Božo Grubić

August 16, 2023 late afternoon. F1B. World Cup, France!
Fly-off.
50 out of 109.
Huge pressure. Focus, focus…
Starting place 29, Slavko with me. We have set up on the 25m line on time. Model – ORCA, Sidus electronics, stooge, winder, rubber band and me. Max 360. Bojan is somewhere down the starting line – around the 35th position.
Nervousness – it’s not going according to plan, the first delay of the start due to the positioning of the measurers. Time is being wasted. The pressure is mounting.
If only Kaća were here!
A new delay. The maximum increases to 480 seconds allowing time to set the timer – I run to fetch the programming device. Then another delay due to the turning of the starting line towards the wind which was slowly weakening. It was dusk already. Živa joined us. I went to get a screwdriver to “release” the horizontal. Slavko was about to do the same without me!? It was clear that we’ll start right away, before the cold and the increase in humidity in the air. The first rubber band above 1.6 torque. At the beginning of the fourth minute, I threw out the model!

Almost vertical, very good transition from the bant phase to the first turn of the spiral path! We entered the last turn without hesitation – the rubber band “pulled” well. It flew straight into the wind, wing down, slightly right into a turn, propellers stopped – exactly where they should have!
Complete path very good!

There was still a free flight left!

During the third minute of the flight, I went to the position 25 m where my stooge and winder were, I lowered my knees to the ground, put my face into my hands and waited.

And who knows what might have happened if we didn’t “play” as a team in my 7th start. And we “played” indeed! At the start I was second behind Bojan, on time. Extremely “inconvenient” position, start number 3. The wind was blowing down the starting line, faster than at any time that day – from us to position 38! Everything was happening behind our backs. There was no one left in front of us at the start – only lanes. Time went on. Next to us, the Canadians monitored thermo probes on two tablets for their competitor. Some models flew down the line, but far away. We were waiting.

The Canadians started – they had the “promise” of both tablets. Then I started. At Toza’s suggestion, I lowered the model, but mechanically I started the timer, the model was in my hand, the propellers started and they cut the index finger of my left hand – blood! I took that engine off and winded a new rubber band. There was no moment I wanted. Another round and – bang. Give me the third rubber band! It didn’t last either – at least not as long as I wanted. I’ve run out of rubber bands!
My world came crashing down! I “ate” a good portion of time. Toza and Slađa still had to start after me. I was walking, like a zombie, towards our camp. Let the two of them fly. I need at least 5 minutes to complete at least two half-tube motors, rubber band holders, Piserchio hooks… Slavko shouts “Stop, where are you going, come back!”, Toza: “You have our rubber band, take it and wind it” and adds: “But please, only 5% less tight”.
And we did the maximum!
Then the two of them made successful flights in just 9 minutes!
We did it!

And what if we didn’t have compatible half tubes for rubber bands, Piserchio hooks and rubber band holders?
A few years ago, we agreed that, in addition to these elements, we should use the same batteries on GPS and electronic timers in terms of capacity and dimensions and with the same connectors, the same type of transmitter (with the same connector) and GPS receiver – so that we could support each other. It turned out to be a strategically correct deal!

At complete darkness, we headed to the technical control, Toza, Slavko and I, the whole national team was at the door, the first congratulations from the head of the control, then of our whole team!
Our triumph!
I call Kaća on the phone, we share joy together!

And, as our people say, “every miracle lasts three days”, all that remains is for me to wake up – but I’m still dreaming!

Slobodan Midić

Here are represented three most commonly used MID wing airfoils in F1A free flights models:

* MID 102 N (wing root) * MID 102tip N (wing tip)

* MID 103 N (wing root) * MID 103tip N (wing tip)
* MID 117 N (wing root) * MID 117tip N (wing tip)

The letter “N” in the airfoil designation means that the airfoils have been tested and redesigned, so small corrections have been made to the shape of the airfoil most around the nose and trailing edge. This resulted in better continuity of the polar lines of airfoil, without any peaks or discontinuities, so the airfoils are renewed.

Since the F1A models are light, they fly at low speeds, so the air flow around the airfoil in free flight occurs in the critical field of transition from laminar to turbulent state. In order to prevent the spontaneous separation of the laminar boundary layer from the upper surface of the wing (which would lead to a sudden jump in air resistance during the movement of the profile), turbulators were applied to all these profiles on the upper surface of the wing at 50% of the wing chord, which forcefully transform the laminar air flow into turbulent, which allows the streamlines to reattach to the wing surface again but now as turbulent flow. Those turbulators are placed along the entire wing span and are usually 0.3-0.4mm thick nylon strings or 0.35×1.5mm plastic strips. Placing the turbulator closer to the leading edge increases the air drag in flight and reduces the finesse of the model, because then the model loses altitude more quickly. Without turbulators all above airfoils are very bad for flying as F1A wings.

The Reynolds number characteristic of the air flow around the wing of the F1A model in steady gliding flight is Re=40000 and therefore for each profile for the F1A class of models a profile polar is given for that dimensionless number.

In addition, the profile polar is important for the Reynolds number Re=200000 or Re=200k, because it characterizes the profile for a vertical shot when ejecting the model from a 50m long towing rope, at a speed of 50m/s. A good indicator of the profile is that the polar has the as possible lowest drag coefficient Cdo at zero lift Cl=0.

A further improvement in profile characteristics is achieved by making wings with flaps that can be deflected up and down to specific angles by means of suitable servos built into the wings (or less often in the fuselage). Flaps are actually rotating moving surfaces of the trailing edge of the airfoil, usually the width 25%-30% of the wing chord.

Flaps rotate around longitudinal wing hinges usually located on the lower surface of the wing. From the polar diagram of flapped airfoils, it can be seen that when the flap is deflected up by -15 degrees, the resistance of the profile Cdo at zero lift Cl=0 drops drastically to almost half of the value compared to the wing without flaps, which is used to achieve the highest possible height after releasing the model from the string during high start.

On the other hand, lowering the flaps down by a small angle, e.g. +5 degrees, the camber of the airfoil increases and the model climbs better in the thermal column of air.

In order to make good use of all those additional possibilities of flapped wings, a microprocessor timer must be used, which must be expertly programmed according to the existing weather conditions at that moment in the competition.

The main caracteristics of MID airfoils are given in the Table 1. and 2.