National Free Flight Society

SEN 2678

  1. In the Press
  2. Closed Loop Control FB
  3. Fab Feb Heads Up
  4. North American Cup

In the Press
Over the Edge of Beyond with Fantham and 91 cups of coffee. = Excellent report on the 2019 World Champs by CHE in the January 2020 Aeromodeller. Good description, photos, inside stories with CHE wisdom.

Closed Loop Control
From: FB
Can Tezcan: Just my curiosity! In the 70ies there was a beautiful Wakefield, Reliant, with mechanical Torque Control VIT. It has some spring mechanism in the rubber tube end and a platform where the Stabilizer sits on top. According to rubber torques, it was changing the incidence of stab. But because if the heavy mechanics in the tail it was not very popular.

So now my question. Today to make such a mechanism with electronics is just a simple job. One pressure sensor on the rubber bobbin at the end, simple electronics and one servo can solve all.

Peter Martin: It’s open-loop control, so would be fine. Closed-loop is banned, however! I’m not sure the pressure/torque sensing is as simple as you believe, however. The forces at the rear bobbin would be liable to destroy lightweight sensors.

Dylan Roberts: Is it truly open loop… All torque controlled features seem closed loop to me,…. Input to the control system is the rubber torque. If this varies so will the output.

Peter Martin: Yes, it is truly open loop. There is no setpoint, or error feedback in the example you gave. For example, in a full-size aircraft autopilot, the speed, heading and altitude setpoints could be selected and the control system adjusts thrust/elevators/rudder/ailerons to accel/decalage/climb/descend/turn based on error from setpoints. If the plane encounters jet-stream, crosswinds or turbulence, the control system is programmed to adjust thrust and control surfaces to maintain speed, heading and altitude.

The analogy to your example is a pilot at cruising height where the plane is programmed with thrust/elevators/rudder/ailerons in fixed relationships. If the plane encounters a disturbance which knocks it off course, there is no compensating back on-course, it just keeps going. The reason is that it’s open-loop, not closed-loop.

In free flight, we normally program our models in a fixed relationship with time, the only difference with torque-based control is that the relationship is fixed to torque not time. It’s still fully open loop with no compensation for errors. I understand that there’s some confusion on the matter of closed loop, but hopefully that clears it up.

Dylan Roberts: while I agree with you that the time-based aerodynamic controls are open loop. the prop pitch on a modern F1B model is closed loop…. There is a setpoint (cam) an input (rubber torque) and an output (propeller pitch) So a closed loop control system is allowed as long as the input is not something related to the aerodynamics like air speed, pitch or bank?

Peter Martin: No, a variable pitch prop definitely isn’t closed loop! The cam position is proportional to torque (assuming the spring angular deflection is linear) and prop pitch is dictated by rotary position on the cam, all 3 are in a fixed mechanical relationship – The input is rubber torque and the output is prop pitch, but they are NOT setpoint, input and output of a closed-loop system.

The closest you could come to a closed-loop system on a legal free flight model aircraft is to consider its inherent aerodynamic stability – For example, you set the angle of attack (setpoint) of the wing by trimming tail incidence (and/or CG), then the pitching moment coefficient provides negative feedback to give a restoring pitching moment (input) when the actual angle of attack (output) deviates from desired.

What has been banned is active sensing which operates flight controls. The exact wording from the CIAM General Rules 2019 is “Closed loop control systems with active sensors and operating aerodynamic flight controls or moving mass are not allowed, unless allowed in the class rules.” Can Tezcan’s suggestion has an active sensor and operation of aerodynamic flight controls, but it isn’t closed-loop, therefore it’s definitely legal.

… Glad to help! I didn’t want to mention earlier, but I’ve been a professional Control Systems Engineer for the last 20 years. So I can spout on this topic till the cows come home. ///

Editor’s Comment
The above piece was selected from FB by Aram Schlosberg.

The explanation by Peter Martin is very good and explains closed loop very well.
As an historical note I understand that some of the original wording was provided “at the last moment” by Dan Tracy.  Dan who was a noted aerospace engineer (he had a title  “Technical Fellow” at Boeing ) was an adviser to the AMA delegate at the CIAM where the original “no closed loop rule” was voted in. Dan said (from my memory) that the FF delegates wanted to rule out “automatic stuff” but did not know how to express that in technical terms so Dan helped them do that and introduced the term closed loop.  Which we see today from Peter’s comments has a very clear meaning, even if it takes someone like Peter to explain it to us!


Fab Fab Heads Up
Firstly a reminder that if you are taking part in the FAI events, both World Cup and Mini events you should pre-enter at  You do not enter for the AMA events at the Ike online.

Note that a FAI License is only needed for the World Cup events, not for the Mini events.

In processing the entries received so far (over 100) we have noted that over 20 have FAI licenses that are currently valid but expire before the date of the contest – so check yours!


The Toronto Free Flight Group
Presents the 2020
North American Cup
A Premier FAI World & America’s Cup & Team Championship
Dates: Tuesday, February 11th 2020 with Wed & Thursday February 12-13th as reserve days
Place:  Bissonette Mirage Field, Lost Hills, CA USA
Events: Individual F1A, F1B, F1C, F1Q & 3 Person Team F1A, B & C (Teams can be mixed nationalities)
Entry Fees:  $30.00 for F1A, F1B, F1C, F1Q – Team entry included in fee
Juniors free all events!

AMA liaison CD: Brian Van Nest
Co-Organizer: Peter Allnutt: 527 Philippine St, Taft CA USA +1(661) 763-5039
Organizer: Tony Mathews: 11233 Hwy 118, Haliburton, Ontario, Canada K0M2S0  +1(705) 854-0698 (cell)

Tuesday February 11th
F1A/B/C/Q Round 1 8:00 am – 9:00 am.       Rnd 1: 240 sec max F1A/B/C/F1Q
Round 2 9:00 am – 10:00 am      Rnd 2: 240 sec max F1A/B/C/F1Q
Round 3 10:00 am – 11:00 pm   Rnd 3: 180 sec max F1A/B/C/F1Q
Round 4 11:00 am – 12:00 pm   Rnd 4: 180 sec max F1A/B/C/F1Q
Round 5 12:00 pm – 1:00 pm     Rnd 5: 180 sec max F1A/B/C/F1Q

F1A/B/C/Q Fly-offs start earliest 3:30 pm to 6:00 p.m. Latest (weather permitting – or Sunrise Wed or Thursday morning)
Trophies for 1st, 2nd and 3rd place F1A, F1B & F1C & F1Q. Medals for 1st, 2ndand 3rd place Teams.
Medals for top Junior in each event also! Champagne and snacks will be available Tuesday evening starting at 6:30 pm. Final awards ceremony will be after the final flyoffs.
(Sunrise for Feb 11th – 6:51 am Sunset – 5:36 pm – 17:36)

Deviations from the FAI sporting code and special instructions:
Competitors MUST provide their own timekeepers. Round 1 and 2 max time will be 240 sec or less depending on the weather conditions. Fly-off Max times and start time will be decided on the day with the Jury depending on the weather.
Team scores will be decided by the total time of the 3 contestants including flyoff times (ie: not as per the sporting code).
Note: A Starting “area” will be utilized without pole positions (depending on entry numbers)
Link to the pre entry form (Cut and paste it into your browser):