Flight model

from Wikipedia, the free encyclopedia
Remote-controlled model airplane Twin Star flying over the space

A flight model is an aircraft that is operated in model form, i.e. in reduced or miniaturized size, mostly for the purpose of amusement or leisure. It is either a reduction in size of a prototype from passenger aviation or a purely functional model that only appears in model flight in this form. Functional model airplanes (for example from plastic model making ) as pure standing models are not considered in this article.


TAM 5 from Maynard Hill, 2003
A transatlantic flight model in classic balsa and foil construction.

Airplane models are mostly built and flown as a hobby . They are available on the market either as a kit or in various levels as "almost ready to fly" (ARF for almost ready to fly ), "ready to fly" (RTF for ready to fly ) or "ready to fly with built-in radio receiver" (RTB for ready to bind , ready to establish the radio connection between transmitter and receiver: English binding ) sold. Some model builders develop and build flight models themselves. Flight models are available in sizes from a few centimeters to spans of several meters. A typical model sailor has a wingspan between 1.3 and 4 meters. Motor-powered model aircraft typically have wingspans between 0.8 and 2 meters. The range of drives currently available includes rubber motors , electric drives , piston motors and gas turbines . ( Deflagration jet engines are mostly prohibited.)

Model flying is practiced as a hobby, as well as in national or international competitions as a sport. If it is a remote-controlled model, it is usually operated within sight of its controller. A flight model is usually controlled by just one person. The teacher-student flight is an exception. The flight model is controlled by a primary transmitter (teacher) who can temporarily transfer individual or all control functions to a secondary transmitter (pupil). This procedure is mainly used when learning to fly a model.
There are clubs by and for those interested in model aircraft.

Historically, model flying is older than person-carrying aviation and is viewed by the International Air Sports Association FAI as a separate branch of aviation.


The history of the flight model is closely linked to the history of aviation . Long before the first manned flights, the fundamentals of flying were tested with model aircraft. The mathematician and philosopher Archytas of Taranto , * 428 BC BC, is mentioned in the 2nd century AD by Aulus Gellius in his book Noctes Atticae as the builder of an airworthy bird model. The " dove of Archytas " is described as a wooden replica of a dove created by mechanical skill and other arts; which had flown because it was kept balanced in the air and was driven by a current of air enclosed and hidden inside. In the fourth century BC Chr. Play Chinese children already with a toy , the first known model of a helicopter (rotorcraft) can be considered. The Chinese top consisted of a round rod into which bird feathers were inserted in a slightly crossed shape. By turning the rod between the two palms of the hand, the springs finally generate enough lift to raise the top into the air.

model airplane by Alphonse Pénaud, 1871
  • 1804: Sir George Cayley builds a successful glider model based on his aircraft concept from 1799.
  • 1811: Albrecht Ludwig Berblinger (Schneider von Ulm) failed to demonstrate his proven airworthy hang-glider due to unfavorable wind conditions.
  • 1848: John Stringfellow lets a steam-powered model airplane fly about 36 m freely in a factory hall.
  • 1857: After successful model tests, the French brothers du Temple de la Croix receive a patent for a powered airplane.
  • 1871: The Frenchman Alphonse Pénaud builds his Planophore , a free-flying powered airplane model with a rubber engine.
  • 1875: The Englishman Thomas Moy tests a tethered powered airplane model with a steam engine drive and 4 m wingspan.
  • 1877: First flight of a steam-powered model helicopter by Enrico Forlanini
  • 1879: The Frenchman Victor Tatin builds a powered flight model with propellers and a compressed air motor that successfully takes off from the ground.
  • 2003: The flight model Trans-Atlantic model 5 of Maynard Hill crosses the non-stop flight to the North Atlantic .
Classic wooden construction around 1950

Building model airplanes as a hobby came up before the First World War . A book by PL Biegenwald with the title "Flugmodellbau" was published as early as 1913. The rubber engine was primarily described as the drive and the possible use of compressed air, CO 2 and combustion engines was mentioned. The electric motor was considered completely unsuitable for model aircraft. In the illustrated book of the German Historical Museum “Das XX. Century - Photographs on German History ”, on page 91 there is a photo from Berlin, dated 1919, on which around ten ready-to-take off water model aircraft, obviously with a rubber engine, are shown. In the beginning, building materials were mainly plywood , thin pine strips and paper for wing coverings ("tensioning paper "), later balsa wood was widely used , and today mainly plastics such as fiber composite and foam molded parts and " iron-on film " for covering.

In the 1930s, internal combustion engines were introduced for model airplanes. B. the "Kratmo". The first successful experiments with radio remote controls were carried out in the second half of the 1930s. After 1950, radio remote controls became more widespread through commercial manufacture, first as tube devices that required large and heavy batteries, and later with transistors. But only the proportional systems introduced in the mid-1960s , in which each rudder precisely follows the deflection of the stick movement on the transmitter, with which especially model aircraft can be controlled precisely and safely, made model flying popular.

Finished fuselage and styrofoam-balsa sandwich surfaces with foil covering on the Carrera Draco3003 flight model around 1980


Model aircraft are initially roughly divided into model airplanes and model helicopters (see there).

Flight models can be differentiated and subdivided according to various features, for example according to the flight principle (lighter / heavier than air), the drive (without drive, combustion or electric motor drive, rocket drive), the control (without control, line-controlled, (radio) remote-controlled, program-controlled), according to the type of use (slope or thermal glider, aerobatic models, speed models, scale models, vertical take-offs), the construction (prefabricated models, modular models, built according to plan) or the main material used (wood / rib construction, molded foam models, full GRP models) .

So that international competitions can be held, the international interest group CIAM of the FAI ( Commission for International Model Aviation of the International Air Sports Association ) divides model aircraft into competition classes . Records can also be recognized internationally within the classification by the FAI. There are also national competition classes.

The aviation law of the respective country also divides model airplanes into different classes, for which different legal restrictions apply.

Flight principle

Lighter than air

Remote controlled model hot air balloon

Models with static buoyancy are filled with a lifting gas or with hot air.

Flying without a load-bearing inner skeleton:

A model airship flies with a supporting inner skeleton (lifting gas or hot air). Model balloons and airships are usually remote controlled. Competitions have not yet been held.

Heavier than air

Remote-controlled helicopter model Bell 222 with electric drive

Models with dynamic buoyancy

Models that generate a force that counteracts gravitation, which is neither static nor dynamic lift, and which can reach a significant flight altitude (i.e. do not exploit a ground effect or, for example, require magnetic interaction with the ground):


Remote controlled models

A remote-controlled model airplane is a model airplane that is equipped with a receiver ( receiver , RX) and wirelessly (without a fixed connection) controlled remotely with the help of a transmitter ( transmitter , TX) via a carrier signal. In general, radio signals are used as carrier signals ( RC aircraft , radio controlled , see also radio remote control ), in the entry-level segment also infrared .

The transmitter sends a radio signal to the receiver in the flight model, which transmits the position of the control sticks or switches on the transmitter. The receiver then controls the electronic / mechanical steering gear ( servos ), which z. B. move the control surfaces.

  • Remote-controlled flight models are divided into the following competition classes by the Fédération Aéronautique Internationale (FAI):
    • F3 - remote controlled flight
      • F3A - Remote-controlled motorized aerobatic models up to a weight of 5 kg
      • F3A-X - Remote-controlled, original-like motorized aerobatic models up to a weight of 10 kg
      • F3B - Remote-controlled thermal glider models
      • F3C helicopter
      • F3D - pylon racing models
      • F3F - Remote-controlled hillside glider models
      • F3G - remotely controlled motor gliders
      • F3H - glider models - cross-country races
      • F3I - Remote-controlled model glider tow
      • F3J - Remote-controlled thermal glider models - continuous flight
      • F3K - hand launch glider models
      • F3P - Indoor aerobatic models
      • F3S - speed flight models
    • F4 (AH) - prototypical model flight ("Scale")
    • F5 - radio-controlled model aircraft with electric motor
      • F5A - aerobatic models
      • F5B - electric model gliders
      • F5C helicopter
      • F5D - pylon racing models
      • F5E - solar model aircraft
      • F5F - electric glider models - "entry-level"
      • F5J - electric thermal sailor

Free-flying model aircraft

Free-flying models fly without being controlled by a pilot, but self-control (program control / magnetic control) is possible.

Indoor flyer of the F1D class
DAeC 2002

Free-flying models are divided into the following competition classes by the Fédération Aéronautique Internationale (FAI):

  • F1A - glider models (A-2 class)
  • F1B - model aircraft with rubber engine ("Wakefield")
  • F1C - model aircraft with combustion engine up to 2.5 cm³ ("I" - formerly known as Roman 1)
  • F1D - hall flight models
  • F1E - self-steering glider models, e.g. B. Magnetic control Magnetic flight
  • F1G - model aircraft with rubber engine ("Coupe d'hiver")
  • F1H - glider models (A-1 class)
  • F1J - model aircraft with combustion engine up to 1.0 cm³ (1/2 A class)
  • F1K - model aircraft with CO 2 engines
  • F1L - hall flight, ECB models
  • F1M - indoor flight models (beginners)
  • F1N - hall glider for manual launch
  • F1P - flight models with combustion engine up to 1.0 cm³, max. 1500 mm span and min. 26 dm² wing volume
  • F1Q - model aircraft with electric motor, battery weight max. 120 g Ni. or 90 g Li.

Further classification options for free-flying model aircraft result from the type of use, the structure, the material used or other similarities:

  • Sailors usually have a wingspan of around two meters . You start with a high start line. All free-flight models must be equipped with a timer ( see program-controlled models ). Usually u. a. controlled the horizontal stabilizer of the model to initiate a forced landing, which prevents flying away in strong thermals .
  • Self-steering slope sailors; Before the manual start, the compass-controlled rudder is adjusted so that the model flies firmly in the direction of the compass against the uphill wind and returns to the set flight direction in the event of deviations due to the magnetic control.
  • Motor sailer
  • Rubber motor models are glider models / gliders that are launched by hand with a propeller driven by a twisted rubber cord and rise to a height of approx. 70 meters. If the energy of the rubber strand is used up, the propeller blades fold against the fuselage and the model flies on as a glider model.
  • Balloon helicopter helicopters are flying long with the compressed air of a balloon up to 20 seconds
  • Indoor flight models are extremely light models (masses below 1 gram can be achieved, but a minimum mass of 1.2 grams is required for competition classes), powered by a rubber motor. The models “sneak” through the air. Depending on the height of the hall, flight times of up to 30 minutes can be achieved in the competition class. Because of the required hall height and air calm, competitions are often held in the salt mine.
  • Paper planes are objects in flight , which are mainly made of paper.
    • Paper airplanes are "normal" model airplanes made of paper, cardboard and cardboard.
    • Folders consist of one, rarely several sheets of paper. World records are also held in this class (e.g. GBdR - longest flight time: 27.9 s, Takuo Toda , April 11, 2009)
  • Rockets are model aircraft powered by rocket motors. Mostly in the classic rocket variant, but occasionally also converted glider models.
  • Balloons and airships filled with gas or hot air
  • Lifter

Magnet control

The direction control takes place here with the help of the earth's magnetic field . A compass needle (a rotatably mounted magnet ) always points to the north, and, depending on the rudder is controlled. In the event of a deviation from this neutral position, the magnet causes a restoring torque on the rudder. As a result, the flight model always keeps the same (sky) direction. By presetting a rotation angle ("offset angle") against the north direction, any other direction can also be selected.

A microcontroller control (program control) can follow an electronic compass - a smooth transition to program control.

Program-controlled models

In its simplest form, a program control consists of a mechanical or electronic timer. These are common in almost all free flight classes, among other things to ensure a reliable landing after the maximum competition scoring time has expired. Complex forms of these timers can certainly control several functions (elevator, rudder) and thus implement more complex flight sequences (flight phases), for example in the start phase (colorful).

The miniaturization of navigation, radio and computer components also allows autopilot and program controls for model flying. The self-steering free flight models differ from the controlled models in that after the start and thus the beginning of the free flight, it is not possible for a pilot to influence the model settings.

A well-known example is the Trans-Atlantic Model 5 from 2003, which completed over 99 percent of its transatlantic flight under program control.

Line-controlled models / fettered aircraft

In line-controlled tethered aircraft , the “pilot” holds the motorized model with outstretched arm on control lines (taut, thin steel ropes) that operate the elevator and guides the model in a circle using centrifugal force , rotating synchronously.

  • Tethered pilots are divided into the following classes by the Fédération Aéronautique Internationale (FAI):
    • F2A-FAI speed or speed models
    • F2B - aerobatic models
    • F2C - team race
    • F2D - Combat or (in German) foxhunting models
  • Aerobatics
  • Fox hunt - a meter-long strip of crepe paper is attached to two flight models (of the same length). The two opponents stand together in the middle of the circle and try to cut as much as possible away from the other's trail with their own propeller.
  • Speed ​​or racing model - only the pace counts. The models are often asymmetrical and have only one wing on the inside of the circle and the elevator on the outside of the circle.
  • Prototypical replicas or models
SIAI-Marchetti SF 260 as an original and as a remote-controlled model

Possible classification without reference to competition

  • Glider models; Replica of a glider prototype or purely functional model without a prototype.
  • Motor glider; z. Sometimes several meters span, often with an electric motor
Model of an mx2 , a 3D aerobatic
stunt plane with a wingspan of 121 cm
  • Aerobatics - or 3D flightable models, which are characterized by large rudders and large rudder deflections in order to be as agile as possible. 3D-airworthy models have a motorization that can propel the aircraft vertically upwards, ie the thrust is greater than its own weight. They are mainly used to demonstrate spectacular sightseeing flights, but the suitability for clean aerobatics often suffers. Models for classic (dynamic) aerobatics therefore have moderate rudder sizes and a slightly different overall design in order to better show classic aerobatic maneuvers.
  • Pylon racing models. Flight models that are designed for maximum speed and that are used in pylon races.
  • Prototypical replicas (aircraft models); also often referred to as scale models. The aim is to create a replica that is as true to scale and as possible in detail. However, wing profiles may be changed (restricted without distorting the appearance too much) in order to take into account the changed aerodynamic circumstances due to the downsizing. National competition classes such as semi-scale allow deviations from the prototype, provided these are not recognizable from five meters away.
  • Model airliner
  • Slowflyer and Park Flyer , light and slow flying models, also as trainers.
  • Indoor models, extremely light and manoeuvrable models that are used for indoor flying in indoor areas or in sports halls.
Shockflyer EXTRemA 330S
from Ikarus (0.8 m wingspan)
  • Shock Flyer , a special indoor design. Light and very manoeuvrable, with powerful motors, therefore suitable for 3D .
  • Rocket planes (or rocket gliders), converted aircraft models with remote control.
  • Model helicopters , differentiation between trainer models for sporting use (3D, aerobatics) and scale models as detailed replicas of the original helicopters.
  • Paramotor models
  • Remote-controlled airships and balloons filled with gas or hot air

Remote controls

There are different forms of remote control:

Radio remote control

Transmission of the control functions by means of a high-frequency electromagnetic field ( HF ) by a radio remote control .

A radio remote control system basically always consists of a transmitter with its own power supply, a receiver, the steering gear (servos) and the power supply for the receiver and servos. A distinction is made between hand, desk and pistol transmitters. With two control sticks, each with two degrees of freedom (cross stick), the four most important control inputs can be passed on to the control functions of the flight model. As a rule, these are height, side, aileron and engine for fixed-wing aircraft and pitch, roll, yaw and pitch for helicopters. Additional functions can be activated using switches, rotary controls or sliders.

Radio remote controls for (flight) models are subject to certain legal and regulatory requirements. In Europe ( EU ), the 35 MHz-A frequency range with 20 assignable channels is generally reserved for model aircraft. Other frequency ranges (27, 35-B, 40, 41, 72, 75, 433 MHz, 2.4 GHz) may differ from country to country or are not exclusively reserved for model aircraft. For some years now, signal transmission in the 2.4 GHz range has been used more and more, which avoids interference from double channel occupancy. In order to avoid channel double occupancy, the use of a frequency board is mandatory on model airfields .

There are now approaches to transfer the control functions from a smartphone to the flight model using an app via Bluetooth . In this case, the image signal from a video camera mounted in the model can be transmitted back to the smartphone and displayed on the screen. Flying with the help of a video transmission of the pilot's perspective is known as FPV flight .

Line remote control (tethered flight)

The model is controlled remotely using two or more lines (mostly steel cables). For this, the model inevitably moves on the surface of a hemisphere, in the center of which the pilot stands (the model generally flies on a circular path around the pilot). Due to the limitation to a few lines, only one or two control functions (flaps, elevator, rarely motor throttle) are common.

Infrared remote control

Transmission of the control functions by means of a modulated light signal (e.g. infrared ). Since light signals do not require approval, this type of remote control is often used on very affordable models. Because the infrared transmission is disturbed by sunlight, the IR control is only suitable for indoor models.

Other remote controls

Other possible remote controls work e.g. B. acoustic ( ultrasound ), but are not in use today.


Collection of model airplane engines
  • Propeller : Converts an axial rotary movement into a propulsive force in the direction of the axis of rotation. The propeller moves freely in the air or is shrouded without guide vanes ( ducted propeller ). The lift rotor of a helicopter is a special case .
  • Impeller : also converts a mostly axial rotary movement into a propulsive force. With an axial impeller, the air mass is accelerated by a jacketed rotor . A system of guide vanes, the so-called stator , ensures a swirl-free flow to the downstream rotor. The rotor and stator blades of the impeller are similar in principle to at turbines conventional turbine wheels and blades.
  • Combustion engine : piston engine or rotary engine with one or more cylinders and with different cubic capacities . Depending on ignition principle different motor types differentiated (are diesel engine , Glühkerzenmotor , gasoline engine ). Engines with multiple cylinders are often named according to their arrangement (e.g. radial engines ).
  • Early electric direct drives. Right row: Keller motors; left: Geist and Bühler motors with motor attachment
    Early gear motors with folding propellers from Graupner , Robbe and Carrera
    Electric motor : Conventional brush motors or powerful brushless electric motors ( brushless motors ) as internal, external or internal external rotors. Historical short note: It was not until the use of samarium cobalt by the Dipl.-Phys. Heinz Keller-built Keller engines helped the "rather weak drifts to a previously unknown temperament" (quote from 30 years of retrospective of the DMFV ). Other manufacturers are now building motors with the most powerful neodymium-iron-boron permanent magnets to date . There are even manned planes as electric planes . Just as important was the introduction of high-current NiCd battery cells in the mid-1980s by the Sanyo company and the development of high-load speed controllers with power transistors.
  • Turbine jet engine : model gas turbines which, in principle, like their larger models, work as one- and two-stage jet engines. Furthermore, models with gas turbine drives have left their niche existence and offer their own fascination, especially with their prototypical sound. The recoil drive in the form of black powder propellants, water, steam or compressed air rockets is also used for special purposes .
    • Turbojet : Propulsion is determined exclusively by the exhaust gas flow from the turbine.
    • Turbofan : The turbine drives a fan that provides the majority of the propulsion.
    • Turboprop : The turbine drives a propeller , which is responsible for most of the propulsion.
  • Ramjet / pulse jet engine : Pulse jet engines were z. B. used in the Second World War on the Fieseler Fi 103 . Functional models working according to this principle are also used in model airplanes. Due to their high volume, the use of pulse jet engines is subject to severe restrictions.
  • A special type of drive is the electrostatic drive based on the Biefeld-Brown effect in the lifter .


Flight models that use the airspace are legally considered aircraft in Germany and form a separate aircraft class . The operation of model aircraft is regulated by Section 16 of the Aviation Ordinance (LuftVO).

Flight models with a mass of less than 5 kg can be flown without a special ascent permit (exception: rocket propulsion) with a minimum distance of 1.5 km from airfields. When using an internal combustion engine, a minimum distance of 1.5 km from residential areas must be maintained. Suitable model airfields are often operated by model building associations. A demarcation of leisure / model flying to other (e.g. commercial) purposes is not (no longer) mentioned in § 16 LuftVO.

Aircraft models with a maximum take-off mass over 25 kg must be approved individually ( Section 1, Paragraph 1 of the Air Traffic Licensing Regulations (LuftVZO)). Similar approval regulations apply as for man-carrying aircraft. The operation of such model aircraft requires the possession of a model pilot's license and is only possible on approved airfields.

In addition, air traffic control clearance must be obtained from air traffic control within the control zone of airfields ( Section 16a LuftVO).

In Germany in 2005 Planes are noisy since July 1 Aviation Act no longer be exempted from compulsory insurance for aircraft. Therefore, since then, the same claims for liability insurance apply to model aircraft as to man-carrying aircraft, this applies above all to the liability sums. While the special model flight insurances have adapted quickly to the new requirements, many general insurers have ignored the new regulations or completely removed the insurance of model aircraft from general liability insurance. Anyone who operates a flight model in public airspace without or without sufficient liability insurance is committing an administrative offense. The following associations, for example, provide information on model flight insurance : DMFV , DAeC .

For model aircraft between 5 and 25 kg, see also Unmanned Aircraft # Legal situation .


  • Rudolf Gulich: RC - model aircraft construction for beginners. Augustus Verlag, Augsburg 1999, ISBN 978-3-8043-0262-4 .
  • Rudolf Gulich: RC model aircraft with internal combustion engines. Augustus Verlag, Augsburg 1999, ISBN 978-3-8043-0263-1 .
  • Werner Frings: Practical model engines. Wellhausen & Marquardt Medien, Hamburg 2007, ISBN 978-3-939806-00-4 ( model aviator book ).
  • Gerald Kainberger: The big book of model flying. Theory, technology, practice. Verlag für Technik und Handwerk, Baden-Baden 2010, ISBN 978-3-88180-793-7 ( vth specialist book ).
  • Manfred-Dieter Kotting: Modern remote controls for RC model aircraft. Receivers, servos, accessories. Verlag für Technik und Handwerk, Baden-Baden 2008, ISBN 978-3-88180-780-7 ( vth-Fachbuch = FMT-Fachbuch ).
  • Lothar Schäfer: Aerobatic Workbook. Basic knowledge for stunt pilots. Wellhausen & Marquardt Medien, Hamburg 2009, ISBN 978-3-939806-13-4 ( model aviator edition ).
  • Dieter Schlueter : remote-controlled helicopter. With Schlüter comeback petrol helicopter and comments on further development. 14th revised and expanded edition. Neckar-Verlag, Villingen-Schwenningen 2007, ISBN 978-3-7883-4126-8 ( model book ).
  • Martin Simons: Model Flight ABC. Basics, aerodynamics & tips. Verlag für Technik und Handwerk, Baden-Baden 2006, ISBN 3-88180-735-7 ( FMT reference book ).
  • Michal Šíp: The great manual model flight. Geramond, Munich 2008, ISBN 978-3-7654-7348-7 ( model aviator edition ).
  • Alasdair Sutherland: Basics of model flying. Know more, fly better. Verlag für Technik und Handwerk, Baden-Baden 2001, ISBN 3-88180-711-X ( vth-Fachbuch = FMT-Fachbuch ).
  • Heinrich Voss: practical model turbines. Wellhausen & Marquardt Medien, Hamburg 2010, ISBN 978-3-939806-04-2 ( model aviator edition ).

Web links

Commons : Models of aircraft  - collection of images, videos and audio files

Individual evidence

  1. The long-distance record achieved with the transatlantic flight was revoked by the FAI in 2009 because the category "autonomous flight" no longer applies to model flight
  2. § 1 Paragraph 1 No. 8 LuftVZO
  3. ^ Daniel Stocker: Learning to fly models. glocknerhof.at, accessed on November 29, 2018 .
  4. ^ DI Christian Faymann: List of Austrian model flight clubs. Austrian Aero Club, accessed on November 29, 2018 .
  5. ^ Aulus Gellius , Martin Hertz, Carl Hosius: A. Gellii Noctium Atticarum Libri XX . BG Teubner, Stuttgart 1969. Page 352: “Nam et plerique nobilium Graecorum et Favorinus philosophus, memoriam veterum exequentissimus, affirmatissime scripserunt simulacrum columbae e ligno ab Archyta ratione quadam disziplinaque mechanica factum volasse; ita erat scilicet libramentis suspensum et aura spiritus inclusa atque occulta concitum. "
  6. http://www.modellflugimdaec.de/leistungssport/elektrosegelflug/f5f
  7. http://www.modellflugimdaec.de/leistungssport/elektrosegelflug/f5f
  8. Thomas Wiesiolek: F1E magnetic control - function and structure. rc-network.de, December 19, 2010, accessed April 10, 2018 .
  9. NEWS FOR AVIATION DRIVERS (NfL) PART I Langen, March 2, 2006 NfL I-59/06: " The operation of model aircraft with intermittent jet engines (pulse engine, thrust tube, Schmidt-Argus tube) or ramjet engines (ram jet) can only may be permitted if it has been proven by measurements that the immission guide values ​​are not exceeded in the residential areas concerned. "