30 Minute Dawn Mulvihill

See a picture of the Bronco

 

Proposal

To design, build and operate an elastic powered, free flight, model airplane conforming to the rules of Dawn Mulvihill, which will remain aloft for 30 minutes, double the duration of current designs in this class of model airplane. 

Summary 

This proposal is aimed at a goal of evaluating a given power source and designing a propeller to maximize the energy available.  The power source in this case is 1/8 in. rubber strip stranded together to make a large rubber motor.  The model it will power has the constraints of a 300 sq. in. wing and must be rubber strip powered. The classification for this type of model is known as Dawn Mulvihill. These models are designed to be flown very early in the morning when the effects of thermals are minimal to non-existent.  Current flight durations for this event range from 9 to 14 minutes and it is believed that durations can be increased by at least double.

Background 

Categories of Model Aircraft

 Model aircraft are broken down into three basic categories, Radio Controlled, Control Line, and Free Flight.  A Radio Controlled (also known as Remotely Controlled) aircraft is flown by a pilot who sends commands to the aircraft via a transmitter at his location.  He controls the attitude, direction and even the engine speed of the aircraft.  Similar to a Radio Controlled model, the pilot of a Control Line model can control the climb and decent of his model through a cable or multiple cables directly connected to flight controls in the aircraft.

 Free Flight models do not have the luxury of a pilot.  Once the model is released it must self correct to remain in the desired flight pattern.  This presents some unique challenges that are not present in “controlled” aircraft.  Forces on flight control surfaces must be set to balance during different stages of the flight or the model may spin or dive. 

Rules of Mulvihill 

Mulvihill is a class of Free Flight governed by only a few simple rules.  The model must be rubber (strip) powered and the projected area of the wing cannot exceed 300 sq. in.  These models have maximum official flight durations of 5 minutes each for the first three official flights with the maximum increasing in 1-minute increments each successive flight.  Dawn Mulvihill has the added challenge of no limit on the maximum official flight duration and the model is flown at 7:00 AM when the effect from thermals is minimal to non-existent.  The winning models over the past few years weighed 2 to 3 ounces and the winning flight durations were between 9 and 14 minutes.  Copies of the plans for two of these models are included in the Appendix to this proposal.

 Hypothesis 

It is believed that the current designs flown in Dawn Mulvihill are not as efficient as they could be and 30-minute flight times should be attainable.  The basis for this hypothesis is that currently there are Free Flight models that are flying in excess of 1 hour indoors where the ceiling is limited, while outdoors, where the ceiling is unlimited, times of less than one quarter this duration are the norm.

 The key area needing improvement is utilization of the available energy.  Like any other type of motor, a rubber motor has a power curve that it follows when it is outputting the power to the propeller.  Current outdoor propeller designs are not designed to maximize thrust at all points on the power curve.  Instead they are designed to be a compromise that will work well during the middle portion of the power run but will be inefficient at the beginning and end.  For current Dawn Mulvihill aircraft, the propeller is only providing a climbing thrust for 3 to 5 minutes and the rest of the flight the model is gliding with the propeller folded.

Testing 

There is quite a bit of data that will need to be collected.  First, all of the current designs will be evaluated to determine a starting point for the design in regards to airframe weight and wing loadings.  Next, all current manufactured rubber strip will be tested in order to determine the power curves that each can provide.  Propeller designs will be tested next to match a propeller to the particular rubber strip that will be used.  Research utilized by designers of indoor models will be evaluated to determine if the same techniques can be applied to an outdoor model.

 Three models will be constructed. The first will be smaller than 300 square inches and will be built to evaluate the chosen design.  Changes in center of gravity and angles of declination of the flying surfaces will be evaluated to determine optimum positions.  This process is currently in progress and a copy of the plans for this design is included in the Appendix of this report.  A full size model will then be constructed to test the results found with the smaller model.  The final model will then be constructed and fitted with the final propeller design. The model will be flown in the Lucerne, CA area to record average durations during the early morning hours.  This last model will subsequently be flown in competition at the West Coast Free Flight Championships in Lost Hills, CA occurring in late September of 2004.

Conclusions 

This project is an ambitious one.  Many of the competitors flying in the Dawn Mulvihill event have been building and flying models for up to 50 years and it will be difficult to improve on their designs.  It is hoped that by identifying the power curve of the rubber motor, the propeller design can be matched to this curve and less energy will be lost in the conversion of the potential energy of a wound rubber motor to thrust to propel the model into the air.  By studying the techniques used by the indoor flyers and adapting them for outdoor applications, it is hoped that the model will achieve the goal of an unassisted 30-minute outdoor flight.

Appendix:

“Bronco”, Designed by Daniel Heinrich

Model to be used for project


 

“Lean Machine”, Designed by Clarence Mather

Current high time in event: 14 min. 14 sec.

 

“Ro-Maxer”, Designed by Bud Romak

Average flight time 10 min.