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Palouse Ridge Runners R/C
Newsletter
Pullman Ridge Runner Moscow
============================= June 2003 ===============================
Remember, no flying on Sunday June 1 until after 4pm.
Next
Meeting:
The June club meeting will be at John Sawyer’s flying field on Tuesday, June 3. Flying will start about 5pm with a short business meeting at 7pm. The alternate meeting place in case of rain is the Jack-in-the-Box in Moscow at 7pm (contact Dave Walker for information (208) 882-9180).
Agenda:
By Don Hart
I was leafing through some of my old Model Airplane News issues and ran across two articles by Andy Lennon on techniques to match your prop to the engine and airframe. He has some interesting ideas on how to go about selecting a prop.
You’ve probably noticed that engine manufacturers often have horsepower ratings in their ads – something like 1.89hp at 18,000rpm. This sounds impressive until you realize that this statistic, while true, was probably on a 46 two stroke running a 9x4 prop. Andy Lennon would like to see the engine manufacturers list the rpm at the maximum torque, a much more useful statistic.
If you’ve seen the hp and torque curves in a typical r/c sport engine review, you’ve noticed that the rpm for maximum torque is well below the rpm for maximum hp. High performance engines intended for racing are the exception. Engines such those from Jett and Nelson may have both maximum hp and torque at about the same rpm, some where in the 16,000 to 19,000 rpm range.
According to Lennon, it’s torque that turns your prop. He suggests that you pick a prop that will load your engine near the rpm where your engine delivers maximum torque. Not only will you get the most from your engine, a larger prop is more efficient, giving you better performance.
Of course, if you run your engine with too large a load, you risk damage from overheating, especially if your plane has a cowl.
Think about how much air your prop moves – this is the thrust that moves your plane. A way to compare props is to calculate the volume of air per minute. You probably remember the formula for calculating the area of a circle:
Area = pi x radius-squared.
Pi is roughly 3.1416. We want to use diameter (d) rather than radius, so the formula becomes
Area = 3.1416 x diameter-squared / 4.
Factor in the prop pitch and rpm and do a bit of algebra, and Lennon ends up with a formula for
volume of air per minute = diameter-squared x 0.7854 x rpm x nominal-pitch
For a APC 10x9 prop at 10,710rpm, the air volume is
10 x 10 x 0.7854 x 10,710 x 9 = 7.57 million cubic inches per minute
A computer spreadsheet should allow easy comparison of props. Lennon has a table for four two stroke engines on props that load the engines near their max torque rpm and max hp rpm:
High torque rpm is 10,710 on a 10x9 prop. Air volume is 7.57 million cubic inches per minute.
High hp rpm is 17,900 on a 9x4 prop. Air volume is 4.55 million cubic inches per minute.
High torque rpm is 7,060 on a 14x14 prop. Air volume is 15.21 million cubic inches per minute.
High hp rpm is 14,180 on a 11x7 prop. Air volume is 9.43 million cubic inches per minute.
High torque rpm is 9,139 on a 14x14prop. Air volume is 19.69 million cubic inches per minute.
High hp rpm is 10,484 on a 15x8 prop. Air volume is 14.82 million cubic inches per minute.
High torque rpm is 5,290 on a 20x10 prop. Air volume is 16.61 million cubic inches per minute.
High hp rpm is 9,280 on a 15x8 prop. Air volume is 13.11 million cubic inches per minute.
As you can see, in all cases the larger prop at the max torque rpm moves lots more air, giving much more thrust than the smaller prop at the max hp rpm.
Lennon states that tuned pipes increase both torque and hp, explaining why pattern and speed fliers use tuned pipes.
Lennon also gives an example of practical application of this information. He had two buddies, both flying planes powered with the ST G90 engine. One was a pattern ship, and the other was a 700 square inch P-47. They were both using 13x6 props running between 11,000 and 13,000 rpm. Lennon talked them into using larger props to get the engines loaded nearer max torque. The pattern flyer found that a 16x6 gave him excellent performance at moderate speed with much lower noise levels. The scale flyer found that a 16x8 gave the best performance on the P-47 with improved climb, shorter takeoff and a low reliable idle.
One problem with a larger diameter prop can be restricted ground clearance. You may have no choice but to increase pitch rather than diameter.
Dave Gierke, another contributor to Model Airplane News, came up with the “propeller load factor” or PLF. This is calculated as
PLF = diameter-squared x pitch
The advantage of PLF is you can determine different sizes of props that should put about the same load on an engine. Here are some examples from Lennon’s article:
14x14 PLF = 2744
15x10 PLF = 2250
16x10 PLF = 2560
16x8 PLF = 2048
16x12 PLF = 3072
I calculated a few more:
9x4 PLF = 324
10x6 PLF
= 600
11x7 PLF
= 847
12x6 PLF
= 864
13x6 PLF
= 1014
16x6 PLF = 1536
Let’s say a modeler has a 46 two stroke that gives max torque at 9,800 rpm. A light-weight, slow plane should use a large diameter, lower pitch prop. A heavy-weight, fast plane might use a smaller diameter prop with greater pitch to match the flying speed. To get the same prop loading factor on the two planes using the same engine, the light plane might fly very well on a 12x6 with a PLF of 864 giving 9,700 rpm. The heavy plane might fly best on a 10x9 with a PLF of 900 giving 10,700 rpm.
Here are some torque and rpm statistics that Lennon got from past MAN engine reviews.
Four stroke engines:
Enya 41 max torque rpm = 10,300
Saito FA50 max torque rpm = 11,500
OS FS61 max torque rpm = 9,000
Enya 80 max torque rpm = 8,000
OS FS90 max torque rpm = 7,200
Thunder Tiger 91FS max torque rpm = 6,850
OS FS120 Surpass max torque rpm = 8,333
Saito 150 max torque rpm = 4,993
Saito 100 Flat Twin max torque rpm = 6,500
Two stroke engines:
OS 10 FSR max torque rpm = 11,000
OS 25 FSR max torque rpm = 8,000
OS 32 FSR max torque rpm = 7,548
Thunder Tiger 36 max torque rpm = 9,800
Nelson 40 max torque rpm = 18,500
OS 40 FSR max torque rpm = 9,000
MVVS GES/R 40 max torque rpm = 13,100
Tiger Shark 40 max torque rpm = 7,900
SuperTigre 46 max torque rpm = 8,223
OS 46 FX heli max torque rpm = 9,000
Sport Jett 46 max torque rpm = 15,850
MDS 46 ABC max torque rpm = 9,798
Enya 60X ring max torque rpm = 9,800
Webra Silverline 60 max torque rpm = 6,900
ASP 61 max torque rpm = 4,645
RJL K60 max torque rpm = 9,500
Fitzpatrick 61 max torque rpm = 8,100
Fox Eagle 74 max torque rpm = 9,600
SuperTigre G90 max torque rpm = 5,923
Webra 120 max torque rpm = 6,745
OS 140 RX max torque rpm = 8,677
DA 3W-24 max torque rpm = 5,880
Irvine 150 max torque rpm = 5,500
Moki 180 max torque rpm = 4,600
Zenoah G38 max torque rpm = 5,040
Zenoah G45 max torque rpm = 5,500
If you’re lucky you can find a review for your engine, with torque and horsepower curves, and rpm readings on props of several sizes. If not, you can use a review of an engine of the same displacement as a starting point.
A tachometer will help you find a prop that gets you near to the right load for maximum torque. PLF calculation can be used to find other props with similar load factors. This will narrow the range of props to try on your plane to give the best performance. Rpm will increase at flying speed as the prop unloads, so static thrust is only a starting point for optimum prop selection.
Of course, your mileage may vary depending on the weight, drag and wing area of your plane, as well as fuel, air temperature and humidity. Make sure you engine is well broken in on the recommended low-load prop before loading it for high torque performance.
Good luck with selecting that optimal prop!
Sources:
“The Right Combination” by Andy Lennon, Model Airplane News August 2000
“Choose the right prop for your engine” by Andy Lennon, Model Airplane News June 2001
By Don Hart
The May meeting was held at the Jack-in-the-Box in Moscow.
Beginning in June we will be meeting at Sawyer Field. Flying will begin about 5pm with a business meeting at 7pm. Be there the first Tuesday of each month through the summer.
Les Grammer, PRR treasurer, reported that the club has $2694 in the bank.
The spring fun fly and BBQ is schedule for Saturday, June 7 at noon. Dave Walker will provide venison and beef burgers. The club will provide pop and condiments. Bring a salad or desert to share.
Jeff Nelson will be contest director for the bean-bag drop event. For details, contact Jeff. There will be lots of opportunity for open flying as well.
Joe Bolden, Mel Colvin and Pat Gates are presenting classes to kid through Moscow Parks and Rec. Joe needs plastic egg cartons for building material. Contact Joe if you can help out.
Joe Bolden met the new director of Moscow Parks and Rec at one of the model building classes. The new director is a former helicopter pilot who has had positive experiences in the past with model flying fields in parks. Dave Walker will meet with the director for a more in-depth discussion of the possibilities.
The Thursday evening flying sessions have begun! Flying begins about 5pm and goes to dark.
Dave Sidmore is an Air Force retiree who spent 20 years as a radio tech. He is a beginner at R/C flying.
Richard and David Nelson (father and son) are also beginners who are interested in learning to fly R/C sailplanes.
Dave Walker presented techniques to improve landings with your high wing trainer. Dave recommends the standard rectangular landing pattern beginning with the downwind leg parallel with the runway, a 90 degree turn to the cross wind leg and another 90 degree turn to final.
Try to fly through an imaginary “goal post” target at the end of the runway. Reduce throttle on each leg, using the elevator to control air speed and the throttle to control descent rate on final. Flatten the glide at about 3 feet by slightly increasing up elevator.
When the plane reaches about 12 inches above the runway, use the elevator to hold the plane off the ground as long as possible without actually gaining altitude. Use the rudder to keep the plane lined up with the center line and to keep the wings level. Remember to steer with the rudder control after the plane touches down.
Dave stressed the importance of using rudder for turns and to keep your plane lined up on the runway. Rudder turns keep the wing more level than aileron turns. Rudder is also more effective at the low airspeeds experienced during landing, avoiding stalls. On final, steer the tail – move the rudder stick in the direction you want the tail to move.
Dave also covered dead-stick (no power), head wind and cross wind landings. Cross winds will push the plane toward the edge of the runway. Dave recommends using the rudder to point the nose into the wind on cross wind landings. You don’t need to hold the rudder over as the plane will tend to hold a crab into the wind, maintaining the flight path down the center of the runway. Use the rudder to correct any drift away from the runway center line.
Dave will gladly answer questions and provide hands-on instruction in landing techniques.
Don Hart brought his QHOR built from instructions found on the internet. The basic “airframe” was built in about 2 ½ hours from coroplast corrugated sign material, a piece of aluminum channel, and a chunk of “poly” plastic cutting board for the engine and landing gear mount. The plane is quick and cheap to build and flies well on a plain bearing 40 engine. This plane might be considered a “high tech” derivative of the “Pizza Box Flier” Don built and flew last fall.
June 8 – Four Mounds Spring Fun Fly, Spokane
June 14 – Deer Park Fun Scale
July 12-13 – Spokane IMAC competition
July 19-20 – Orrin Crooks Memorial glider meet (see Bruce Bumgarner for details)
August 9-10 – Coeur d’Alene IMAC competition
August 16-17 – Spokane electric fun fly
September 7 - Four Mounds Fall Fun Fly, Spokane
September 20-21 – Jump-Off-Joe float fly, Chewela, WA
Go to the PRR web site and click on the “events” link for more information.