Airplane PDQ Airplane PDQ is a low cost, easy-to-use conceptual design tool for general aviation aircraft.  The initial layout is created with an “Airplane Sizing Wizard”.  The designer enters basic performance requirements and chooses from a wide variety of configuration options.  The wizard completes the sizing calculations and generates a 3D model of the aircraft.  The designer can then refine the aircraft using dialogue boxes and CAD commands.  The first 3D model can be created in less than 15 minutes! Airplane PDQ also generates detailed analysis reports.  The reports include: Design Summary, Geometry, Weight Estimate, Weight and Balance, Drag Breakdown, Performance Charts, Trim and Aero, CG Limits, and Design Check.  The analysis tools will audit the configuration and advise the designer to help ensure the aircraft is safe and practical.  It’s like having an aircraft design consultant review the specifications.  These tools allow the designer to quickly investigate “what-if” scenarios for different modifications, and the aircraft can be optimized before construction begins. The CAD module provides basic 2D drawing commands.  AutoCAD users will feel right at home but neither interface is particularly friendly, especially if you have no prior CAD experience (the cryptic icons alone may scare the uninitiated).  After PDQ creates the 3D model, the designer reviews the overall configuration.  Fuselage shape and component locations can be revised by dragging control points in the top, front or side view.  Menu driven dialog boxes allow exact parameters to be entered for the major aircraft components.  3D views can be turned on to visualize the configuration but can not be used to edit the design.  The User Guide is excellent for beginners and advanced users alike. I decided the best way to evaluate Airplane PDQ was to model the Cozy IV and to compare its performance predictions with actual flight data.  This was especially important for a non-standard canard configuration.  There were several iterations but the final PDQ models for the Cozy IV and the Apollo are shown here: This exercise resulted in several “lessons” about the July 2004 release of Airplane PDQ, which are: 1. There are six methods for estimating the aircraft empty weight and these predictions ranged from 1198 to 1483 lbs, compared to the actual weight of 1179 lbs.  Most methods grossly over-estimated the forward wing weight.  PDQ does allow each component to use a different method, so the user can refine the weight and CG estimate assuming they have an actual understanding of what each aircraft component will weigh. 2. Component CG estimates are based on each component’s physical location except for the “Equipment Group”, which includes avionics, electrical, flight controls, and furnishings.  CGs for this group appear to be biased towards the tractor configuration (ie: too far forward for a pusher design).  We had to override some component weights to get the empty weight CG estimate within four inches of the actual CG. 3. The initial estimate for the Cozy’s neutral point was 99.6” whereas the actual location is 105.4” from the origin.  This error resulted from modeling the strake’s root chord as a constant section across the fuselage.  The strakes had to have 60 degrees negative sweep across the fuselage to get the neutral point to 104.5”, which is 0.9” from actual. 4. Airplane PDQ predicts a fuselage pitch attitude of -0.12 degree at 190 mph at 7,500 MSL, whereas some Cozy owners report the aircraft cruises at +1.0 degree pitch attitude.  This may result from PDQ’s inability to account for canard downwash on the main wing, which reduces lift and requires a more positive pitch attitude. 5. PDQ’s airfoil database is limited to the 4, 5 & 6 digit NACA series.  Airfoil Optimizer has a much larger database and includes instructions for exporting airfoils one at a time to PDQ.  Custom airfoils must use data generated by an airfoil analysis program such as XFLR5.  This may result in some inaccuracy since the performance data is not generated by a wind tunnel, as the NACA data is.  In any case, I wish PDQ had a larger airfoil database. We never would have known about items 1 through 4 had we not reverse engineered the Cozy.  My advice is to model a real aircraft for comparison and don’t rely on PDQ as your only analysis tool for unconventional configurations. With the Cozy model now fixed, I moved on to performance predictions.  The table below compares Airplane PDQ’s predictions with the CAFE foundation’s measured performance data for the Cozy IV.  I’ve included PDQ’s predictions for the Apollo as well.  All data is based on a gross weight of 1900 lbs and a Lycoming O-360 (180 hp) engine. We can see that PDQ is very good at predicting top speed and climb rate but does less well for canard stall speed.  It over-predicts L/D ratio (based on airfoil data from XFLR5) and provides conservative Fwd and Aft CG limits relative to the neutral point.  If we had not corrected the Cozy’s neutral point, the CG limits would be off quite a bit.  Since the Apollo does not use strakes, its NP prediction should be more accurate. Even though prediction accuracy is not consistent, Airplane PDQ remains a worthwhile investment.  The real power of this software is not the ability to predict final performance, but rather, the ability to optimize the performance of an aircraft relative to its baseline design.  By experimenting with different airfoils, wing area, span, incidence, dihedral, decalage, etc, a designer can determine how much these variables affect performance and then optimize them.  This type of comparative analysis is extremely valuable whether you are creating a point design or trying to maximize total performance. Site Map Email the Designer Copyright © 2012 Apollo Canard