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Thursday, February 20, 2014

P30 Rubber Power: A Beginner's Guide (all you need to know to get going!)


My modified Roger Dodger P30, after recovery from a tree. Twin fins had broken off, but otherwise unscathed
I haven’t flown a P30 in ages and intend to change that this summer. As a prelude, here are some things I've learned about it.

Rules and Designs

The P30 rubber class originated in the USA in the 1970s. It was intended to provide a simple entry point into rubber power for novices, yet a challenge for expert “rubber fanciers”. Specifying a ready made plastic propeller meant that carving one was not required, a daunting task for newcomers. The combination of a 10g rubber motor [1], 40g model weight [2] and 30” maxima for projected span [3] and overall length presented a brainteaser that experienced aeromodellers would relish. Clearly, those rules were good, as here we are nearly 40 years later and P30 competitions continue to be well attended in many countries.

Model designs vary from simple traditional stick and tissue construction, to complex forms using carbon fibre and mylar covering. Rolled sheet balsa fuselage construction is common and once mastered is a quick way to build. Motors are typically 4 or 6 strand, or 3mm or 2mm rubber, wound to something in the range of 600-1700 (yikes!) turns. It is wise to balance the prop and take steps within the rules [4] to ensure it spins smoothly and disengages cleanly to freewheel.


This is a Roger Dodger built to the plan, with a single fin
My modified version has twin fins and a slightly altered rear fuselage
Here you can see what I did to modify the rear end

Flight Patterns


Anyone who has built a simple stick rubber model and flown it with a neutral trim will know that it has a tendency to turn left under power. The left turn is mainly from the model’s reaction to motor/propellor torque. For P30 and many outdoor rubber classes, turning to the left on climb is generally considered a no-no because it is not very stable and does not launch as high as climbing right. In a left turning power climb, trimming to the left with thrust adjustment, rudder trim and using high powers, can all lead to dangerous left spirals. Most P30 flyers trim their models to turn right on the climb. In a right climb, the motor/propeller torque constantly rolls the model the the left, lifting the inside (right) wing to encourage a spiral climb (rather than a dive!) to the right. On the glide, the freewheeling propeller acts like a front fin that makes the model glide right. Consequently, a right-right pattern is common. However, some flyers trim for a right-left pattern.
Another factor to think about is whether the fin is above or below the fuselage, because the spiralling airflow behind the prop causes rolling forces, both under power and during the glide as the prop freewheels. A study [5] showed that having the fin under the fuselage rather than on top imparts a strong right trim under power, diminishing rapidly as the power decreases and then turning into a strong left trim during the glide phase.  


I would suggest only trying right-left where the model designer specifies it, the fin is underneath the fuselage and/or the model has a strong natural tendency to fly left when test gliding. I’ve never tried right-left with a P30 though, because it sounds like a difficult trim to achieve and many experienced flyers insist that right-right is best.

Trimming and Flying


As ever, trimming starts at home. With the rubber motor in position, CG should be as indicated in the plan, but if you do not know the CG position, then 50-60% from the LE is as good a place as any to start. Ensure flying surfaces are flat and warp free, or whatever trim tabs, wash in, wash out, stab tilt, etc, as are described on the plan are present and correct. It may be useful to input a tad of down thrust and right side thrust to the prop from the get go while you are still at home. This may be done by shimming the nose block, or adjusting screws if there are any.


Before setting out to the flying field, you will need a winding stooge, blast tube, stuffing stick (for loading the motor) [6], winder, a selection of pre-weighed prepared motors, various shims and tabs, a smorgasbord of stuff for running repairs including glue and tape, blue tack for weighting, rubber lube (castor oil, silicone paste, soap, glycerine, etc), optionally, a camera, pole and streamer and something to carry it all in. You will also need a bag of patience, to help you wait for a dry relatively calm day for test flying!


Then out to long grass to test glide with good old trimming-style throws from the shoulder. Some people advocate test gliding without the prop, but with a weight instead to give the same CG. That may be useful to help understand the model’s character, but many others test glide with the prop in freewheel position. After all, that is how it is going to glide in practice. Do not alter CG at this stage, but adjust wing or stab incidence to achieve a good stable glide with no stall or dive. Most models seem to have some decalage, not 0-0.  If you can, then test glide from a few feet up to get more airtime. When the model flies in wide circles to the right, you are ready to move on to the next stage - power. However, at this stage, some flyers are looking for a left glide circle, as noted above.


Start with about 150 turns. Watch how the model behaves under power and adjust thrust line accordingly. Add down thrust for power hangs, right thrust to keep it turning right, noting that right thrust can sometimes cure stalls. If right thrust is getting large, you may employ a fin tab for a bit of right, but use that rudder trim sparingly. Increase the number of turns, until you are happy at high-ish power and then you could try a new motor for a full power test. It’s not a great idea to try a full power flight for the first time during competition - something bad and unexpected may happen! How do you know what is full power? A common way is to test wind a similar motor to destruction and then take some high percentage (e.g.~90%) of that to be a practical “full” power. Another is to use a torque meter. Build experience and know what rubber feels like before it blows!


Try out different motor strand combinations to figure out what works best for your model. Does it perform better with a punchy powerful climb, or does a longer dribble of power yield more flight duration? The advantage of the former (typically a 6 strand motor) is more height, and the latter (usually a 4 strand) is a longer power run and possibly easier trimming as the difference between power and glide phase is less stark.


In essence, trimming is about finding a compromise between all the variables that gives the best results. Sounds simple, but there are quite a few variables:
  • Decalage
  • CG
  • Thrustline
  • Power profile (motor construction and winding)
  • Trim tabs, flying surface warps and stab tilt
There is more than one way to skin a cat, and there are many different approaches to finding this compromise. My suggestion is to be logical, organised and try to change only one variable at a time, otherwise it is easy to get lost.


Enjoy and Look After your P30 Model

If you are intending to compete at meets, then practice in a variety of weather conditions so that you know how best to handle them when the day comes. That said, there is nothing wrong with being a fair weather flyer and competing only in postal events.


Use a DT and set it to a sensible time on every flight, especially on a thermic day. I have watched my P30 fly away with the DT set to 2mins - because that turned out to be way too long for the size of the field I was in! Write your phone number on the model. The most effective DTs are arrangements that detach the wing and have a line from the wingtip to the rear of the fuselage. Pop up stab and pop up wing are common.
A DT is very important, as P30s fly away easily. This is a button timer for my pop up stab DT
Store your model in a protective box. Try to avoid the flying surfaces warping during storage, for example, by banding them to something flat.


P30s are immensely satisfying and fun to build and fly. Now I cannot wait to get out there and fly mine again!



FOOTNOTES

[1] Same as F1G (Coupe d’hiver). The 10g weight includes lubricant.
[2] Measured without rubber motor.
[3] Projected span is the measurement from one tip of the built model wing to the other tip. It is not as measured flat on the plan e.g. adding the lengths of each panel.
[4] BMFA Rules: “Only a commercial plastic propeller with a maximum diameter of 9.5 inches may be used. The hub may be modified to fit the shaft and for freewheeling purposes but not for folding. Plastic may be removed from the surface of one blade for balancing purposes only. The diameter, pitch and blade shape may not be altered.”
[5] Andrew Longhurst, Free Flight Quarterly, Issue 39, April 2011, pp.14-15.
[6] The first three of these can be easily self constructed.



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