Dipl. Ing. Falk Pätzold Technische Universität Braunschweig, Institut für Flugführung February 21 st 2014

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1 1/14 Results of Flight Performance Determination of the Lak 17a FES (S5 3117) using the comparison flight method in Aalen Heidenheim Elchingen, August 20th and 21st 2012 Dipl. Ing. Falk Pätzold Technische Universität Braunschweig, Institut für Flugführung February 21 st 2014 Overview The company LZ Design d.o.o. (Slovenia) developed an electric propulsion for gliders called Front Electric Sustainer (FES). The electric motor and the propeller are installed in the nose of the fuselage. In non operating mode the two propeller blades rest against the outer surface of the fuselage. It is expected that this has a small effect on the gliders drag budget but the quantity this change of drag is not know properly. In four flights on two sequent days the influence of the propeller was investigated by determining the flight performance of the gliders with installed and dissembled propeller blades. Figure 1: DG 300/17 (D 1633) and Lak 17a FES (S5 3117) while flight trials

2 2/14 1. Description of flight trials The Lak 17a FES S was investigated on August 20 th and 21 st 2012 at the annual IDAFLIEG Sommertreffen. As usual the DG 300/17 D 1633 was used as reference glider for the flight performance determination applying the comparison flight method. All comparison flights of this investigation took place at the airfield Aalen Heidenheim Elchingen (EDPA) in southern Germany. Basic data of Lak 17a FES value source flight weight during investigation 400.8kg measured centre of gravity during investigation 272 mm aft reference datum certified: mm aft reference datum measured AFM wing span b m measured wing area S 9.8m² AFM Table 1: Basic data of Lak 17a FES (S5 3117) Flap Settings: Denomination Deflection of inner flap L Table 2: Flaps settings of the Lak 17a FES (S5 3117) Investigated configurations and aims of the investigation As depicted in fig. 2 in non powered flight the two propeller blades snuggle on the outer surface of the fuselage in a horizontal plane. To determine the additional drag both propeller blades were temporary replaced by a nose section representing the unmodified glider (see fig. 3) Figure 2.a+b: Propeller blade positions in powered flight (left) and in resting state (right) [1]

3 3/14 Figure 3: Fuselage nose without propeller blades Figure 4: Ventilation inlet Additionally the ventilation inlet is located in the nose of the fuselage. The effect of the ventilation state shall be investigated exemplary. The aim of the investigation is the determination of the additional drag due to the two propeller blades in rest position and the ventilation state in an operational relevant speed range. A partial performance investigation is appropriate to achieve this goal. Flap setting 0 was investigated from 100 to 160km/h and flap setting 1 from 160lkm/h to 190km/h with both the propeller blades installed and dissembled and ventilation in open position. From flap setting 0 the closed ventilation was investigated for both propeller assembly states. Overview flights and measurement sections On both days three flights in very viable weather conditions were done starting at sunrise. The airfield altitude is located at 586m above sea level and the maximum altitude in all flights was about FL95. Depending on the measuring program each flight lasts about one hour with a gross measuring time of about 30 minutes. The third flight of each day was planned for experimental investigations but both were not successful due to synchronisation problems of one data recorder. For all flights the weight of the DG 300/17 was maximised using water ballast to equalise the flight performance of both gliders as far as possible. The duration of the measurement section of constant speed was set to one minute to get an appropriate number of independent sections within the few flights. Flight No Date Aug 20 th 2012 Aug 20 th 2012 Aug 21 st 2012 Aug 21 st 2012 Investigated configurations Lak 17a FES Without propeller blades; flaps 0, and 1; vent open/closed With propeller blades installed; flaps 0, and 1; vent open/closed Pilot Ref DG 300/17 m F,Ref [kg] x SP,Ref [mm] aft ref. datum Pilot Vgl Lak 17a FES m F,Vgl [kg] x SP,Vgl [mm] aft ref. datum Usable sections A. Dilcher A. Kubasik A. Dilcher A. Kubasik As Flight No. 1 A. Dilcher A. Kubasik As Flight No. 2 A. Dilcher A. Kubasik Table 3: Flaps settings of the Lak 17a FES (S5 3117)

4 4/14 2. Preliminary results The data evaluation is based on the principles presented in [2] with several modifications. Traditionally the results of the flight performance investigations are named to be preliminary as future findings on the comparison flight technique in general or the investigation described here may be used to re evaluate the flight performance of the Lak 17a FES. Output quantities Using the airspeed calibration of the DG 300/17 the calibrated airspeed (CAS) corresponding to each basic airspeed (BAS) of the Lak 17a FES can be determined. Note: The indicated airspeed IAS is equal to the basic airspeed BAS if the airspeed indicator is error free. The core result of the investigation is the aerodynamic caused vertical speed w as function of the calibrated airspeed CAS. This measured relation is converted to glide number L/D, lift coefficient c L and drag coefficient c D for different depictions of the flight performance: 2 2 V w V c L / D V {1} CAS CAS L ( ) = = CAS w w cd c L 2mg = {2} ρ S V 0 F 2 CAS where is: m total mass g earth gravity (9,80665m/s²) ρ 0 air density at sea level according standard atmosphere (1,225kg/m³) S reference wing surface c = D L D c L For some depictions the total drag coefficient c D is reduced by the drag coefficient of the induced drag c Di assuming an elliptic lift distribution. This results in a more sensitive depiction of the residual drag. 2 c L ( c D c Di,ellip ) = c D {4} π Λ where is: Λ wing aspect ratio For determining the angle of attack AOA the pitch angle Θ is measured and the air path inclination angle γ a is calculated from the determined flight performance. The difference of both angles is the wanted angle of attack: 1 γ a = arctan {5} L D AOA = Θ γ a {6} The AOA refers to the horizontal plane in weighing reference attitude. {3}

5 5/14 The measurement equipment for the flight performance determination includes a differential pressure sensor that measures the pressure difference between the cabin behind the pilot and the static pressure of the onboard system to determine the cabin pressure. It is assumed that the airspeed error is solely caused by the error of the onboard static pressure. This assumption is based on the widespread knowledge about flow probes that the static pressure is primarily error prone. Mostly this assumption leads to plausible results. So the pressure difference can be corrected by the static pressure error and is related to the calibrated dynamic pressure: c p,cabin,corr ( p p ) cabin stat,onboard measured stat,onboard,error = {8} q calibrated p At a pressure coefficient of zero the cabin pressure is the same as the undisturbed static pressure at this altitude. All determined quantities for the airspeed calibration, flight performance, angle of attack and cabin pressure can be combined in several ways. The most common diagrams are shown. Airspeed calibration For the airspeed calibration three different diagrams were chosen. Figure 5: Airspeed calibration, CAS BAS depiction

6 6/14 Figure 6: Airspeed calibration, (CAS/BAS) BAS depiction Figure 7: Airspeed calibration, (CAS/BAS) AOA depiction The airspeed calibration of the Lak 17a FES S depicts no abnormalities. The differences between the configurations are due to the accuracy of the method.

7 7/14 Sink rate diagrams Figure 8: Sink rate vs. calibrated airspeed at reference weight L/D diagrams Figure 9: L/D vs. calibrated airspeed at reference weight A clearly determinable but small drag increase due to the presence of the two propeller blades can be seen. The effect of the ventilation is very small respectively without the propeller blades negligible.

8 8/14 c L c D diagrams Figure 10: c L c D diagrams c L (c D c Di ) diagrams Figure 11: c L (c D c Di,ellip ) diagrams

9 9/14 Lift slope curves Figure 12: c L AOA diagrams (c D c Di ) AOA diagrams Figure 13: (c D c Di,ellip ) AOA diagrams

10 10/14 Cabin pressure outputs Figure 14: c p,cabin,corr CAS diagrams Figure 15: c p,cabin,corr q c diagrams

11 11/14 Figure 16: c p,cabin,corr AOA diagrams The determined cabin pressure coefficients are comparably to other gliders. 3. Summary and acknowledgements In four flights on two days the effect of the two propeller blades of Front Electric Sustainer (FES) on the flight performance Lak 17a FES S was investigated for two flap settings in different speed ranges. The effect of the ventilation state was investigated for one flap position. The results show a very small increase in aerodynamic drag due to the propeller blades in an expectable order. Thanks to Luka Žnidaršič for providing the Lak 17a FES and the technical support while the preparation of the flight tests. The flight performance investigation of gliders at the IDAFLIEG summer meeting is a joint effort of the Institute of Flight Guidance at Technische Universität Braunschweig, the Interessengemeinschaft deutscher akademischer Fliegergruppen IDAFLIEG e.v. and the participating institutes of the Deutsches Zentrum für Luft und Raumfahrt DLR e.v.. Sincere thanks are given to all! 4. Contact In case of any questions, comments and suggestions don t hesitate to contact me via f.paetzold@tu braunschweig.de 5. Literature [1] electric sustainer.com/technology.php, Feb 19 th 2014 [2] Wende, G.: Flugleistungsvermessung von Segelflugzeugen, TU Braunschweig, Institut für Flugführung, Dissertation, 2003

12 12/14 6. Appendix 6.1 Glider condition log Flight performance determination of Lak 17a FES (S5 3117) at IDAFLIEG Sommertreffen 2012 Version: February 21st 2014

13 13/ Weighing log Flight performance determination of Lak 17a FES (S5 3117) at IDAFLIEG Sommertreffen 2012 Version: February 21st 2014

14 14/ Flap setting log