Wright Stuff C

[vehicleguy]

So first time experimenting with a 3 bladed propeller yesterday. It worked after some trimming, but it is not amazing. I had to transfer a lot of clay from the nose towards the center of gravity, and even then, the plane would rise and lose altitude quicker than expected. I also believe the prop was unbalanced, but I'm not sure what effect that had on the flight.
I believe I read something about using a shorter rubber band for a 3 bladed prop compared to a 2 bladed prop. Can anybody confirm if this is true?

[coachchuckaahs]

So first time experimenting with a 3 bladed propeller yesterday. It worked after some trimming, but it is not amazing. I had to transfer a lot of clay from the nose towards the center of gravity, and even then, the plane would rise and lose altitude quicker than expected. I also believe the prop was unbalanced, but I'm not sure what effect that had on the flight.
I believe I read something about using a shorter rubber band for a 3 bladed prop compared to a 2 bladed prop. Can anybody confirm if this is true?

VG:

At the speeds these props turn, you need to be balanced. An unbalanced prop transfers a lot of energy to the plan in the form of vibration.

The three-bladed prop, if made with the same blades and pitch as the 2 bladed prop, will need thicker rubber, but will turn slower.

Coach Chuck

[coachchuckaahs]

One more: don’t try to add more than a few turns at the very end of the process (no stretch).

Cannot agree more! We break a LOT of rubber by not winding hard enough at full stretch and then trying to hit our turns later. When that rubber is screaming at you, its hard to not give in! Missed winds at full stretch CANNOT be recovered later in the process!

WE wind about half our turns at full stretch. This year's rubber sizes mean about 0.3-0.4 torque at full stretch. Then as you walk in, do so at a speed to maintain that torque for the first half of walk in. At that point the torque will start sharply rising. Pick up walking speed, with a goal to "land" the winder at about 2X the walk in torque. Do not try to fit more winds once you have "landed". That will break rubber!

It is also important to break in the rubber. The Super Sport is pretty good about allowing fairly high torque in break in, in fact we often break in at the highest torque we expect for that size rubber. Our break in consists of one or two hard winds. You will find the rubber takes a few more winds each time you use it. As the rubber becomes well used, you need to limit both winds and torque. Early in rubber life, we allow torque to guide us. After more than a few flights, the torque may not get as high, and you can easily exceed max winds and break it, so keep track of both.

With the thin rubber this year, if winding reasonably hard, you may only get 2-3 good flights before the rubber breaks.

Also with the thin rubber, small nicks in the rubber are a greater percentage of rubber width than with larger rubber. Therefore it is critical to use sufficient lubrication when tying the knots, or you will fail within the first few full winds, and will fail near the knot.

Rubber breakage is a normal part of the process if you are winding hard. However, you should get a few good hard winds before it breaks!

Coach Chuck

[calgoddard]

You can win any WS competition with 2009 or later Tan Super Sport (TSS) rubber, assuming it has been properly stored and lubricated. Keep it away from heat and UV radiation.

When I fly in adult Penny Plane competitions I can get about 5-10% longer times flying with TAN II rubber. Due to my dwindling inventory of TAN II rubber I often compete indoors using TSS rubber. I never compete outdoors with TAN II rubber as the motors are relatively large, e.g. 10 grams in F1G and P-30, and use of such high quality rubber would be a waste.

Manufacture of TAN II rubber ceased around 2002 and what little remains is hoarded by expert fliers and used very sparingly. Recently, 40 grams of May 99 TAN II rubber allegedly sold for over $1,500.

[lechassin]

Are you saying you climb higher with smaller (.06 v. .062) rubber? That seems opposite of what I've been learning, ie. if you need more climb you may need a higher weight/cross section rubber? I'm now confused?

This is only our second year so I suspect our plane doesn't know any better lol, but that's exactly what's happening: A 2.8 gram 0.0625 motor climbs to 25 feet at 0.25-0.3 in.oz torque (less going right because the climb banks less).

The 2 minute flighte used 2.8 grams of 0.060 at 0.2 in.oz or less, and we still hit the banner, dropped, then climbed again. I don't know why it's backwards seeming. Maybe the thinner motor distributes power better, so less banking, less flaring, less power stalling, etc... I'm not sure.

Edit: all else aside, the motor-making and winding advice we've gotten on this forum is the single largest thing that increased our flight times (Thank You!)

2nd Edit: the number of blades doesn't correlate with how thick the rubber needs to be. A lower pitch three-bladed prop can pull harder than a high pitch two-bladed prop at lower power input. For anyone making our prop, if you're uncertain how to optimize stuff, just use this formula: make the prop exactly like ours, make the hook-to-hook length 18", make a 2.8 gram (67" strand of TSS) motor and wind it as described above to 0.6 in.oz (about 4300 turns), then unwind to 0.3 in.oz (leaving about 4100 turns). If the plane is an 8.0 gram biplane trimmed for a smooth cruise, you can trim the climb as I described a few posts up and it'll fly 1'45" at 25 feet.

[coachchuckaahs]

The number of blades, if it is the only variable changed, does indeed correlate to the thickness of rubber used. My comment was:

The three-bladed prop, if made with the same blades and pitch as the 2 bladed prop, will need thicker rubber, but will turn slower.

Now, if you reduce pitch, whether a two-bladed or three bladed prop, you will need to reduce the linear density of your rubber (cross sectional area). The primary noted purpose of going to 3 blades was to allow shorter motor sticks by loading the rubber more, thus thicker rubber.

In either case, you will find an optimum pitch angle for the blades. I suspect this pitch angle, for identical blades, will be similar for 2- or 3-bladed props. In that case, going from 2 to 3 blades will increase the load, and therefore need thicker rubber.

Coach Chuck

[lechassin]

The primary noted purpose of going to 3 blades was to allow shorter motor sticks by loading the rubber more, thus thicker rubber.

Well now I'm confused! It suddenly appears that we've gone down opposite paths! I apologize, I misunderstood your post because I got stuck looking at it from my point of view.

We would use an even longer, thinner motor if we could. We don't because we are hampered by two design parameters that have reached a critical point, motor stick length and prop flaring:

#1 We can't make the motor stick any longer than 18.5" or it becomes too weak and/or too heavy. That limits our motor length to 35-40" depending on how much slack we're willing to accept. Since 2.8 grams of rubber is our optimum, we're stuck using 0.060-0.0625 rubber (to reach that magic 2.8 grams at our fixed length). That rubber thickness determined our prop pitch, based a gross estimate of knots remaining (we fine tune the length of any motor based on a precise count of knots remaining, and that changes depending on ceiling height and chosen aggressiveness of winding).

#2 After we thinned to 0.060 rubber, the average rpm during the climb was lower. The prop doesn't flare enough and we climb too high. Any attempt to make the prop flare more results in a shaking/vibrating mess.

For us, the use of three blades was independent from all that. For equal power input, we just prefer to spin three lower pitch blades, rather than two higher pitched blades . We based this on the observation that high pitched props are used on planes where high speed is the goal (even though it takes a while to get there), and low pitch props are used on planes where thrust is preferred (even though top speed suffers).

[lechassin]

For discussion on prop pitch philosophy:

Rare Bear air racer: https://abpic.co.uk/pictures/code-numbe ... are%20Bear Different props over the years but all very high pitch, 4500 hp needed to turn them, big plane to carry that engine. World record 528 mph but it takes a while to get it up to speed.

STOL (Short Take-Off and Landing) competition: https://vimeo.com/115727943. Very low pitch prop, only 100 hp needed to turn it, small plane is enough to carry the engine. World record 10 foot takeoff but the top speed is a joke.

Both planes are extremes, a series of compromises to achieve a particular goal. IMO, WS planes are more like the STOL plane, yet the prop pitch I see everyone else using looks more like the racer's. That contradiction is where I got the idea to run the lowest possible pitch, and add the third blade to load the thicker-than-wanted motor as much as possible without increasing the pitch.

[coachchuckaahs]

Each blade design will have an optimal pitch angle. This can only be determined through testing. In general, due to the very slow speeds of the prop (even this year's tiny props are very far below sonic), the best performance pitch tends to be higher than expected, IMHO. Several times in past years we took an ok performer to the next level simply by increasing pitch.

Your mileage may vary, but don't be afraid to try different pitches as well as different prop planforms.

Coach Chuck

[calgoddard]

In the hobby of rubber powered airplanes, the pitch-to-diameter ratio (P/D) of the prop is a very significant parameter that needs to be optimized. In the WS 2020 rules the maximum allowed diameter of the prop is 8 cm. For reasons I won’t explain here, you should employ the maximum diameter prop that the WS 2020 rules allow. That leaves the optimum pitch to be determined.

In general, for maximum efficiency, the blades of the prop should have a true helical shape. It is conventional to calculate the P/D of a prop with helical blades by measuring the angle of one of the blades at the “point of interest”, which is usually the three-quarter ( ¾ ) radius (R) position. The P/D of a helical blade prop can be calculated with this formula:

P/D = Tan Ɵ (π)(r/R)

Where

R = radius
P = pitch
D = diameter
r = radius at point of interest
Ɵ = angle at point of interest

I am not sure what the angle is for lechassin's 3-bladed prop at the ¾ R position. Assuming his prop has blades with a true helical shape I calculate that the P/D of lechassin's 3-bladed prop is about 1.8. In my experience, that is a very good P/D for an indoor duration stick model with a conventional 2-bladed prop.

There is an easier way to calculate the P/D of a prop with helical-shaped blades. Use the following formula, which I attribute to the late, great master builder and flyer, John Barker, of the United Kingdom. Measure the angle (Ѳ) of one of the blades at the ¾ R position. Then P/D = tangent (Ѳ) x 2.356.

The blade outline and the area of the blades also need to be optimized, but in my experience, these are less important factors than the P/D of the prop.

Generally, you would like to minimize the RPM of the prop to maximize efficiency. Induced aerodynamic drag increases exponentially with speed, so the faster the blade tips spin, the more drag they encounter. I believe either Brian T or Coach Chuck previously explained how an ever-increasing number of blades on a prop reduces its efficiency. However, the severely limited prop diameter in the WS 2020 rules probably dictates using more than two blades to get enough total blade area to slow down the prop.

Adding blades and/or increasing blade area adds weight, which is not a problem as long as the model only exceeds the minimum allowed weight by a few hundredths of a gram. Adding weight on the front end of a model is usually a good thing in terms of trimming the model so long as decalage and CG location for a given air frame are optimized.

Increasing total blade area and/or pitch of the prop requires that the length of the rubber motor be shortened while maintaining the same optimum rubber motor weight. Indirectly you are increasing the width of the rubber motor, but length and weight are more accurate measurements than width. Increasing the width of the rubber motor can lead to undesirable motor stick bending at high launch torque. The various parameters of a free flight rubber powered model are subject to trade-offs in terms of cost-benefit.

You must diligently use a torque meter and record all your data on flight logs in order to be highly competitive in rubber powered airplane events.

Once an indoor model has been properly trimmed, the key to winning a conventional contest is matching the rubber motor to the prop in terms of length and weight, and proper winding. The WS 2020 rules also dictate that you must be able to adjust trim to fly efficiently in opposite orbits on consecutive flights.