Of course its notvjust power to weight but drag too, and important is wing performance, the amount of lift generated. Can someone talk about thiswing in any detail?

Originally Posted by mustflynow2

Of course its notvjust power to weight but drag too, and important is wing performance, the amount of lift generated. Can someone talk about thiswing in any detail?

I don't know about detail but along with drag comes a lot of lift. It will fly pretty slow with good control. No mater how much power you use you can't cram it through the air fast.

Hi -

What the AXi table is proposing is about 370 watts for a model that is close to 6 lbs in weight. I have flown a trainer on about 400 watts for a little less than 6 lbs and it flew OK as a trainer but it was not really adequate power. The style of flying you would like to do is similar to what a Piper Cub would do; however, aerodynamically, the Piper Cub probably has considerably less drag (narrower fuselage, struts rather than wires). The problem is, that it will take power to move a draggy airplane through the air, so the comparison may not be quite as straight forward as the table suggests. If your flying field is a wide open space without any tall obstructions nearby, I would probably give it and see how it goes. If your field is more limited, the extra power from a 4S set-up would not hurt.

Good luck,

Teo

PD. High lift airfoils may generate more induced drag, but I am not sure that the airfoil on the Bristol is anything special; I think that it is simply flat bottomed (Clark Y would probably be a better choice). Built lightly enough, the plane should be able to fly slowly. In my experience, a blunt leading edge can delay the onset of a stall significantly enough to be noticeable (I used to fly control line stunt and blunt leading edges seemed to corner a little better). I am not sure that much of this applies to the Bristol.

Hello again,

I just ran the numbers that you provided and the 7.5g/W rule would yield about 575 watts for the Citabria Pro I recently finished. The model weighs 9.5 lbs and has a 80" wingspan. The current power system is an AXI 4130/16 turning a 17x12 prop off a 5S pack. With that set-up, I get about 950 watts on the meter. I have flown the model and it is reasonably light for its size (the box said it should weigh between 11 to 12 lbs, I made some changes) and although it will take off in a hurry (about 20-30 ft run) it is not overpowered by any means. It will loop from level flight, but to make a round loop requires full power through the top of the loop; otherwise it just drops off and gives you an egg shaped loop (like the real one). Cut off the power, and it slows down in a hurry thanks to its thick wing, struts, windshield and large cowl. The thing that surprised me a little was the need to keep the throttle at 1/2 to 2/3 power for level flight and even then it is not moving very fast. That is what it takes to overcome the drag. On 575 watts given the 7.5 g/W rule, I would probably have to fly at almost full throttle just to stay airborne. I would say take the rules with a grain of salt.

Teo

Originally Posted by viva_peru

Hi -

What the AXi table is proposing is about 370 watts for a model that is close to 6 lbs in weight. I have flown a trainer on about 400 watts for a little less than 6 lbs and it flew OK as a trainer but it was not really adequate power. The style of flying you would like to do is similar to what a Piper Cub would do; however, aerodynamically, the Piper Cub probably has considerably less drag (narrower fuselage, struts rather than wires). The problem is, that it will take power to move a draggy airplane through the air, so the comparison may not be quite as straight forward as the table suggests. If your flying field is a wide open space without any tall obstructions nearby, I would probably give it and see how it goes. If your field is more limited, the extra power from a 4S set-up would not hurt.

Good luck,

Teo

PD. High lift airfoils may generate more induced drag, but I am not sure that the airfoil on the Bristol is anything special; I think that it is simply flat bottomed (Clark Y would probably be a better choice). Built lightly enough, the plane should be able to fly slowly. In my experience, a blunt leading edge can delay the onset of a stall significantly enough to be noticeable (I used to fly control line stunt and blunt leading edges seemed to corner a little better). I am not sure that much of this applies to the Bristol.

I understand to your arguments. But on the other hand I have Sopwith Tabloid (1,3m wingspan, 2,2 kg) before maiden. And for this model is this setup tested and recommended:
AXI 2826/10
prop. 15x10
battery 2S LiPO !!!

And, the model with this setup is able scale fly with 1/2 throttle and the flight length is 20 min by model description. Because this proppeler and battery are very atypical for this AXI (for 2826/10 are typical setups: prop 11x8 and 4S for trainer to 2600 g or prop 14x7 and 3S for sailplane to 2600 kg) I consulted it directly with model author and he confirmed it. I would not believe it, but it's so.

I get where they are going, for electric motors, current is a measure of torque, with low voltage and high current you can spin a relatively large prop; albiet, slowly. The higher the voltage, the more quickly you get to the current limit and hence as the voltage goes up, the prop diameter goes down. The trick is to have adequate pitch speed and thrust to be able to fly the model.

Good luck with the testing and keep us posted of how it turns out.

Teo

I will inform about test results of course. But actually is a long period of windy (and actually rainy) days and it is not good for any testing.

So I tested Bristol with AXI 2826/12 and 3S. And the same result - I had to fly with full throtthle. After the throtthle reduction, the model began to decline. So I will try 4S and as second option, I borrowed engine with lower RPV - XM4250EA-8 (RPV 510) and 16/6,5 prop.

Can I ask, where you have CG on BUSA Bristol? I met a colleague on the airport and in his opinion the behavior of the model (declining model after the throtthle reduction) may be due to the fact that the center of gravity is too far ahead. He owned the model in the past, and he mentioned that (perhaps) shifted the center of gravity backwards. I have the center of gravity exactly as it is on the plan now.

For the CG, the answer is it depends. If you need to carry up elevator while in flight it could be an indication of a forward CG. The best way to find out is to start shifting the CG rearward and see if you find a point which the model does not drop the nose so abruptly. A forward CG can also become evident during the landing flare where you simply run out of elevator to hold the nose up (say, with 10-15 degrees of elevator deflection). The alignment between the motor , wing and stab could also be off but that would be harder to sort out. By default, a flat bottom airfoil will have some positive incidence if simply laying flat on top the fuselage.

For the lower Kv motor, I would try a more highly pitched prop, 16x6.5 will not be enough. I would try a 16x12 or 16x10 and see how it goes (I have a model with a 16x10 prop on a motor with a KV of 515 and on 5S it pulls about 55 amps). On 4S you will be pulling quite a bit less so I would try the 16x12 first (assuming that they are easy to get).

Good luck,

Teo

Originally Posted by mkope

Can I ask, where you have CG on BUSA Bristol? I met a colleague on the airport and in his opinion the behavior of the model (declining model after the throtthle reduction) may be due to the fact that the center of gravity is too far ahead. He owned the model in the past, and he mentioned that (perhaps) shifted the center of gravity backwards. I have the center of gravity exactly as it is on the plan now.

Howdy fellas, I have recently repowered mine with a zenoah 20, i know heavy up front, and am carrying alot of up trim. Model weight is 3300g dry so not overly heavy, it flys fine apart from running short on elevator as expected. So i believe the plan has the cg at 3 13/16, i make to be 97mm, im ahead of this a little but feel it needs to go back quite a bit, i guess its always going to fly a bit faster than a lighter model but am hoping good balance will help with elevotor efficency esp when landing. Anyone balancing back beyond 3 13/16? I hope to get this model flying really well

This is my 2nd Bristol. With an under camber wing they create a lot of lift. To go up you add power, to descend you lower power. With our overpowered modern models with semi or fully symmetrical wings we blast along and use the elevator for climbing and diving. I balance on the plans location and trim for very slow flight and good glide. If need be I move the balance back a little to maintain elevator control. To control rapid climb under power I use pretty much down thrust on the motor. To land I keep a touch of power and try to fly it a foot off the runway and just let it sink to the ground. 2 wheel landings work best for me.

After next tests with Dualsky motors (mentioned above), I returned to AXI 2826/12 engine, but with different battery - 4S 4000 mAh. I had to make some changes - battery is bigger and it does not fit under motor cowl, so it is in the fuselage now. But finally I'm happy with how the model flies. Just fly to 1/2 throthle and the model holds the horizon. Flight times have also been prolonged.

I too am happy with my flights today, its amazing how much better it landed into a bit of breeze with the cg at 3 13/16ths. I added 70g of lead to the skid tringle under the fuse. I also had a bit of slop in my elevator control snake that i rectified. There is still a slight amount of up trim but not the amount from the previous day. The zenoah has it overpowered for sure but into the wind at low throttle a scale air speed is produced, holding the horizon as you put it mkope, flying hands free that looks very nice. I think when my motor gets a few more tanks through it i will increase the pitch, n run lower rpm, giving a more relaxed feel. With a 12oz tank i am probably able to fly close to 30mins, but 10 mins is plenty for me so maybe an 8oz will suffice.
When i flew mine electric i had my battery right behind the firewall, requiring removal of the wing each flight. Mkope, do you have a hole in the firewall to put your battery into under the cowl? I used to do this on a gee bee to get cg right, battery down the length of the fuse through the firewall right behind the prop under the motor, or do you put it across the fuse in front of the firewall?

I must remove wing for replacement battery too. I replaced steel wires between fuselage and low side of wings by stretch material and it is easy remove wing now. In my Bristol is not possible insert battery under the cowl, because there is dummy rotary engine.