All-Things Speed Flying!
Introduction
Speed flying in helicopters has been around for a while now in various forms, but only in the last year or so has it begun to gain more widespread acceptance as a pastime within the hobby. The concept is rather simple: How fast can a helicopter travel through a course of known length? It’s not unlike the Bonneville Salt Flats racing that is done every year out in Utah, for those that are familiar. Straight-line speed is the name of the game and we want to see how fast you can go in both directions to eliminate directional and wind bias from your final score.
Let’s get started with the models…
Basic Model Setup
Size and Configuration
While there are some very speed-specific models out there, really all you need is a standard pod-and-boom heli to get started. Historically, speed flying has been focused primarily on the larger models in the 700mm blade size range, but recently and with the huge increase in robust and aerodyamic smaller helis available to the consumer, there has been an interest in the so-called “mini” classes categorized by blade sizes between 200mm and 630mm. Fully-fuselaged models are the pinnacle of speed flying and also tend to offer two-stage, heavy-duty drive trains capable of withstanding the extreme power levels of today’s motors, but if you’re just getting started, then I wouldn’t worry too much about these right now, as they’re expensive and relatively rare. So, for now just choose one of your models and let’s roll with it…
Center of Gravity
First and foremost, your model has to have a nose-forward CG in order to excel in speed flying. Why? In order to make effective use of the main blade disk as a propulsor, it needs to have a negative angle of attack (tipped forward) to the oncoming air. Applying maximum collective to a model that is level with the horizon will translate into a climb-out, not any appreciable forward speed.
To do this, the easiest ways are to either move your batteries forward or add lead weight to the nose of the canopy using some double-sided tape or more permanent epoxy if you’re committing to really focus on speed with this model. Check your CG by holding your model by the main blade grips in a knife-edge orientation, thus allowing it to rotate freely about the main shaft. Check from both directions (nose left and nose right) to be sure that your drive train gearing and one-way bearing aren’t affecting the results. What you’re looking for is to have the nose pointed down at a minimum of 5-10 degrees. You’ll have to play around with the exact angle that optimizes your flight performance, as each model is different.
Head Speed
Now that we’ve got our CG set correctly, let’s talk about head speed. Most 3D helis (relative to 700mm blades for reference) turn the head at between 1900-2100rpm. This is great for 3D, but very slow for speed flying. Check with the manufacturer of your model and blades to determine what they recommend as the maximum safe head speed and shoot for 50rpm below to put some margin between you and the limit. This may require you to increase your pinion tooth count or in extreme situations, increase your motor kv. For now, we won’t worry about ESC tuning, so let’s just stick with the standard 20% throttle head room on your governor and see how it goes.
Flybarless Gains
This one turns out to be pretty simple most of the time. You’re turning your head and tail much faster, thus there is more “mechanical” or inertial authority in the system. What does this mean? You don’t need as much electronic gain to compensate. Start by lowering your head and tail gains by 5-10 points depending on your flybarless system and see how it goes. You’re looking for a fast tail wag and a head or blade disk shake/wobble during high-speed flight. If you get this, then keep decreasing the gains.
There is also a secondary benefit to reducing tail gain. Because your model is moving at high speed (if you’d done it correctly), the pod-and-boom shape has a natural tendency to “weathervane” or align itself with the direction of flight, thus reducing the need for tail compensation of main rotor torque. By reducing the tail gain (and tail blade size in a more advanced setup), you are reducing the amount of work that the tail puts into holding the model’s heading. This, in turn, reduces the amount of power needed by the tail to do its job and as a result, you have that excess power available to put to the main rotor for more speed.
Blade Pitch
This is a tricky parameter to get right and it’s the single largest reason why beginners suffer from the dreaded “pitch-up” otherwise known as retreating blade stall. Let’s talk a little physics before we go any further. It may help for you to get out a pen and paper to draw what I’m describing because you’ll really be able to see what’s going on that way.
Let’s look down on the disk of our heli from above. The head rotates clockwise and the nose points in the direction of flight, which for this exercise will be up towards the top of the page. Now, draw your main blades directly over the left and right sides of the model (90 degrees to the boom and flight path). Label the left blade “advancing” and the right blade “retreating.”
Ok, here’s where it gets fun… Imagine that you were able to shrink yourself to about an inch tall and stand on the tips of your blades with a wind speed meter facing the leading edge while the model is flying through a speed pass. What would you see?
On the advancing blade, you’re going to measure a total wind speed coming towards your leading edge that is a combination of the speed of the blade tip through the air and the forward speed of the helicopter. They’re both in the same direction, so they add together to make a bigger number.
On the retreating blade, you’re ALSO going to measure a total wind speed coming towards your leading edge (which is facing towards the back of the model) that is a combination of the speed of the blade tip through the air and the forward speed of the helicopter…EXCEPT that on this blade, they’re in opposite directions, thus they subtract from each other to make a smaller number.
This highlights two problems unique to rotorcraft: 1) reaching critical mach number on the advancing blade, which can cause a significant increase in drag (compressibility effects) and 2) reaching too low an airspeed over the retreating blade airfoil that, for a given pitch or angle of attack, can cause stall.
The second one is where we’re really concerned. If the retreating blade stalls and we remember that gyroscopic precession causes action 90 degrees behind the input, then the retreating blade sees a zero lift over the tail boom while the advancing blade now sees positive lift above the nose. The result? A pitch-up…and potentially soiled shorts.
Why are we talking about this in the blade pitch section? Because everyone things you need 48 degrees of pitch to go fast…ok, slight exaggeration, but you get my point. Going to high pitch without sufficient head speed will put you at higher risk of retreating blade stall (RBS).
Furthermore, a lot of beginners mistakenly think that keeping the head speed low will actually be “safer” for them to learn on. Yes, this is true to a point, but if you do manage to nail a solid pass and get some decent speed and your head speed and pitch are unbalanced, then you’re going to hit RBS and may poop yourself or worse, destroy your helicopter. So, let’s start by keeping the collective pitch below 12-13 degrees. Actually, 11-12 degrees is a perfect starting point to learn how to fly speed and then once you get better, you can move up from there.
Blades
Once you understand everything we’ve just talked about, you should be able to pick up on why blades are so important in speed flying. There are two main features we want to look for: 1) blade weight and 2) CG. A good rule of thumb for speed blades is to go with something that is 15-20% heavier than typical aggressive 3D blades in that size range. For a 700-class model, shoot for blades in the 225-250g range. Some good examples are Radix 710 V2’s, Rail 716’s, and DH 711’s. Once you get the hang of it, you can find a few speed-specific blades in that size range like the SAB 720 Speed’s or the famed MicroBeast X-Blade 713’s, which are currently the best of the best for that size.
Regarding CG, we don’t want an aggressively CG’d blade like you’re used to seeing in 3D. Instead, we prefer a blade who’s span-wise CG is further out towards the tip and who’s chord-wise CG is neutral. Again, refer to the list of blades above for examples of this…
With tail blades, you should plan on dropping one size below what is typically run for the heli size you’ll be flying. As I mentioned in the gains section, you want to minimize the power lost to the tail rotor and since weathervaning is going to help you maintain heading, you don’t need those big 3D tail blades.
Flying a Speed Pass
The easiest way to get started in flying speed is to use the good old stall turn. Get your heli oriented to fly lengthwise along your field and give yourself some altitude. Start by flying slowly towards one end and begin to pull up into the stall turn. Once you get to the top, get your tail spun around so that the nose is pointing downward and then fly through the stall turn and smoothly pull up at the bottom to enter the straight line pass. Simple, right? Let’s dissect this a bit more…
When you enter the stall turn, you need to make sure to feather out collective as you feather in cyclic or else you’ll get yourself into a situation where you’re either pulling over the top to do a loop or in extreme and high-speed cases, you’ll over-current your ESC because of a combination of collective and cyclic pitch inputs.
Ok, so you’ve made it through the pull-up. Make sure that your model is pointing straight up in knife edge and isn’t drifting to one side or the other. At the top, take the second or two of zero speed to check the heli’s orientation. Sight off of the disk and the tail. Is the disk in knife edge? If not, then get that nose down so that you don’t gain any horizontal distance until you pull up and into the run. Is the disk rolled to one side? Well fix it or else you’re going to be flying at yourself or away from yourself. Neither are desireable… How’s the tail look? Make sure you’re pointing straight down or when you pull into the run, you’ll “crab” to one side, which kills speed and can cause drift in your course.
Now you’re into the straight run and you just need to make sure that you keep it in the same place, right? No change in altitude, no change in direction, no small, speed-robbing corrections. Well, if you’ve set your CG, head speed, and pitch correctly, then you should have almost a hands-off straight-line run through the course.
If you find that the model has a tendency to balloon up, then you need to adjust your CG and check your collective pitch again. Adding more forward cyclic in during extreme cases of balloon may actually result in RBS because you’re adding even more pitch to the retreating blade, but this will only happen at relatively high speeds. There are also some parameters in your flybarless system that will likely help compensate for the tendency of the model to balloon, but you need to work on the mechanics first before you start messing with the electronics.
Practice your stall turns on both sides because no one ever won a speed competition with a REALLY FAST left-side run. Remember, your score is an average of your best left and best right runs, so you need to be able to do both equally well.
More Advanced Maneuvers
Once you’ve mastered the stall turn and want to mess around with the more advanced methods of transition, then look no further than the reverse-half Cuban 8. Now, before we get into this, let’s talk generally because what I’m about to say applies to all speed flying transitions.
First, ANY maneuver that allows for minimal reduction in speed from the previous pass while transitioning into the next pass is preferred to one that doesn’t allow this… There are two things that need to be balanced when exiting a run and transitioning to the next entry: 1) Speed and 2) battery consumption.
Battery consumption is the easiest to think about in that, the more that you consume of your battery in maneuvers outside of the course, the less voltage holding capability (read: headspeed and load maintenance) your system has to allow for a solid run.
Speed is the more challenging to figure out if you haven’t thought it through completely. The most obvious thing that speed does for you is to get you through the transition quickly so that you can minimize battery consumption outside of the course.
The other thing it does for you is maximizes your entry speed into the course when harnessed through the proper maneuver. A reverse half Cuban 8 (or any looping maneuver for that matter) benefits from high entry speed because of the fact that the rotational speed through the loop (how many degrees of loop the model traverses per second) translates directly into tangential speed (straight-line speed) at course entry.
Why? Your initial entry speed into the course or tangential speed is calculated as follows:
v_entry = radius of loop * rotational speed (in radians/s)
Or more simply, v_entry = 2 * Pi * radius of loop / time it takes to get through the loop
So, looking at the second equation, the larger the loop and/or the shorter the time it takes to get through the loop, the higher the entry speed is… Ok, so how do we maximize entry speed? Again, it’s a balance of a number of different things.
A larger loop does increase speed and gives the pilot more time to sight and position the model correctly, but also takes longer to complete, which increases battery consumption. If you choose to make the loop smaller, then you’ll have to make sure that you have a higher entry speed and a more rapid transit time through the loop, but you’ll minimize battery consumption. On the other hand, you better be precise with placement because a smaller and faster loop doesn’t give you much time to get things well aligned for course entry.
To clear up confusion on the stall turns, when you see more accomplished (I won’t refer to them as pros because the competition circuit isn’t nearly as evolved as the 3D world) pilots do a stall turn, it is typically on their first run.
Why? They don’t have the entry speed from a previous run to take advantage of and trying to get it by running high pitch early on will eat up the battery. As such, they save their battery for the subsequent runs by slowly and at low head speed, getting to an altitude where a stall turn will get them a decent amount of speed to enter the first run. Once they’re in the course, they should have enough speed to exit into a good reverse half Cuban 8.
Getting your RHC8 down does take guts, but in fact, doing slow ones makes it more difficult. I found that I needed to just bite the bullet and get into full-speed passes with RHC8 transitions to get the feel for it.
Sight your line so that when you exit the course, you can execute an immediate 45* pull-up to start the RHC8. Make sure that you’ve brought collective down before hand or else you can pull more current on a pull out of the course than during the actual run if you’re not careful.
Next, roll over, check your tail positioning, adjust and then rapidly feather in as much collective (preferably all of it) as you can while maintaining a clean line through the loop. Once you’ve got your collective set, then it just comes down to holding the loop and exiting at the right time and angle such that the model is in straight and level flight through the course.
A lot of people have trouble with applying full collective early on in the loop and wait to do it right at course entry, which results in a momentary ballooning of the model followed by restoration of the proper nose-down angle for the run…this reduces speed and uses up battery in a non-productive way.
It’s easier said than done and I still make this mistake ever so often, but if you find yourself within 30* of the end of the loop and entry into the course and you’re not at full collective, then don’t bother. It won’t get you much if any more speed and it’ll chew through precious battery capacity that you could use more productively on a well-executed pass later in the flight…not to mention that everyone will know that you got over-zealous with pitch.
Competition
Now that you’re an expert in speed flying, let’s talk about the competition scene. There’s not much of one yet, but we are seeing an increase in interest around the world and in particular, in the US. The main competitions that are available are the IRCHA Speed Cup, The Poting Speed Cup, and the newly formed OHB Speed Cup. There are also smaller events that are holding grass-roots style events, so keep an eye out in your area or get one started yourself.
The official governing rule set for heli speed flying is covered in the FAI F5 – 203, which is an F5 Open category covered in SC4 Volume ABR 14 Section 2.5. Feel free to dig into it if you’d like more detail, but for now, I’m going to give you the general feel for things.
For the big models (>600mm blades), the typical course layout is a 200m long course over which the time between the two gates is measured via a variety of means to determine the average speed of the model. For more serious competitions, the FAI ruleset is followed, which includes a 100m pre-stage area on either side of the 200m course. This forces the pilot to enter the pre-stage at a level attitude (not in a dive) such that the actual speed through the 200m course is not significantly affected by any speed picked up in a dive prior to entry.
For those competitions that either don’t have the space or the inclination to be so serious, a standard 200m course without pre-staging is used. The general rule is still that the model must not be diving into the course, but there is no need to be level for a full 100 meters prior to entry into the course.
The courses are usually laid out with two sets of pylons that define a 200m long course left to right and a lane front to back between the two sets. While flying within the lane is not necessary in all competitions, those which make use of more advanced camera-based timing systems have a limited window in which the model is reliably detected and this usually corresponds to the lane defined by the pylons.
For the smaller models, there’s been much more variation in course size based on their relative newness to the competition scene.The typical course is either 100m or 120m, both of which work very similarly. The justification for the 100m course is that it is exactly half of the standard 200m course where the 120m course is derived out of a desire to maximize the amount of time the model is in the course such that timing error does not represent a significant portion of the measured value. At the current time of writing this, the speeds of these smaller models are low enough to the point where I personally don’t see value in forcing the pilot to fly an extra 20m, as the measured times are on the order of those observed in 200m competition with the larger models.
A typical class break-out would be as follows, although variations on this theme are perfectly fine and should be left to the discretion of the event director.
200-499mm blades, 100m or 120m course, 6S lithium polymer voltage limit
500-630mm blades, 200m course, 12S lithium polymer voltage limit
631-820mm blades, 200m course with 100m pre-staging, 14-16S lithium polymer voltage limit
While the models in the first two categories are almost always pod-and-boom configurations and are almost universally referred to as mini classes, the larger models have started to show up with full fuselages on them. Most notably, The Minicopter Diabolo Speed, the Henseleit TDS, the TDV fuselage updgrade for the Henseleit TDR1, and other various custom designs. In this largest class, there is usually a further segregation between fuselaged models into a so-called Unlimited class and higher-power, custom pod-and-boom models into an Open or Pro-Modified class, and stock pod-and-boom models into a sportsman class.
What about actual speeds? What can you expect out of your model? Well, at the time of writing this article, the top speeds in these classes as measured in recognized competitions were as shown below and represent averages of left and right passes measured via a chronometer and/or a camera system.
200-499mm blades:
- 96.9mph/156.1kph un-official, 6S Goblin 380, 2015 IRCHA Speed Cup, Mini A Class
500-630mm blades:
- 133.9mph/215.6kph un-official, 12S Gaui R5, 2015 IRCHA Speed Cup, Mini B Class
631-820mm blades:
- 177.6mph/286kph official world record, 14S Minicopter Diabolo Speed, 2015 Ballenstedt World Records, F5 – 203
- 202.4mph/326kph un-official, 14S Minicopter Diabolo Speed, 2015 France 3D Cup, Unlimited Class
Wow! That was a lot of discussion, huh? Well, I hope after reading through all of this you have a much better feel for what the world of speed helis is all about. I will say that some of this is not necessarily absolute and may be subject to debate amongst more experienced speed pilots, but generally speaking it should get you well on your way towards becoming an experienced speed pilot. I think I covered everything, but if you think I missed something, then feel free to let me know and I’ll get it updated.
Also, look for my upcoming tech tip on how to put on your own grass-roots speed competition complete with a description of how to set up the course, do the judging, run the pit, and build the pylons. As always, if you have any questions, please feel free to contact me at .
– Justin