Sunday, October 10, 2021

Free Flight Quarterly Website Stats

Jumpline, the hosting service for Free Flight Quarterly went offline for major upgrades on October 9th. On the morning of October 10th Jumpline was back online but access to the Free Flight Quarterly website was not available as of 11:00 AM EST,

Friday, January 20, 2012

Rubber Model Trimming via PGI and TOP

Over the course of many years Jean Wantzenriether has developed and published an approach to trimming and design modification of high performance rubber powered free flight models.  One key aspect of his method is to avoid the use of variable position surfaces except for de-thermalization. The name first given to the method was PGI, standing for Piqueur Gravite' Incidence which refers to the key aspects of thrust line relative to the center of gravity, center of gravity location and wing incidence relative to the thrust line.  Later refinements and additions to his method led to the name TOP,  for Three axis Optimal Permanent equilibrium.  TOP, like PGI, avoids variable positions surfaces. TOP additionally provides guidance effecting both the lateral axis and longitudinal axis.  

Starting around 1980 Jean's publications on PGI and TOP have appeared in many places such as Vol Libre, The National Free Flight Society Symposiums, and Free Flight Quarterly.   "Insights on the Dynamics of Rubber Powered FAI models", a book published by Free Flight Quarterly, contains a comprehensive overview by Jean of PGI and TOP.  In this book one can find not only the PGI and TOP methods, but also other extremely interesting analyses of the design and operational aspects of a contest rubber model, in particular the role of the horizontal stabilizer and on optimum propellers.  "TOP Fundamentals" an article from this book may be read by opening the following link, TOP Fundamentals .  Additional information on the book and how to buy a copy my be found at FFQ Books .   


Sunday, August 28, 2011

Ilbesheim 2011

Ilbesheim 2011

One of the largest turn-outs in the 2011 World Cup scene, participants from 23 nations including Mongolia, Argentina and the USA and challenging weather conditions with over 160 registered participants. A recipe for a big World Cup showdown.
After a full month of rainy weather and possibly the worst summer weather ever recorded in western Europe, the forecast looked very promising. Sure, the weathermen had promised good weather many times before, which lasted about half a day, but this time the forecasts left no doubt. We would have summer weather throughout the Ilbesheim weekend. But just like the many previous promises, also this time we wouldn’t keep it dry. Notably the southwest of France had proven to be a serious low-pressure system generator, pushing areas of vicious thunderstorms towards the Benelux countries and Germany. And this time would be no different. Thursday night, a heavy weather system swept over Belgium, which caused the death of five youngsters at a music festival when a big tent collapsed. In fact, we could witness the weather system from a distance and although it approached us slowly, only a bit of rain touched as Thursday night.

Bad weather moving in from the west

Like with the previous low-pressure systems, its trailing end brought wind and turbulence and in Ilbesheim it was not different. The first round on Friday morning started therefore with a gusty wind, carrying the models towards and across a railway track. The driver of a passing train could see several modelers walking parallel to the tracks and he must have alerted the local police. At the start of round two, which was delayed due to the ever-increasing wind speeds, the police in fact arrived. Luckily they only warned the contest director, but if any one would be seen near the railway tracks again, they would press charges. This left the organizers no other choice but to relocate the starting line. And so it happened. A new location was chosen and the wind had turned as well, this time heading away from the railway track and motorway. The downwind area was not obstructed by trees, windmills or villages which otherwise made the selection of a starting positions a daunting exercise. We managed to fly the remaining 4 rounds in wind speeds of 5 m/s. Thermal activity was mild and most models landed some 1 km away from the launch. Some interesting flights could be seen. Allard van Wallene exploded a stabilizer during acceleration and Roland Koglot spoiled a launch for a sub 3 minutes flight. Nevertheless, everybody kept pushing on and by the end of the day some 11 flyers in F1A managed a clean score. At the end of the last round the sun was already close to the horizon, and Ansgar Nuttgens decided to have the fly off the next morning at sunrise.

Robert Lesko with beautifully crafted all 'Textreme' carbon flapper.

Saturday morning dawned with perfect conditions. A slight overcast sky and only a light drift towards the motorway. We gathered at the same position where we flew round 1 the day before. Out came the LDA ships and flappers for a performance showdown. Soon the 10 minute slot was on its way. Dirk Halbmeier and Allard towed upwind in the direction of Ilbesheim while the others decided to head for higher grounds at the south end. Soon Dirk made a perfect launch with his LDA ship. Allard followed and launched shortly after. But the pattern was off and the model shot sideways at a 45 degree angle and overbunted. A few stalls followed and the brand new LDA ship ended at some 60 meters altitude. Nevertheless, the glide was remarkably fine for a 5 min 30 flight and 4th spot. Meanwhile also Kosonoshkin had launched followed by Rene Limberger. Slowly the models where circling back to earth without any signs of lift. Kosonoshkin touched down in a winning time of 402 seconds followed by Limberger (382). Halbmeier settled for third spot (361). All flying LDA models, a sign that flappers are quickly heading for oblivion.

History and future? Allard’s flapper and new LDA ship.

In F1B Silz won (358) over Stefanchuk (345) and Rosonoks (342).
Shortly after, the sunrise competition started. It was announced that the moment thermal activity could be noticed, the competition would be finished. In fact, this morning showed the best weather of the weekend. No wind, sunny and warm. Many made use of this weather to trim their models. Many flights were made, most landing close to the starting area. Although no real thermals were present, the sun did its best to warm up the air and some flights had some assistance of more than buoyant air. The rules said that the best and worst flight time would not be counted for the result. Jorg Schellhase won the event with his flapper.
The morning of the Daedalus cup greeted us with some fine weather. But dark clouds were moving in from the south and we only managed one round before the competition was interrupted for some hours to let the rain storms pass through.

Rene Limberger pointing to Mammatus clouds moving in

Temperature had dropped some 10 degrees and the rest of the day it would climb back to some 25 degrees. The wind direction was again heading straight for the motorway. To be better safe than sorry, Ansgar decided to reduce the max flight time to 2 minutes for rounds 2 and 3. The wind had been dropping gradually and rounds 4 and 5 could be flown with 150 and 180 seconds respectively. Conditions were not hard, although again some LDA ships could be seen dropping fast after a perfect launch to barely make the max. Needless to say, 5 rounds combined with flights of 120 seconds would result in a huge fly off. Everybody was wondering if such a fly off could at all be organized. Participants had to organize their own timekeeper. After another move of the flight line, the poles were quickly positioned and pole positions assigned. Shortly before the start of the fly off, the timekeepers were shifted one spot to secure timing impartiality. All went like clockwork and soon the horn sounded. 29 flyers (!) towed up to find a safe spot to circle. Brian van Nest quickly moved away from the pack and launched a few minutes later. Kosonoshkin towed passed the parked cars at the other end of the line. Allard was towing his new LDA ship and made a perfect and high launch with some 3 minutes to spare. Despite some light turbulence it landed at 6:58, the strobe leds making it perfectly visible against the dark skyline. Soon the other times were known and only Koglot and Kosonoshkin came close with 6:56 and 6:50 respectively. Again, all flew LDA models but Max Herwig came close flying a regular model to a well-deserved fourth place after having lost his LDA model in a cornfield at the German nationals the week before.
The F1B fly off soon followed against a setting sun. Without apparent helpful air, Danko Sokolic won (331) over Rigault (296) and Seifert (288).
If there would be a price for bad luck, Kees van de Ven would have won it. When packing up for changing starting position, he locked himself out of his car with the keys inside. Things had to be organized to regain access, and some steel wire contraption was welded in the Schmal winery workshop to be able to reach a door handle through a forced gap at the window frame. Finally he managed, but the F1A fly off was already on its way and he arrived just when the 10 minute slot had finished.
Ansgar Nuttgens and his crew organized a great event. The weather didn’t always cooperate and the nearby motorway didn’t make it easy to choose a safe starting position. Nevertheless, we had a great time. The Schmal winery in Ilbesheim proved to be a great location for camping, social events and price giving and seeking shelter for the rain. Food and drinks were served for modest prices. I’m looking forward to the video footage taken by a professional filmmaker. A DVD is to be released soon and can be ordered at

Monday, April 11, 2011

Internet Changes In Progress - April 11, 2011

Sergio Montes has a new email address.  You can reach him at

Thursday, January 6, 2011

Not a member of NFFS--The National Free Flight Society?

Don DeLoach, Editor of the U.S. National Free Flight Society's "NFFS Free Flight Digest" made the following announcement, valid for the year of 2011    

Announcing a *Special* offer for new members: two years for half price!

Roy Hanson has made a generous offer: He thinks so much of the NFFS and the its publication, the NFFS Free Flight Digest he is paying out of his own pocket HALF the cost of EVERY NEW member who signs up for two years.  $29 dollars gets you two years if you are 19 or older ; $9 dollars if you are younger.  Non-U.S.  members pay just $37.50 for two years. 
Each Digest issue is at least 40 pages, delivered six times a year.  Each issue is crammed with Free Flight how-to, plans, contest reports, photography and much more.   The Digest covers every aspect of Free Flight: AMA, Indoor, FAI, FAC, SAM, Nostalgia--the works.

If you are a former member who has let their membership lapse for a year or more, now is your chance to re-up for half price.   To make it easy, you can even hit the following link and use your credit card. 

Tuesday, December 28, 2010

Model Propeller Constants - Weick, Durand and Lesley

"Aircraft Propeller Design", written by Fred E. Weick and published by McGraw Hill Book Company in 1930 makes reference to wind tunnel test results for a series of propellers with pitch to diameter ratios between 0.5 and 1.1, see pages 105 to 108 . The tests of these 13 propellers performed by W. F. Durrand and Everett Lesley are reported in "TESTS OF THIRTEEN NAVY TYPE MODEL PROPELLERS", NACA TR 237, 1926 .   You can find out more about the tests at Durand Lesley Propeller Collection and download the test report at NACA TR 237 .

In 1994 I, Chris Stoddart, developed and posted to the internet a mathematical model based on these tests and Fred Weick's Representative Section / Blade Element theory,  The model made use of MiniTK a free version of the TK Solver, a mathematical modeling and problem solving software system based on a declarative, rule-based language, commercialized by Universal Technical Systems, Inc. , see TK_Solver at Wikipedia  and UTS .  The free version can still be used as a DOS application in Windows and with Linux.  You can download MiniTK from  MINITK and the mathematical model from Weick.TK .
Weick.TK allows the examination of many interactions on propeller performance on parameters such as efficiency, power and thrust as effected by pitch, diameter, velocity, and rotational speed and other factors.  The program can select an optimal propeller to match a flight condition. It gives reasonable answers to queries ranging from rubber power to control line speed.

The Rule, Variable, and Unit sheets of Weick TK appear below.

Rule Sheet
Status Rule
Comment ;Program -, Written by Chris Stoddart, July 23, 1994

Satisfied eta=TAN(PHI)/TAN(PHI+GAMMA)
Satisfied C_p=QGF*B*WR*J^2*((TAN(PHI)+TAN(GAMMA))/(TAN(PHI)*SIN(PHI)))*C_L
Satisfied C_s=J/C_p^0.2
Satisfied C_s = V*(rho/(P*(N/(2*pi()))^2))^.2
Satisfied eta=C_t*J/C_p
Satisfied BETA=ATAN((4*PoverD)/(3*PI()))
Satisfied TAN(PHI)=V/((3*D/8)*N) ;-- use without inflow
Satisfied C_L=0.355+4.3*ALPHA-3*(ALPHA+0.075)^2 ;-- use without inflow
Satisfied GAMMA=0.47*ALPHA+0.0073/(ALPHA+0.075) ;-- use without inflow
Satisfied GAMMA=ATAN(1/LovrD)
Satisfied T=(rho*C_t*N^2*D^4)/(2*pi())^2
Satisfied P=(rho*C_p*N^3*D^5)/(2*pi())^3
Satisfied Q=P/N
Satisfied M_0=T/P
Satisfied J=V/(D*N/(2*pi()))
Satisfied PoverD = Pitch/D
Satisfied WR = Chord/D
* Undetermined condition IF OPT = 'yes THEN ALPHA = .8/57.3

Variable Sheet
Status Input Name Output Unit Comment

File Name: July 23, 1994

.579 D
m Diameter

.722 Pitch
m Geometric pitch

Chord .06948 m Chord at 3/4 distance to tip

2 B

Number of Blades

.12 WR

Width ratio at 3/4 distance to tip

PoverD 1.24697754749568
Aerodynamic Pitch over Diameter

8 N
rev/s Angular velocity

5 V
m/s Forward velocity

T 1.06051082397811 N Thrust

w Absorbed Power

Q .129313391262165 N*m Absorbed Torque

M_0 .163155511381247
Figure of Merit [T/P in N/w]

J 1.07944732297064
Advance ratio V/(N*D)

eta 81.5777556906235 % Efficiency

C_t .120262549489696
Thrust Coefficient

C_p .159132947457634
Torque Coefficient

C_s 1.55901190738972
Speed/Power Coefficient

BETA .486760145073151 rad Geometric Angle of Attack

PHI .429595614242357 rad Relative Wind Angle

ALPHA .0571645308307935 rad Aerodynamic Angle of Attack

C_L .548405092943241
"Lift Coefficient" Correlation

LovrD 12.1526639689179
"Lift/Drag ratio" Correlation

GAMMA .0821015133831694 rad Angle who's tangent is Drag over Lift

1.226 rho
kg/m^3 Air density

.366372781256872 QGF

Overall torque grading factor (0.366)


To find optimal Pitch given D,HP,N,V

enter 'yes, otherwise leave blank

Units Sheet
From To Multiply By Add Offset Comment
m in 39.37

ft in 12

m mm 1000

kg g 1000

rad/s RPM 9.549296585513721

m/s ft/s 3.280833333333333

w hp .001342281879194631

rad deg 57.29577951308232

N*m lbf*in 8.850748065226473

lbf*in ozf*in 16

lbf ozf 16

w J/s

N lbf .2248090247334889

N/w lbf/w .224809024733

lbf/w ozf/w 16

rev/s rad/s 6.283185307179586

ft/s MPH .6818181818181818

w HP .001342281879194631

lbf*in gf*cm 1153.16

- % 100

lbf gmf 454

ozf*in gmf*cm 72.07250000000001

Friday, October 8, 2010


In issue 37 Sergio Montes and Andrew Longhurst present an article to celebrate "The 60th ANNIVERSARY OF SCRAM, Laurie Barr’s LIGHTWEIGHT CONTEST MODEL".


The printed edition of issue 37 contains a full size plan on 4 pages.  As a service to our subscribers that prefer not to unstaple their printed edition or those that receive the Digital Edition the links below provide access to a a full size plan and files for printing on typical personal printers on either A4 or Letter size paper.

Full size plan

Tiled plan for A4 size paper

Tiled plan for Letter size paper