As the model moves through the air, changes in direction and changes in wind speed cause the rotor blade to deflect further. The rotor head is working normally. Scale rotor heads normally use negative delta to assist in counteracting this motion. While there seems to be a common misunderstanding of whether the correct term is positive or negative, for the purpose of our discussion, we will use negative delta to describe a rotor head that adds a small amount of opposite pitch to help stabilize the rotor blade. You can generally think about delta as a geometric influence on blade pitch as it refers to the relationship of the control ball location and the hinge point of the blade grip. So, returning to our traditional rotor head, it relies only on negative delta to stabilize the rotor blade.

This is where the OF rotor head is superior in design. On an OF rotor head, as the rotor blade deflects upward, past the normal 15 degrees of pitch, the flexible beam takes over and applies an increasing amount of resistance against this deflection, until the flexible beam stops any further rotation of the blade grip. Now remember, the OF rotor head also uses negative delta to stabilize the rotor head which means you have two active systems working together to stabilize your rotor head.

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Article 1.002 - What rotor head rpm should I be using with my OF head?

This is another of those simple questions that doesn't really have an answer because there are so many factors that are different even on two models of the same scale helicopter. The factors that we are talking about specifically are the weight of the model, the choice of engine, the type of exhaust, the number of blades, the rotor blade airfoil, the day's temperature and the windspeed at the time of flying the model. So rather than tell you what your rotor head rpm should be, we will simply state the maximum recommended head speed as a guide.

You can figure out which class your model fits into by taking the standardized OF rotor head span of 240mm and adding 2 x rotor blade length. For example a 5 blade OF rotor head, turning 670mm blades = 240 + (2 x 670) = 1580mm or 1.580 meters.

1.5 meter rotor diameter, maximum of 1,300 RPM
1.8 meter rotor diameter, maximum of 1,100 RPM
2.0 meter rotor diameter, maximum of 950 RPM

These are maximum rotor head speeds and your actual hovering and forward flight speeds will be below these values. As you add more rotor blades your head speed will generally be reduced further unless the engine power is increased and your gear ratio is changed to meet the increased load on your engine. Are there situations where your head speed will exceed the maximum? Yes, it can happen, especially in a models that do not use a rotor speed governor during fast decents. Momentary overspeeding is unfortunately very common and can be extremely dangerous if not corrected.

I would say that the majority of scale models are flown in smooth manuevers that prevent overspeeding in general, but getting the rotor head setup right on a new model is always tricky. This become further complicated when the modeler is unfamiliar with the components and is not receiving adequate help. What do we mean by adequate help? Reading the newsgroups and Internet forums is a very rich source of information and prevents or outright solves many common mistakes. But is cannot replace a experienced modeler's role to inspect the rotor head and be your spotter during your first flights. Now it is quite likely that you simply do not have anyone in your area that has experience with OF rotor heads and you are the first with the OF rotor head system. Our recommendation is to contact us if you are having problems and we will do our best to help you directly or put you in contact with a modeler closer to you who has experience with the OF rotor head system.

As a final comment on rotor head speed, if you are not using a tachometer, we strongly recommend you invest in one. They are not cheap but using the tachometer will ensure that you are not exceeding the maximum rotor speeds provided by OF. In terms of a diagnostic tool, it is quite excellent as it gives you accurate RPM readings normally within 10 RPM and is a good way to track your engines performance over time. Again there are variables that will change day to day, but overall you will learn about which head speeds your model flys best and which speeds to avoid. Common to many larger scale models, especially gasoline and turbine, these fuselages will resonate at specific frequencies which are easily matched to certain head RPMs. A good example I have seen personally is the gasoline Bell 47G which exhibit cracks in the windshield and visible vibration in the landing gear when the head speed is too high.

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Article 1.003 - Is there a special way to install my multi blades to the head?

OF rotor blades are numbered to ensure that they are attached in the correct order. It is as simple as following the numbers. On the 4 and 6 blade rotor heads the rotor blades are attached in pairs because each pair is balanced to its mate. So on the 4 blade head, blade 1 -> 3 and 2 -> 4 where blade pairs are mounted opposite to each other. On the 6 blade head, 1 -> 4, 2 -> 5 and 3 -> 6 in the same manner of opposite pairs mounted to the rotor head. On the 3, 5 and 7 blade rotor heads, all rotor blades are factory balanced together as a set.

What you will find from your own experience in flying multiple rotor heads is that there will be a unique installation order for the blades to your rotor head. Due to slight differences in pushrod linkage length and small amounts of free-movement in the control system your rotor blades will track better in one installed configuration than another. What we talking about is easiest to understand in a simple two blade rotor head. We all have noticed that the exact same rotor blades installed to the head will not track the same when the individual rotor blades are swapped on the blade grips. We normally mark each blade and the corresponding rotor grip to ensure we install the rotor blades to the head in the same configuration day to day.

In a multiple rotor head, this takes longer but the results are the same. On the 4 and 6 blade head, you have verified that each rotor blade is identical to its mate, both in weight and in center of gravity. Matching pairs are installed to the head and during initial spoolups the blades are tracked. Sometimes you are lucky and the all blades track beautifully the first time and on other occasions, blade tracking is close but can be better. The way to start solving this is to track which blade is attached to which rotor grip. Write it down. Then, starting with matched pair of blades that won't track perfectly, swap the blades on the head. Spool up and check tracking. Normally it will either get better or get worse. If swapping blades makes the tracking worse, then record your findings, and switch the pair back to their original positions.

The next step is to swap the pairs of rotor blades on the head, essentially moving all rotor blades together to the next grip on the head. Be careful to ensure you still have matched pairs after moving them. This can be a tedious process, but in the end you will have the optimal installed configuration. Mark the grips and record the install order and this will ensure that day to day you will have consistent blade tracking.

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Article 1.004 - How much should the rotor blade bolt be tightened?

We all have learned that rotor blade bolt tension is important and likely have experience with models where the blades have been too loose and too tight. As the OF rotor blades are significantly heavier than the same length blades used in sport models it is recommended to set the blade tension a little higher. What do we mean? Unlike a sport model with two main blades, it is not convenient to tip a scale model over and apply finger pressure to the tip of a blade when positioned parallel to the ground. This is considered the rule of thumb for sport models as a base then each modeler will have a preference. We firmly believe this adjustment is best done by hand.

As a general rule of thumb for OF multi rotor heads, after each blade is installed, hold the rotor head firmly in one hand and you should be able to move the blade by pushing with one finger from the other hand with some resistance. The goal is to have all the blades set the same, or have the same amount of resistance when moved. Some modelers have been successful using a modeler's torque wrench, but it has been our experience that each blade grip, its matching blade bolt and locknut will differ very slightly. We have found that the torque wrench will get you close, but the final adjustment by hand is necessary.

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Article 1.005 - Are there any common symptoms to watch out for while testing?

As with all new projects, the first time you go out to the field to spool up your new OF rotor head and blades is very exciting but should be done with caution and with assistance to help with fine tuning of the rotor head and the model. As there are many reasons you may experience problems, we have broken the following sections into categories to help you understand why and how to resolve common problems.

Engine - We do not recommend running a brand new engine in any scale model. The engine you have chosen for your scale project should be well run-in in another mechanics where you have good access the carburator and you are comfortable with making carburetor changes day to day as the temperature or humidy changes. The engine type will determine how long the run-in takes, but gasoline engines normally take longer. The danger with a new engine, is its rather rough running and an incorrectly set needle valve can cause a flame out which at low altitudes in a scale model can be disasterous. Just don't do it.

Engine Exhaust - Scale models commonly use mufflers and modified exhaust systems to fit into their fuselage. While problems here are minor, they strongly influence how well the engine runs, especially for engines that rely on back pressure to provide a consistent flow of fuel to the carburetor. Always inspect your model after each flight to ensure there are no leaks from the exhaust manifold and that the bolts securing it remain tight. Any fuel leakage or exhaust residue is a good indication that a problem is likely to occur.

Blade Tracking - Upon following the recommendations in our faq to get the blades to track properly, there are still additional areas that can contribute to poor rotor blade tracking. Look for wear or any distortion in the ball links that attach from the swashplate to the blade grips. Double check the length of these pushrods also, hunting down tracking problem can easily result in one or a few linkages being significantly different lengths than others.

Fuselage Vibration - All models have some vibration. There are different generators for vibration which are the engine, clutch, and cooling fan assembly that produce high frequency vibration and there is the rotor head that generates low frequency vibration. High frequency vibration that is generated by the engine can be minimized by ensuring that all rotating shafts have a runout of 0.002" [0.051mm] or less using a dial indicator. Low frequency vibration can be harder to see but there are always signs. In some fuselages, this vibration is heard in the fuselage as resonance. We have to determine whether a fuselage audible vibration is due to an inbalance in the rotor head or the head is balanced but a fuselage construction problem exists where the fuselage is insufficiently supported. Sometimes, the landing gear connection to the fuselage is not strong enough and a vibration occurs. It is not sufficient to only have the landing gear contact the fiberglass fuselage. The landing gear must have some rigid connection to the helicopter mechanics and more importantly to the upper frames that support the rotor head.

Helicopter Vibration - Model helicopter reasonance is real and can be seen on both sport and scale models. Most reasonance we have personally experienced is with rotor blades that are too tight on the rotor head and in models where the landing gear is loosely coupled to the fuselage. This type of vibration is normally seen as a violent shaking of the helicopter and can be linked to specific rotor head speeds. The best method to handle these situations is to apply more pitch to the rotor head to reduce the time that the rotor head remains at a particular speed. While very rare, this occurs in very low head speeds during spool up or shutdown and can normally be corrected by swapping blades in the rotor head. There are many factors that can contribute to model helicopter reasonance but some models are more likely to be exhibit resonance than others. Most importantly, if you are unable to solve a reasonance problem, don't continue to fly the model, get help. Contact us about your problem and we will do our best to help you work through the issue.

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Article 1.006 - In a hard landing I cracked the tip of one multi blade, what should I do?

This is an unfortunately expensive experience. Modelers new to helicopters are at greater risk as they tend to stay closer to the ground where this can happen in a flash. A gust of wind, especially hovering in ground effect or a slow turn too close to the ground can result in a blade touching the ground. Discard the damaged blades. On rotor heads with 4 and 6 blades, it is only necessary to change the matched pair. On 3, 5 and 7 blade heads, unfortunately you need to change the whole set. While you may only see a small crack in the tip of the blade, there will be extensive internal cracking throughout the length of the blade. It will fail and if it fails when there are other modelers around, it can be potentially fatal. It is far cheaper to replace the blades now than the whole helicopter if the blade fails in flight.

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Article 1.006 - What is a multi-mixer and do I need one?

To be brief, it is recommended. The multi-mixer for the rotor head does what our tail rotor gyro does in stabilizing the tail rotor in flight. It normally attaches between the servos and the receiver and can be used on an older transmitter that does not support CCPM (cyclic collective pitch mixing). The key to the mixer's design is that gyroscopes can be added to the pitch (elevator) and to the roll (aileron) channels to stabilize the model during flight. Mixers like the Helitronix also have an electronic swashplate phase adjustment built-in, so that your rotor head linkages remain close to linear. The phase then becomes a data point instead of manually changing the swashplate follower's position on the main shaft.

You don't require a mixer to fly a multi-rotor model, but it sure helps in dealing with the unfamiliar and strange tendancies that makes flying a multi-rotor head model a real challenge. The OF rotor blades also contribute the most to making your model fly smooth. The heavy, leading edge weighted blades are the real secret in good performance, but the mixer compensates for almost everything else. Traditionally, multi-rotor models have been very sensitive to pitching up in forward flight which can happen when you least expect it, but more of a problem is the tendancy for the model to roll to the side as the head RPM changes. So many scale models were converted back to a simple two blade head for flying.

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