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Maxford 70" F2B Bristol Fighter ARF

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Maxford 70" F2B Bristol Fighter ARF

Old 02-11-2015, 04:23 AM
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Wile E
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Default Maxford 70" F2B Bristol Fighter ARF

I didn't do a full build log, but do have some photos and observations. Here is my completed aircraft:



I'm going to should write the rest up first and then post readable, rather than rambling prose, with illustrations, stay tuned.
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Old 08-22-2017, 11:38 AM
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Crossle32F
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Default Maxford Bristol F2B - wing angle of incidence?

Hello, has anyone had their Maxford Bristol F2B flying yet?

We (my son and I) have tried twice, first attempt a wheel collapsed on the take-off roll (grass field), and it ground looped.

Second attempt (with replacement wheels from Williams Bros.) it got in the air, but is so underpowered could not climb out and turn before going out of bounds of our field, which is ~200 m long. Got it turned and on the ground with minimal damage, but it was all very dodgy.

Motor prop and ESC is the recommended set purchased from Maxford with the ARF. Batteries are as recommended 2x 4S in series . Draws 580 watts at full throttle, held static, and voltage holds at around 30V. All up weight as predicted at ~11 lbs. So it is 50W/lb which is marginal but should be enough to at least climb out faster than it does. The instructions say the motor is oversized since nose weight is needed anyway, so the last thing we would expect is for it to be underpowered.

I suspect the wing angle of incidence may be wrong. How did you set the angle of incidence? It is not a flat bottom aerofoil, and the instructions and assembly video does not show a neutral reference line on the root rib (there is a fleeting glimpse of an electronic level being offered up to the root rib, but you can't see the reference points it is set to). I ended up using the flat bottom edge of the rib cutouts as the aerofoil neutral line. I emailed Maxford USA and got a reply but they did not seem to understand or address the question.

We bought this ARF on one of Maxford's periodic half price sales (on the airframe only), and it seemed at first like a great value as the airframe and covering looked really good; couldn't buy the materials at my LHS for even close to the price. However as we went on we found lots of problems:
Wing position is not properly defined (as mentioned above).
Lots of hardware like clevis ends and pushrods were stripped or to far out of tolerance to be usable, so had to go buy Dubro replacements.
Wheels are inadequate (see above).
2 of the 4 aileron servos were no good.
Lots of the assembly relies on blind strut-in-slot epoxy joints that cannot be adjusted once cured.
Recommended ESC sold with package has no internal BEC - had to buy and add. Could have bought a better ESC locally.
Decal white areas are too transparent, green body colour shows through.
Strut insertion in wings was tight and led to different 'parallelogram' angles between the port & starboard wing pairs and also skew between upper and lower wings in yaw (looking from above, leading edges were not parallel). Had to cut all the struts, assemble, align, and then drill & rivet struts using splice plates.

Anyway, any thoughts on how to get this thing going would be appreciated.

Cheers!
Old 08-24-2017, 05:27 PM
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Wile E
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Originally Posted by Crossle32F View Post
Hello, has anyone had their Maxford Bristol F2B flying yet?

We (my son and I) have tried twice, first attempt a wheel collapsed on the take-off roll (grass field), and it ground looped.

Second attempt (with replacement wheels from Williams Bros.) it got in the air, but is so underpowered could not climb out and turn before going out of bounds of our field, which is ~200 m long. Got it turned and on the ground with minimal damage, but it was all very dodgy.

Motor prop and ESC is the recommended set purchased from Maxford with the ARF. Batteries are as recommended 2x 4S in series . Draws 580 watts at full throttle, held static, and voltage holds at around 30V. All up weight as predicted at ~11 lbs. So it is 50W/lb which is marginal but should be enough to at least climb out faster than it does. The instructions say the motor is oversized since nose weight is needed anyway, so the last thing we would expect is for it to be underpowered.

Cheers!
Hi Crossle32F,

Looks like I forgot to post more info. Those Maxford wheels are just luggage tires with plywood spacers. I replaced them first thing. Good job on keeping the weight down to 11 pounds, mine weighs between 12 and 13 pounds. Power system is a Rimfire 1.20 on a 6s 5000mAH Lipo for 1200 watts. Prop is a 16 x 8 XOAR scimitar style. That all adds up to 100 watts per lb, twice your power rating. At only 20 amps, you definitely need a bigger prop for more power draw.
Old 08-25-2017, 04:32 AM
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Gtarling
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As Wile E intimates, 20 Amps is way too low a current draw. Try fitting a prop that gets the draw up around 50 Amps if the motor will take it. Don't neglect pitch speed - you'll probably need 8" to 10" pitch.
Old 08-31-2017, 09:39 PM
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Crossle32F
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Default Progress with 17 x 10 prop

Thanks for the responses, here is an update:

I ran the numbers in eCalc (access from Castle Creations website). With the Maxford drive kit (16 x 8 prop) and including my battery leads extension it predicts 866 W and warns:
  • "The Pitch Speed should be at least twice as large as the estimated Stall Speed. Increase your propeller pitch."
.... but as mentioned before, I only get 580W in static test.

Recalculating for a 16 x 10 prop eCalc says 1025W with no warnings, and at 17 x 10 it says 1225W again with no warnings. This is more like it and closer to 100W/lb.

I installed a Zinger 17 x 10 wood prop and got 1060W peak in static test. So power draw is still a bit lower than estimated. but a lot closer with the 17x10 prop than with the 16x8.

To allow loading the batteries from the front with the leads pointing aft, so the connections can be made through the gunner hatch, I made a series battery connector, a shunt (for our local field rules), and a battery lead extension. This creates a lot of extra connectors, wire, and solder joints directly in the power circuit, which is immediately suspect. Everything is EC5 connectors and 10 ga. good quality wire and I added a capacitor pack just going into the ESC. I went through and checked for any unusual resistances in the circuit, and found nothing over 0.1 ohms. In static test at full load I found no unusual heat building up in any of the leads or connections. So, all the excess connection grodge does not seem to be the problem but, if doing it over, I would get the ESC up forward as far as possible (we have a bit of nose weight anyway), connect the batteries at the front, and put the shunt up front through the bottom of the cowl to minimize all the lead lengths.

We also went over the airframe and found all the control 'horn' rod fittings had completely loosened off in the very short running and flight time on the aircraft so far (control horns are a threaded rod with a cup washer and nut on either side). So watch out for this, we will set them all back up again and use red loctite or a dab of epoxy on the threads. If I were to do it over again I would chuck out all the Maxford hardware and find better stuff.

Also one more HS-55 aileron servo is gone already, so that is 3/4 failed before we are really in the air. So again, I would advise against buying the Maxford supplied servo kit and finding some better servos on your own instead.

Cheers, will report again on our next attempt to fly

Last edited by Crossle32F; 08-31-2017 at 09:44 PM.
Old 09-03-2017, 11:13 PM
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Crossle32F
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Default Success with prop change

The 17 x 10 prop seems to have done the trick. Flew well today, got about 7 mins flight time (a 3 min. and a 4 min flight) on the 2x 4S 4200 mAh batteries in series.

We ended up replacing all 4 aileron servos (HiTec HS-55's) with Futabas (a pair of S3107 and a pair of S3156). With all 4 ailerons active the roll response was quite touchy but adding a bit of expo and reducing the rate a bit helped.

The batteries are a tight fit and when warm after flight were very difficult to remove. We have another pair of 4S which have puffed a bit and cannot be used. The battery box is not really big enough for two 4S 4200 mAh. I will have to carve a bit more from the side ribs.

Other than that all is well, the approach speed is quite high when landing, not the slow floating approach one might expect from a WW1 biplane, but not a real problem.

I will still rework the battery arrangement to connect from the front to reduce both leads length and to get some of the nose weight out.

The Maxford pilots are too big, and are identical, so don't look quite right.

Cheers, hope this helps anyone building or thinking of buying this aircraft, my suggestion if you must have it would be wait for one of the periodic sales and buy the airframe and the guns only, then be ready to buy better wheels and hardware/fittings and find your own servos, motor/ESC, & pilots along the lines that Wile E has shown.

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