DaveL322
Posts: 502
Score: 125
Joined: 7/9/2004
Last Login: 5/21/2013
From: Medford, NJ, USA
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I'm jumping in late.......took a week off after the NATs, and then spent a week cleaning the shop and lining up experiments and testing for next year....influenced no doubt by the 74K unknowns, which is even more of an influence in strong winds.
I'm not smart enough to cut/paste multiple quotes to respond to......but.....couple points I would add -
I've flown identical planes side by side on the same day multiple times with the only difference being single prop vs Brenners Contra system. On a radar gun, the top speeds were equal, and when flown through the pattern, the flight times were always within 5 seconds, with the greater influence on time being the pilot, not the powerplant. Without a doubt, 100%, spiral airflow, torque, PFactor, and gyroscopics are real (just like the science says) and ARE AFFECTING our pattern models. I am not suggesting our pattern models can not be designed, trimmed, setup, etc, to greatly minimize the effects, BUT, the effects are real, and do cause trim problems if they are not addressed (and they can not truly be cancelled out), and they are all but eliminated with the Contra system. I say all but eliminated as the relative RPM / torque of each prop on the Contra is not always balanced in transient conditions. Another point on this that I will add.....maneuvers that require rolling left and right, or snapping left and right, or require both positive / negative corners are often among the most challenging....outside of the challenges to the airframes, ergonomics and habits are also factors. The ergonomic factor is simply that our hands are not symmetrical when pushing left aileron vs pulling left aileron (for the Mode 2 flyers), and the habitual factor is that most pilots simply have a preferred direction for rolling, snapping, or a preferred "sight picture" (ie, always show the canopy first when rolling). The Contra very effectively eliminates assymetries in the plane from spiral airflow, torque, etc....but it does not eliminate ergonomic assymetries and pilot preferences. The science behind all of these ideas is easy enough to research, and in my opinion, the only aspect for debate is to what degree it affects our discipline.
If an airplane is trimmed to fly straight and level, and exhibits positive stability (in both pitch and yaw), are not the sum of the vector forces such that the net value is "1G" of lift? 1G being the amount of lift needed to overcome gravity. If the geometric configuration (and airspeed) of the plane is not changed, that 1G lift exists no matter what the orientation of the plane is......and that means barring no other changes, the plane will move to the canopy in uplines, downlines, and KE flight. The airplane does not know what attitude or orientation it is flying with, and has zero capacity to turn off the lift being produced. It is very clear to me that minimizing incidences (downthrust, positive in the wing, negative in the stab, etc) and reducing wingloading are effective at minimizing the pull to the canopy caused by 1G......but minimizing incidences and wingloading do not eliminate the pull to the canopy. That said, I have (and have flown other) planes that exhibit zero movement to the canopy in uplines and KE. The zero movement is not the product of the wings being unloaded, or not producing lift in a given attitude, but the product of changes or additions to the vector forces such that the net value of the 1G lift is altered and the new net value is 0G (zero G). In the vertical upline, the change that is introduced is added power......which IF downthrust is part of the setup will impart movement to the belly....and this also assumes constant airspeed as a change in airspeed would alter the lift from the wing as well. In KE flight, the change is the addition of beta (angle of attack in yaw), and now the fuselage is flying with some amount of slip in yaw (which is not present in level flight, uplines, or downlines). I will suggest that a properly designed fuselage in addition to producing lift in yaw also produces a slight amount of lift (-1G, negative 1 G) to the belly resulting in a net 0G trim condition when in KE. It is very easy to alter KE pitching behavior by changing the fuselage shape, or directing airflow on the fuselage......no changes to CG or incidences are required. I have flown planes that went straight down....and they did not have positive stability in pitch.....miserable to fly for pattern. With a tiny bit of down elevator mixed to low throttle, straight downlines are very possible, predictable, and consistent, and this is a product of changing the vector forces such that 0G is present (reducing the AOA of the wing to zero). My last comment on this is that the effects of 1G on a very well trimmed and setup plane can be small enough such that they are not noticeable in anything other than calm air, even with zero mixes. My Contra Bravo exhibits ZERO need for mixing (even on a 74K unknown), save a small amount of down elevator at low throttle.
Motor vs engine discussion - the science is 100% on the side of motors having more torque. And that FACT alone doesn't mean all pilots will prefer electric motors as again, ergonomics and preferences come into play - ask 10 pilots to put the throttle stick at 25, 50, and 75% (without looking at the TX), and you are likely to get 10 different answers. And the throttle curve / torque / power feel is different with different prop diameters, prop airfoils, motor design, exhaust tuning, cam timing, gearing, KV, etc. The very nature of electric motors is that they actually govern RPM to an extent.....the motor "wants" to run at a given RPM which is dictated by KV * voltage. If the actual RPM is less, the motor pulls more amps to get to that RPM, and if the RPM is more, the amps drop. This means the electric will add power whenever airspeed drops (going to a vertical climb, or adding drag with rudder or aileron inputs), and the electric will reduce power whenever airspeed increases (going into a dive). This is very different than glow which drops RPM some amount when entering a climb, and noticeably increases when going into a dive. FWIW, it is easy enough to do the math to figure out the average current draw of an electric flight - I typically average about 32 amps (~ 90 amps peak) with my geared Neu setup (single prop or Contra) for a mix of P/F13 in variable wind conditions and an average flight time of about 5:45 enter box to exit box and average discharge of 3,600 mah. This is flying at sea level - even at 1,000', the mah used drops by 5% or more (flight times stay the same). Very long slow flights in calm air can be as low as 25 amps average and very big fast flights in high winds can be as high as 40 amps average.
Motor vs engine discussion - the science is 100% on the side of the engines having the greater amount of energy available (since fuel capacity is essentially unlimited). The greatest stress caused by the 74K unknowns (especially in windy conditions) is on the pilots, then the airframes, then the powerplants, and most certainly the electrics feel the stress more than the slimers (tic). And in the case of the US NATs, electric still came out on top, and in my opinion, will continue to do so for all the positives offered by electrics. Do I anticipate some changes in electrics? Absolutely...and I also anticipate changes in airframe designs (electric or glow) for the 74K unknowns, and to better exploit the advantages a Contra system offers. Aside from the aerodynamic demands from the increased amount of KE flight and snaps, the 74K unknowns also put a premium on having a wider speed envelope available - and my personal opinion is that this ultimately favors the electric simply because a good ESC can provide more (electronic) braking than the mechanical braking action of a glow engine.
And perhaps most importantly - anothe Princess Bride quote -
Vizzini: ......INCONCEIVABLE.
Inigo Montoya: You keep using that word. I do not think it means what you think it means.
Regards,
_____________________________
Dave Lockhart
Team Horizon/JR/Spektrum/ThunderPower Castle Creations F3A Unlimited NeuMotors Central Hobbies Tech Aero
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RE: F3A Results - SPOILER - 
