uav longitudinal motion flying qualities applied in airworthiness certification · pdf...

208 Technical Sciences

REVISTA ACADEMIEI FORŢELOR TERESTRE NR. 2 (74)/2014

UAV LONGITUDINAL MOTION FLYING QUALITIES APPLIED IN AIRWORTHINESS

CERTIFICATION PROCEDURE

Róbert SZABOLCSI [email protected]‐obuda.hu

Óbuda University, Budapest, Hungary

ABSTRACT The purpose of the author is to derive new set of dynamic

performances of the longitudinal motion of the UAV spatial motion that could be applied directly during measure and test of the type – and airworthiness compliance of the UAV. Due to missing or not properly defined flying qualities of UAV the importance of this article is undoubted. The article will fill the gap in regulations, and its results are recommended for application for experts and authority staff during measure of the compliance of the UAV flying characteristics.

KEYWORDS: flying qualities, UAV longitudinal control, UAV autopilot,

UAV automatic flight control 1. Introduction There is a large-scale UAV and UAS

types available for evaluation in airworthiness compliance measure procedures. Due to this reason this article focuses only on airworthiness of the automatic flight control systems of the conventional UAVs and UASs. The main goal of this paper is to summarize and evaluate existing regulations from the point of view of theirs availability for measure of compliance of flying qualities to those of airworthiness criteria. The article will address dynamic performances applied in airworthiness certification of the UAV. Due to lack of regulations, or due to weak standpoints defined in written norms author will propose a new system of dynamic performances applied to measure compliance to pre-defined conditions.

The 3D spatial motion of the UAV is supposed to be separated to those of longitudinal and lateral/directional motions. This article will limit its investigations to the UAV longitudinal motion parameters.

The author of this article will define a new system of criteria applied in measure of compliance of the airworthiness of the UAV applied in public (military) aviation in segregated airspace. The author will derive forms of the 3D spatial motion of the UAV, and will derive dynamic performances of the UAV automatic flight control systems.

2. Overview and Related Works The topic of this paper is undoubtedly

actual and is in the focus of attention of many air authorities and research institutes. There are many examples how to derive

Technical Sciences 209


compliance of the UAV designed for public, mainly for military applications. However there are few of those interested in airworthiness of UAV applied in civil applications when UAV is flown in non-segregated airspace.

In [1] one can find basic theory of aircraft flight dynamics, and related theory and applications of the manned aircraft automatic flight control systems. In [2] there is a compiled system of dynamic performances showing both flying and handling qualities of the manned aircraft. This book is derived that the flying and handling qualities defined before for manned aircraft were derived from the point of view of the comfort, or from the discomfort index of the manned aircraft. Easy to see that direct application of those parameters defined for manned aircraft can not be applicable for UAVs or UAS systems [2].

In [5] the author dealt with airworthiness criteria applied during measure of compliance. It had proved that the authority procedures act in the interest of the contractors, and have many advantages whilst there can not be found disadvantages, inspite of the complexity of the probelm to be solved here.

The airworthiness criteria of the UAV and UAS systems are outlined in [3] in general, and in [4] the automatic flight control system of the target UAV called METEROR-3MA is in the focus of attention to measure its compliance to those criteria defined by the contractor.

The airworthiness criteria of the UAV are available in military standard of NATO STANAG 4671. This is still a unique standard available for UAV of public (military) applications. There is an important limitation related to maximum take-off-weight (MTOW) of the UAV defined to be between 150 kg and 20.000 kg. The question to be answered here is how to deal with UAV having MTOW less than 150 kg. Th rest of the new UAV

designs are out of the lower limit. Moreover the is a main trend in minimizaton of the sizes and weights of the UAV with simultaneous growth of the capabilities. Today the micro-, the mini-, and the small UAVs provides capabilities over the large UAVs of the past decades. The other interesting point here to be investigated is the Chapters of the STANAG 4671 standard dealing with “Flying characteristics” / “Controllability and Manoeuvrability”, and its subchapters of “145 Longitudinal control”, and of “147 Directional and lateral control”. These chapters are elmininated by the term and decision of “Not applicable”. Although the standard exists, but its main elements are inactive ones for UAV flying and handling qualities measure during compliance analysis.

3. Review of the Hungarian and

International Legal Environment There is a Decree of Minister of

Defense of Hungary No 21/1998 about registry, production, repair, and airworthiness certification of the aircraft applied in public aviation. This legal source of airworthiness compliance measure is more about procedures than about any kind of technical parameters to be investigated how they meet requirements defined in the form of the flying qualities of the closed loop automatic flight control systems, for instance. There are no regulations still about UAV type – and airworthiness certification process. Neither the procedure nor technical data are available for type – and airworthiness certification. It is easy to agree that there are no regulations available to carry out compliance analysis.

The NATO has prepared a standard dealing with UAV airworthiness certification in 2007. The first version of this standard was circulated in member-countries, and, it was accepted by Hungary in 2008, but not incorporated into domestic system of law by the Official statement of



the State Secretary of the Ministry of Defence of Hungary No 47/2008. The NATO STANAG 4671 standard was published in the year of 2009 for application in military (public aviation) UAV and UAS systems.

It is worth to stop at manned aircraft flying and handling qualities. There is a long-lasting period of compiling norms for several kind of the military aircraft, i.e for VSTOL-aircraft (see MIL-F-83300), for helicopters (see MIL-H-8501A), and for manned aircraft in general (see MIL-C-18244A, MIL-F-8785C, MIL-F-9490D, MIL-HDBK-1797A). The method of the airworthiness measure is outlined in MIL-HDBK-516A military standard. It is easy to see that airworthiness certification of the manned aircraft is traced back for many decades and has a written system of norm applied and successfully evaluated via many aircraft type design, production and maintenance.

4. Flying Qualities of the UAV

Spatial Longitudinal Motion This article limits its investigations to

UAVs of conventional design. It is well-known that longitudinal motion of the aircraft can be derived by its state equation as follows [1, 2]:

, (1)

where: x – state vector, u – input vector; A – state matrix; B – input matrix.

Let us consider the state vector x to have following variables [1, 2]:

u – UAV longitudinal speed [m/s]; w – UAV vertical speed [m/s]; q – UAV pitch rate [rad/s];

– UAV pitch angle [rad]. The UAV input vector is supposed to

have following elements [1, 3]: – elevator deflection;

– change in thrust. Equation (1) represents a multi input

multi output state-space model: there are two available inputs to vary state variables defined in the state vector. The elevator angular position change is mainly for changing pitch attitude, the thrust is for changing longitudinal speed of the UAV. If to analyze the entire longitudinal dynamics it shows that there is a representative transfer function describing pitch rate of the UAV related to change of the elevator angular position. The transient dynamic process consists of two stages. First is the so-called short-period motion, the second one is the long-period (phoguid) motion. The UAV dynamics in this mean can be defined by following representative transfer functions [1, 2]:

, (2)

or

, (3)

where: – – change of the elevator angular position – q(s) – pitch rate;

– – pitch attitude; – – UAV gain; – s – Laplace-operator (complex frequency); – – long-period, low-frequency motion transmission zero; – – short-period, high-frequency transmission zero; – – natural frequency of the undamped phugoid oscillations of the UAV;



– – natural frequency of the undamped short-period oscillations of the UAV; – – damping ratio of the undamped phugoid oscillations of the UAV; – – damping ratio of the undamped short-period oscillations of the UAV; – – time delay.

Using (1) and (2) easy to see that dynamical model of the UAV is the 4th order system with time delay. Due to weak self-damping of the UAV its response to any input will have oscillatory feature. Manned aircraft have stability augmentation system to improve damping of the aircraft so as to avoid phugoid oscillations. For UAVs, if it is necessary to provide good flying qualities, better to use damping

system, too. In this case dynamic performances of the closed loop automatic flight control systems must be derived so as to certify them. Automatic flight control system will terminate system oscillations and will limit oscillations to that of the short period, high-frequency oscillations. If to neglect phugoid oscillations model one can get following longitudinal dynamical model of the UAV [1, 2]:

, (4)

or

. (5)

It is important to derive simplified model of the UAV of (2) and (3), or (4) and (5), because one can refer to them during identification of the UAV mathematical model.

There are many dynamic performances available in automatic control theory to define both time- and frequency domain behaviour of the closed loop automatic flight control system of the UAV. As a rule and practice shows that first loop of the longitudinal angular position control is the pitch stability augmentation system. It is proposed by the author of the paper to have following dynamic performances in the UAV longitudinal control [1, 2]:

– – damping ratio of the closed loop automatic flight control system of the UAV;

– – overshoot of the closed loop automatic flight control system of the UAV;

– – settling time of the closed loop automatic flight control system of the UAV;

– – settling time tolerance band of the closed loop automatic flight control system of the UAV.

In automatic flight control system’s theory one can use following dynamic performances defined on Bode-, or on Nyquist-diagram. The author proposes use of follwing system requirements:

– – gain margin of the open-loop automatic flight control system of the UAV;

– – phase margin of the open-loop automatic flight control system of the UAV.

Dynamic performances defined both in time- and frequency domain are adequate to design the closed loop automatic flight control systems applied ont he board of the UAV. Besides, these parameters can be applied during measure of the compliance during airworthiness certification by any authorities.

Equation (5) defines UAV longitudinal dynamical model, and, this model is often subjected to other simplifications, such as:



– long-period, low-frequency motion transmission zero defined by is neglected;

– the time delay of the UAV is neglected.

The stability augmentation system of the longitudinal motion of the UAV is supposed to have closed loop tarnsfer function as follows:

, (6)

where natural frequency of and closed loop system damping factor of implicitly derive relationship between uncontrolled UAV dynamics and parameters of the on-board automatic flight control system of the UAV.

Le tus find transient behavior of the UAV longitudinal motion having closed loop flight control system parameters as they follow below:

=5 rad/s, . (7) The computer simulation was done

for constant natural frequency of , and varying damping factors of changing

from 0,2 to 2,0. Regarding damping factor, if it is less than 1, the closed loop automatic flight control system is said to be underdamped. If damping factor has value of 1, it is said to be critically damped. Finally, if damping factor is larger than 1, it is said to be overdamped.

In my vision, the damping factor must lie between limits of 0,2 and 2 to allow the automatic flight control system of the UAV.

If to apply to the input of the closed loop automatic flight control system of the UAV a unit impulse function, the response was found and plotted in Figure no. 1.

0 1 2 3 4 5 6 7-2

-1

0

1

2

3

4

Time [s]

q(t)

[de

g/s]

UAV Longitudinal Stability Augmentation System - Impulse Response Functions

Fig. no. 1 Transient Impulse Responses of the UAV with Varying Dynamic Performances



; ; ; ;

These results mainly used for stability analysis of the closed loop automatic flight control systems of the UAV, which is a strong requirement closed loop system must meet. Otherwise the closed loop system automatic flight control system of the UAV can lose functionality, itself. If closed loop automatic flight control system of the UAV is stable, the impulse responses of the UAV

will return to zero whilst time goes ti infinity.

Figure no. 2 shows step responses of the closed loop automatic flight control system of the UAV, when a unit step input signal is applied to the UAV closed loop control system input. The UAV dynamical model applied in computer simulation was (6) with parameters in (7).

0 1 2 3 4 5 6 70

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Time [s]

q(t)

[de

g/s]

UAV Longitudinal Stability Augmentation System - Step Response Functions

Fig. no. 2 Transient Step Responses of the UAV with Varying Dynamic Performances

; ; ; ; Evaluation of the results shown in

Figure no. 2 leads to the following statements:

1. Damping factor of : after defining the minimum and maximum of it, it is useful find nominal range for the damping factor to be as follows:

. 2. Overshoot of the closed loop control

system of , defined by

. If to

consider closed loop system with underdamped damping factor let us define upper limit for the overshoot as .

3. Settling time tolerance band of : this parameters allows to find settling time. It is defined related to the steady-state value of the



response of the closed loop control system. For closed loop automatic flight control systems of the UAV le tus consider following tolerance band of . As tolerance band increases as the closed loop automatic flight control system of the UAV decreases its

4. Settling time of : is a time when transient response reaches and will stay within a band defined as tolerance. Of course, there might be large differences between UAV types and dynamics, so closed loop settling time defined with its upper limit of .

If dynamic performances defined above are accepted to be considered for UAV automatic flight control system airworthiness certification process, they can be applied in measure of compliance of those flying qualities defined in conceptual design phase of the unmanned aerial vehicle system.

There is another important time domain transient response of the closed loop automatic flight control system of the UAV. It is a response of the closed loop control system of the UAV to a periodical square input signals. Results of the computer simulation can be seen in Figure no. 3.

0 1 2 3 4 5 6 7 8 9 10-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time [s]

q(t)

[de

g/s]

UAV Longitudinal Stability Augmentation System - Response to the Double Square Inputs

Fig. no. 3 Transient Response of the UAV with Varying Dynamic Performances to the Square Signals

; ; ; ;

Figure no. 3 shows that UAV automatic flight control system is able to change pitch rate very fast if there is a square signal at its input, and the input signal changes its sign to the opposite one.

Using results of the computer simulation given in Figure no. 1, in Figure no. 2 and, in Figure no. 3 the UAV automatic flight control system behavior can be evaluted very easy. Easy to agree



that due to its complexity, all terms of both classical and modern control engineering can not be applied simultaneously for preliminary design of the UAV automatic flight control systems. In the absence of the human being on the UAV board one can apply dynamic performances degraded to those of derived for manned aircraft. However degradation of the dynamic performances is allowed if and only if

– the closed loop automatic flight control system of the UAV is kept to be stable;

– stability margins are adequate to minimium requirements;

– in airframe critical points (i.e. wing root) forces and moments are minimized to provide maximum fatigue reduction;

– the maximum loads can not degrade other technical parameters, such as parameters of the ground and flight maintenance.

5. Results and Discussion This paper deals with dynamic

models of the spatial longitudinal motion of the UAV used for measure of the compliance in airworthiness process. The models persented here are SISO and MIMO ones, including or neglecting time delay inside the model.

The author summarized general flying qualities of the automatic flight control systems. The large number of possible measures were limited by the author to those proposed by him to be applied in certification processes. This set of dynamic performances includes those flying dynamic performances providing necessary and sufficient conditions applied in type – and airworthiness certification processes lead by authority experts.

The family of the dynamic performances of the UAV automatic flight control system proposed by the author can be summarized as given below:

a. – damping ratio of the closed loop automatic flight control system of the UAV;

b. – percent overshoot of the closed loop automatic flight control system of the UAV;

c. – peak-time of the closed loop automatic flight control system of the UAV;

d. – settling time of the closed loop automatic flight control system of the UAV;

e. – gain margin of the open-loop automatic flight control system of the UAV derived on frequency response functions;

f. – phase margin of the open-loop automatic flight control system of the UAV derived on frequency response functions;

g. – settling time tolerance band of the closed loop automatic flight control system of the UAV.

These performances can be used although in measure of compliance of dynamic performances in angular attitude control systems, too. If to control UAV flight path in large navigation tasks, there might be a need to review dynamic performances defined above.

This article focuses only on spatial longitudinal motion of the UAV. Future work on this topic will be executed on lateral/directional motion flying qualities so as to give a full set of those parameters, or in other words, requirements applied in measure of the compliance of the UAV automatic flight control system airworthiness certification process.



REFERENCES

1. D. McLean, Automatic Flight Control Systems, (New York-London-Toronto-Sydney-Tokyo-Singapore: Prentice-Hall International Ltd, 1990).

2. R. Szabolcsi, Modern Automatic Flight Control Systems, (Budapest: Miklós Zrínyi National Defence University, 2011).

3. R. Szabolcsi, “Airworthiness Criteria of the Unmanned Aerial Vehicle Systems and its Measure of Compliance”, Scientific Bulletins of Szolnok 1 (2013), http://www.szolnok. mtesz.hu/sztk/index.html.

4. R. Szabolcsi, “Airworthiness Certification of the Automatic Flight Control System of the Target Unmanned Aerial Vehicle System”, Hadmérnök 4 (2013), http://www.hadmernok. hu/134_03_szabolcsir.pdf.

5. R. Szabolcsi, “Airworthiness Certification of the UAV and UAS Systems: Friend or Foe?!”, Technical Sciences in East-North Region of Hungary 2013 Proceedings, (June 2013): 1-10.

BIBLIOGRAPHY

Decree of Minister of Defence of Hungary No 21/1998 about registry, production,

repair, and airworthiness certification of the aircraft applied in public aviation. MIL–C–18244A, Amendment 1, Control and Stabilization System: Automatic, Piloted

Aircraft, General Specification, 1993. MIL–F–83300 Flying Qualities of Piloted VSTOL Aircraft, US Washington D.C.,

Department of Defense, 1970. MIL–F–8785C Military Specification – Flying Qualities of Piloted Airplanes, Notice 2,

1996. MIL–F–9490D, Notice 1, Flight Control Systems – Design, Installation, and Test of

Piloted Aircraft, General Specification, U.S. Air Force, 1992. MIL HDBK–516A Airworthiness Certification Criteria, Department of Defense

Handbook, 2004. MIL–HDBK–1797A Flying Qualities of Piloted Aircraft, US Department of Defense

Handbook, 1997. MIL–H–8501A Helicopter Flying and Ground Qualities: General Requirements, US

Washington D.C., Department of Defense, 1961. NATO STANAG 4671 Unmanned Aerial Vehicles Systems Airworthiness Requirements

(USAR), NSA/0976(2009)-JAIS/4671, 2009. Official statement of the State Secretary of the Ministry of Defence of Hungary

No 47/2008 about acceptance of NATO STANAG 4671.

uav longitudinal motion flying qualities applied in airworthiness certification · pdf...

Documents