Allure Unloaded

Out of the box the Allure is arguably “the” best flying monoplane currently on the market. It suits all currently available power systems including glow, electric and contra. All three power systems coupled with the Allure finished 1st, 2nd and 3rd at the 2015 US Nationals so choose your weapon!

I have been flying my EP Allure (Brenner V3 contra) successfully for the last six months. The model fly’s great and has definitely improved my F3A flying and consistency overall. The ready to fly weight of this contra Allure is approximately 4,950 grams. That’s a pretty good weight for a full composite airframe and especially given the physical size of the Allure yet it’s comfortably under the class weight limit of 5,050 grams (inclusive of 1% tolerance). The model did start its life a little lighter than 4,950 grams but a few extra bits have been added like a Tech Aero Throttle Tech unit etc.

Whilst the model fly’s very well at 4,950 grams, I wanted to make it lighter in order to see the effect on flight performance. The thinking was that reduced weight would result in better vertical performance and subsequently lower battery usage. Other than that, we F3A pilots are naturally obsessed with weight reduction.

To bring the weight down there are a number of paths that can be taken. Each path has its pros and cons be they cost money, time, or both. Let’s explore…

Some options to reduce weight may be:

  1. Equipment selection – This has already been done to a large extent with servos and ESC heat sink etc but I could replace the Emcotec SPS arming system. That would however leave a big hole in the side of the fuse. Yuck…
  2. Replace the composite wings and stabs with foam cores or built up panels that are covered with Monocote or Ultracote. Yes, this would reduce weight if done right but would take quite some time to do. Not sure I could replicate the factory color scheme and finish either.
  3. Batteries – I’m currently using Thunder Power 25C G8 5000mAh flight packs and an 800mA Dualsky receiver pack. This combination weighs 1,248 grams. Apart from the airframe itself, this item is the next heaviest component which warrants further investigation.
  4. Replace the current Brenner V3 / NEU contra drive with a Ralph Schweizer CRS / Hacker C54 combination. This would save approximately 100 grams. This option is not cheap and would take quite a bit of effort to retrofit to the Allure. Not unfeasible though.

So, as you can see above, there are a number of ways to reduce weight on the Allure. Each option has its pros and cons but each option is totally possible and if all were implemented a significant weight saving could be had. Just imagine if you selected an Allure builder’s kit and implemented all of the above options. A R.T.F. weight sub 4,500 grams would be very possible!

Some background info:

When I first migrated from conventional two bladed EP to contra, I struggled with the throttle management change. It’s a significantly different feel with the contra and it takes some getting used to. Kind of like a tractor in the up lines and ultra strong braking in the down lines. Consistently, battery usage for a P15 flight was up to 3800+mAh. Choosing to use the Thunder Power 5000 G8 flight packs seemed appropriate. This consumption seemed to align well with figures quoted by Brenner for his contra drive unit. Then I read that European pilots are consuming less than 3000mAh and down to as low as 2600mAh for a P15 flight! At first I thought it was rubbish but then others confirmed these figures. One then starts doubt your equipment but all checked out ok there. It quickly became apparent that the problem was me not the equipment. Basically, it gets down to using more throttle than necessary and flying further out than needed with a contra. One needs to learn to be patient in the vertical up lines and one can also take advantage of the slower flight envelope of the contra by flying a little closer in. Once these two things were cottoned onto my consumption figures dropped significantly. I now use on average about 2900mAh for a P15 flight and have used as low as 2500mAh from the flight pack.

Ok, enough background, let’s get back to our options….. Coming from a technical background, my first thought is to prioritise problems by the biggest offender first. Let’s look at some weights:

  1. Allure ARF airframe – 2,240 grams. Very light for a full composite construction.
  2. Flight and Rx packs – 1,248 grams. Light by most standards. Some people are flying with 1,300+ gram packs.
  3. Contra / Motor / Props / soft mount – 815 grams. Not bad considering it uses two big props.

These are the three heaviest items in order of weight. A Pareto analysis! Google it…

Let’s look at the top three weight contributors in more detail to see what can be done to reduce weight.

  1. Airframe – At 2,240 grams, this is the heaviest item. There’s little we can do about this weight as it’s out of our control. As suggested previously, one could construct foam/balsa wings to reduce the weight. This could potentially take 150-250 grams off the model. A lot of work though.
  2. Flight and Rx packs – The current TP 5000mAh packs and 800mAh pack weigh in at 1,248 grams. The second heaviest component in this package.  thunder-power-5000-packs (2) After a P15 flight I’m using between 2500 and 3200mAh from the pack. This leaves a lot of headroom with 5000mAh on tap and 80% would be a nice 4000mAh. Dualsky makes a flight pack that is marketed as their F3A pack. The capacity is 4600mAh and more importantly the weight is 491 grams. If we use the same 80% rule 3680mAh would be a safe discharge level, there’s still plenty of headroom. The Rx pack consumption is approximately 40mA per flight. Again, the pack is way oversized.  Dualsky also makes a 400mAh 2S Rx pack which is specified as weighing 22 grams. By switching to the Dualsky packs, a flight pack and Rx pack weight of 1018 grams could be achieved. That would be a 230-gram savings! Cost to trial this option; USD$230.00.
  3. Contra / motor / props / soft mount – The current Brenner V3 system with a NEU sans gearbox weighs in at 815 grams including both props and the soft mount. By switching to the Ralph Schweizer CRS with the new Hacker C54 motor a saving of approximately 100 grams could be realised. A fair bit of work would be required to make this option happen. The cost would also be approximately USD$1,800.00. Another option would be to switch to the Hacker C54 on the Brenner CRS. This would require a new soft mount plate and rear support. This would pull 30 grams out of the Brenner / NEU combination. The added advantage would be the cool running of the new Hacker C54 motor. The NEU does get quite hot on the warmer days. This change would cost approximately USD$600.00.

Well, you don’t have to be Albert Einstein to figure out which option was going to give the most bang for your buck and is almost plug and play; Number two of course.

dualsky-4600-and-rx-packs (2)The Dualsky packs were ordered and weighed upon receipt. It was found that the flight packs were better than specified at 486 grams each (without connectors) but the Rx pack was 8 grams heavier than spec at 30 grams. Overall, the total weight was 1018 grams which is a small reduction on the specified weights but more importantly, 230 grams less than the TP packs.

The Dualsky packs are approximately the same length and height as the Thunder Power 5000 G8 packs but are approximately 10mm thinner. This necessitated the packs be laid on their sides in the model due to my retention method. new-packs-fitted-closer (2)

So if you do the math this one change has reduced the RTF weight to 4,720 grams or by 4.5%. The wing loading has reduced from 21.9 oz/sq ft to 20.9 oz/sq ft which is, again, a 4.5% improvement. The cost to achieve this result is approximately $1 per gram. Sounds like a great deal.

The day of testing was Saturday the 24th of October 2015. The weather was fine at approximately 30 degrees centigrade. Wind was from the north (blowing out) and gusting up to 25-30kph and quite turbulent. The flying site was Camden Valley NSW, Australia. It has plenty of turbulence generators like hills close to the field and a drop off under the flight line, some trees and club buildings etc. I should mention the gearing in my Brenner CRS is 9.89:1 (tallest available) and props being used are 22” x 20” front and rear. To have a good reference prior to fitting the lighter packs, the first two flights were flown using the heavier TP packs. This gave me a good feel for the conditions and a reference point to compare against. The Allure handles the wind very well but in the conditions on the test day even the Allure was getting bounced around a bit. The conditions just weren’t nice at all but flying in such conditions often provides the best trimming. There’s nothing quite like turbulent conditions for showing up trimming issues in your model. Now, logic (and myth) says that a heavier model should be better in the windy conditions because it has better penetration. How wrong that assumption is! I’ll explain shortly so read on.

new-dualsky-packs (2)Ok, it was time to try the lighter Dualsky packs…. The packs were fitted in the same position on the battery tray as the TP packs but laid flat on the battery tray. This would have the effect of moving the CG back a little. I fully expected the need to adjust the elevator trim (down trim) after this change. Surprisingly, for straight and level flight, no elevator trim adjustment was required. Strange, but true. Of course, this needs to be rechecked in calm conditions. What was immediately apparent was the amazing improvement to the models stability in the turbulent conditions. The model was rock solid and locked in on rails! The difference in the model was simply stunning! The improvement was so good that I would recommend people do whatever they can to get their Allure down in the 4,700 gram weight range right from the get go. It is achievable with careful component selection. How much energy was taken out of the flight pack? Well, the telemetry told me 2900mAh out of the pack and the maximum current was measured was 72A. Upon checking the flight pack with a Hyperion battery checker, there was a healthy 26% left in the pack after a full P15 schedule and a couple of trim passes early in the flight. No problems there…

To back up my testing, I planned to fly another two flights on the TP packs and then go back onto the Dualsky’s. Unfortunately, after the first flight on the TP packs an axle was sheared on landing the Allure. They are the original aluminium axles and have approx 300 flights on them so not too bad. This ended the day prematurely but overall I was super, super pleased with the effect of the weight reduction.

To conclude, I can categorically say that lighter is better and lighter fly’s better in windy conditions. There is no doubt about it whatsoever. Don’t believe me? Try it for yourself and you will be as amazed as I was. One would think there is a limit to how light one can go before it becomes a detriment. What that limit is remains unknown to me.

I can’t wait to do more testing with these Dualsky packs. Just absolutely awesome!

Jason Arnold.

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