ICE Helicopters   ·   Oct 10   ·   read

Lucky 13 – One of my Favourite PPL(H) Exercises to Teach! But Why?

Helicopter transitioning to forward flight from hover at Elstree

If asked, I am sure most helicopter instructors would say their favourite training exercise involves autorotations – the practice of getting a helicopter to glide gently down without engine power, from my view is both incredible and highly rewarding; the finest thing to see a student enjoy.

Another favourite part of the PPL (H) Training Plan is confined area approaches (and departures!) – which, once mastered, opens up the world of off-airfield operations such as a visit to a hotel or restaurant, it is this flexibility that encourages so many people choose to learn to fly a helicopter.
With hundreds of helipads around within an hour flight of our Ice Helicopters Training hub at Elstree Aerodrome, there are certainly more venues for a lovely lunch or afternoon tea than we could manage the calories to visit!
 
I thoroughly enjoy teaching all of the exercises, but I’ve taken a slightly different stance from the ‘norm,’ and would say my one of my most enjoyable exercises to teach from the PPL(H) syllabus is Ex. 13; Transitions.
In particular, it’s teaching the transition from the hover into the climb. Today, I’ll explain what I find so rewarding about it as well as hopefully giving some educational insight into what’s contained in the exercise itself.
Throughout the first few helicopter PPL exercises, whilst the student is learning the very basics of flying, there will be numerous occasions where the instructor will take control, fly out of the circuit and then hand over the controls once established in forward flight. You may be following us through on the take-off, but it’s not until reaching Ex. 13 that it’s really demonstrated what’s going on.
This is mainly due to the fact that from the novice perspective, a transition to forward flight looks like the pilot is doing something reasonably simple, like this:
  1. The helicopter is pitched forward (nose down);
  2. It begins to accelerate and gain airspeed;
  3. Once it’s going a bit faster, the helicopter will then climb away.
When students become decent at correcting for all the small attitude and height changes during a basic hover/hover-taxi, and they attempt transitioning to forward flight, it usually goes fairly smoothly and feels similar to the above.
However, come Ex. 13, you are shown all the effects which you pretty much subconsciously end up correcting for without even thinking about it. Once the cyclic is moved forward to pitch the nose down to begin the take-off, what actually happens is:
  1. Additional power is applied;
  2. Lateral (left/right) cyclic movement is made;
  3. The cyclic is moved forward, again;
  4. Pedal inputs change (sometimes fairly substantially);
  5. The cyclic is moved forward, again;
  6. Shortly thereafter…forward pressure on the cyclic is then released…!
As you can see, there is a fair amount going on in quite a short space of time, but like most movements in a helicopter, the inputs are so small that you probably wouldn’t even notice them even if you were closely following through on the controls.
It’s this – showing students that what previously looked just like “nose down…walk…run…fly,” but when broken down there are actually so many aerodynamic effects happening to the helicopter which students were previously quite unaware of, that I find so rewarding!
Even when I was learning to fly, I can still remember being fairly surprised to see how much I was previously compensating for without having thought about it.
 
So, what exactly is happening and why?
 
As I’d mentioned, there is a lot going on quite quickly and so it’s important not to go deep into the aerodynamics at each second when teaching the exercise. It’s an early one, so you’d still be learning lots of other things too and don’t want to get overloaded – therefore, a little bit of pre-lesson reading is usually required just to grip the basics.
 
For those who are learning to fly or looking to learn, hopefully the below can explain the inputs I’d noted above in a fairly simple way without being overly technical, nor too vague!
 
Note – depending on the type of helicopter and other conditions, these can happen in slightly different orders and with varied amplitude.
 
1. Effect: Loss of Height
As soon as you tilt the rotor disc forward to start the acceleration, you fall off the “ground cushion” and change the angle at which the rotors are producing thrust. It’s going from solely vertical thrust, to a bit less vertical and now some horizontal which causes the helicopter to descend.
Compensation: Apply some additional power to prevent the sink.
 
2. Effect: Inflow Roll
You have now tilted the disk forwards and are moving in that direction, so the angle at which the airflow is entering the disk is slightly different at the front and the rear. At the front, it’s entering at a smaller/flatter angle whereas at the back it’s entering at a more (relatively) vertical angle. This has the effect of reducing the lift on the blades at the back passing over the tail and increasing the lift of the blades passing over the nose at the front – by the time the blades have either flown upwards or downwards due to the increase/decrease in lift, they are now at the side of the helicopter. This produces a rolling motion which is usually just counteracted for without thinking, but when demonstrated and not corrected for, you can see the relatively high effect this actually has! The direction of the roll depends on whether the blades spin clockwise or anti-clockwise.
Compensation: Apply and hold lateral cyclic opposite to the direction of roll.
 
3. Effect: Translational Lift
Now that the forward speed is picking up and you reach around 10-12 kts, you’re allowing the helicopter (and therefore the disk) to be flown through lots of nice, undisturbed, clean air for the blades to work on. We know that the blades need air going through them effectively to fly, so when you’re giving them a fresh stream of ‘free’ air, they will become more efficient. With the collective position unchanged, the result is that the helicopter wants to climb! We can use that to our advantage, by using the cyclic to convert that tendency to gain height into more acceleration. That’s the goal here, isn’t it?!
Compensation: Forward cyclic pressure.
 
4. Effect: Tail Efficiency Increased
Similar to the above, the tail rotor and vertical stabiliser (or the Fenestron fin) also becomes more efficient with the increased airflow. Therefore, it works more effectively and less anti-torque pedal is required.
In the Cabri G2, which is equipped with a Fenestron, I can sometimes find myself going from lots of right pedal forward in the hover, to left pedal forward instead during the very short 15 second take-off run!
Compensation: Less ‘power’ pedal.
 
5. Effect: Flapback
As the speed of the air is further increasing over the helicopter, you will feel that the nose of the aircraft wants to climb again as the disk starts to flap upwards. No, it’s not Translational Lift Part II, The Sequel; it’s much more exciting – flapback! You’ll have been demonstrated this during earlier exercises (usually Ex. 4 & 5) anyway, but I’ll briefly explain it regardless.
As the advancing blade coming into the forward airflow gains extra airspeed from the forward motion, it gains lift and flaps up (remember the lift formula..?), reaching the highest point at the nose of the aircraft. The retreating blade, going out of the forward airflow and towards the tail, loses airspeed and therefore loses lift so flaps downwards – this one reaches the lowest point at the tail of the helicopter. You could therefore see the net effect as high blades at the nose and low blades at the tail – creating the effect of flapback.
Compensation: More forward cyclic.
 
6. This one’s not really an effect, but I thought I’d finish off by explaining that when you initiate the climb, you don’t pull the cyclic backwards as the rate of climb would become enormous (you could, but this is Exercise 13 and not Apollo 13); instead you just ease back on the forward pressure and allow the aerodynamics to gently raise the nose which starts the climb.
 
What about an approach back to the hover? Well, a slightly different story and a little more to think about in other ways, but that’s for another time!
 
If some of this doesn’t quite make sense, you’re not alone! We highly recommend checking out Geoff Day’s book as it explains it all brilliantly (this isn’t a sponsorship, Google has useful answers too!). Also please feel free to give one of us a call and we’d be happy to help out.
 
By Jack Fox CPL(H)/FI

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