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Jul 21, 2013, 01:58 AM
"Hobby Apparatus" Flyer
Mikey C.'s Avatar
Really enjoyed the video! It's obviously too tail-heavy... I'm thinking that the full-scale model had a bunch more weight up front in the form of the engines. Ordinarily, balancing a traditional airframe layout on the spar would be a good way to go, but since this is anything but traditional, I think you're going to have to experiment to find the sweet spot. If it were me, I'd try adding some weight to the front, and also increasing the prop pitch to give more thrust (to compensate for any weight gain). GL!
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Jul 21, 2013, 10:29 AM
TonyS
I also enjoyed the video!! I hope, after the 2-day rule, you find the damage more easily repaired than starting from scratch. (2-day rule = set damaged aircraft aside, out of sight, for at least two days before considering repairability... the worse it looked when set aside, the longer one should take before assessing repairability.)

And I agree with Mikey's comments. In addition, I think you will need down angle on the engine thrust line, in comparison with a line from center trailing edge through center of leading edge curve.

I am definitely no aeronautical expert!! I believe your choice of airfoil shape will probably have the center of lift near the maximum thickness, perhaps a little aft of that. If that is so, then I think your CoG will need to be very near there, or slightly forward. The sweep in the wing may make calculating where the max thickness is. if it were me, I would measure the wing side of the wing-fuselage juncture.

I look forward to your success with this project!

TonyS
Jul 21, 2013, 11:25 AM
So I'M meant to be in control?
Colonel Blink's Avatar
I am also no aerodynamicist, and as advice is as good as what you pay for it and I'm saying this for nothing, you can guess how good this is!!!!

I agree with the comments above re both downthrust and CG. But I'm not sure I agree with moving up on prop pitch yet. If you increase the pitch it's a bit like gearing up a car or motorcycle in that it's ultimate speed will be higher (if it has the power to pull it) but acceleration lower. Sticking with the lower pitch will give better acceleration.

So far the model hasn't actually really started behaving aerodynamically, so it is impossible (for us numpties) to predict it's requirements when at flying speed; but for me, initially I'd err towards wanting the acceleration in order to get to flying speed as quick as possible, rather than have it wallow longer before reaching flying speed, even if it then went on to achieve a higher speed.

Good luck, and keep at it! Repair it with packing tape and cocktail sticks if need be; don't worry about aesthetics, only aerodynamics. Look on this model as the test bed - when you've discovered what it needs, then you can build the pretty one!!! I look forward to the next flight - I love your spirit of experimentation - partly because I have a good chum who would like me to build a Horton Wingless, which has many similarities to your Burnelli. Only it is much less likely to fly......! The reason for my friends interest is that his name is Bill Horton.....but no relation!

Horton Wingless (9 Oct 1952) (10 min 11 sec)
Jul 21, 2013, 11:26 AM
So I'M meant to be in control?
Colonel Blink's Avatar

Stop press!!!!


Actually, searching here on RCG re the Horton Wingless, it looks like the CG is your answer. Read the guy's report of the first flight of his Horton!!!

https://www.rcgroups.com/forums/show...orton+wingless
Jul 22, 2013, 05:03 PM
Registered User
eagle4's Avatar
Thread OP
mikey, yeah, that was my guess, putting the cg on the spar, but i guess i should just keep adding more weight to the front, see how it goes. what would your thoughts be on putting an aerofoil on the horizontal stabiliser? might help keep the tail up.

tony, down angle? how much would you recommend. I'm still very new to building planes so this is very new to me. as for my CG, it is actually at the thickest part of the wing, my wings are built like an Armin wing from experimental airlines, although i modified it so that it was tapered. the thickness of the wing also changes along its length, getting thinnest a the tip.

colonol, once i get more weight up the front, hopefully i'll have more to report on how the plane starts to behave when the aerodynamics actually kick in. Now that really is a unique looking plane. thanks for sharing

Any other tips? I'm completely open to your sugestions
Jul 22, 2013, 06:44 PM
TonyS
All responses are my opinion, aerodynamicists should feel free to correct errors!! But, don't attack my ignorance as being stupidity.

Quote:
Originally Posted by eagle4
what would your thoughts be on putting an aerofoil on the horizontal stabiliser? might help keep the tail up.
Generally, before an airfoiled horizontal stabiliser (stabilizer in American) could become reasonably effective you would have to reach some forward speed, although, you may have enough prop wash to do the job.
Quote:
tony, down angle? how much would you recommend. I'm still very new to building planes so this is very new to me.
First, after a adjusting the CG forward, if you can, and care to take the risk, a simple test to eliminate thrust angle as a possible problem would be to glide test the model; chucking it out over some tall thick grass. If it flys a nice straight, shallow descent line, CG is close. Then do it again with some power, 25-30% perhaps. Any pitch up/down under power would indicate some down/up change in thrust angle might be needed. Since the plane is so obviously tail heavy in the video it is difficult to know if I think it should have 2-3 degrees down angle/down thrust. The idea is to have the motor(s) pulling straight ahead when the wing is angled to hold level flight a "cruise" speed.[/QUOTE]
Quote:
As for my CG, it is actually at the thickest part of the wing, my wings are built like an Armin wing from experimental airlines, although i modified it so that it was tapered. the thickness of the wing also changes along its length, getting thinnest a the tip.
That is where I would have started, IF that put CG at/near 20-25% of AVERAGE wing chord. When I am experimenting with own designs, I choose that percentage for first flight(s), adjusting aft on succeeding flights as I get the other variables sorted out.

Hope this helps somehow.
TonyS
Jul 23, 2013, 12:44 AM
So I'M meant to be in control?
Colonel Blink's Avatar
Lifting tailplanes* (ie with an aerofoil) were used quite extensively in the Vintage era where a lot of my interest lies; models with lifting tailplanes would sometimes use CGs as far back as 60% of chord.

So I would suggest that a lifting tailplane may help, as it moves the CG requirement aft; with some more noseweight moving the actual CG forward you should hopefully get the two close enough to enable control authority (coupled to your lightning fast reactions and supreme piloting skills) to overcome any aerodynamic imbalances.

Normal (and this a/c is nothing like normal!) thrustline is 2 or 3 degrees right thrust and a similar amount down thrust. Just pack the motor mount with some scraps of ply.

Really enjoying the mental processes, and also as ever I love the RCG worldwide teamwork! I don't know your location (you haven't filled it in on your profile), but I would guess somewhere Antipodean??

Keep at it!



*proper English term
Aug 05, 2013, 10:06 PM
Registered User
eagle4's Avatar
Thread OP
after moving everything forwards, as far as i can get them, and adding some extra bracing to the front, i've managed to move the cg about 1cm, and it actually feels good. i have done some brief throw and catches of the plane and it doesnt have any pitching tendancies, ( i tried it before doing my repaires and it definately had a nose up tendancy) I moved the rear servo fowards into the fuselage, and used a wire for the elevator, i fear i may get too much slop, but i am trying to minimise it. I am contemplating extending the elevator to give me a bit more surface area back there incase i need it. not sure when i'll get a chance to get it in the air next, but we'll see.

and yes, i am an Aussie but i am living in Montreal, Canada these days
Aug 06, 2013, 01:49 AM
So I'M meant to be in control?
Colonel Blink's Avatar
Looking forward to the next test flights - with a current model, I am finding that each 'fix' brings it closer but then shows another issue- a bit like after the jet goes over, then you can hear the neighbour's stereo

Still, Google Chrome wasn't built in a day!
Aug 28, 2013, 05:12 PM
pro vjb
the1pope's Avatar
Your UB-14 looked really good. Pretty sure you're right about the battery shifting at launch. The lurch straight up seemed to make that clear. Hope to see it again later.
Oct 13, 2013, 12:25 PM
Registered User
eagle4's Avatar
Thread OP
Well, its been a while, but I finally got a chance to take my Burnelli UB-14 out for a re-maiden. I had the battery right up against the nose, i moved everything as far forward as possible, even removed the elevator servo from the tail and moved it quite forward and just used a long pushrod to access the elevator. I managed to move the cg forward about 1-2cm infront of the spar. so right at the leading edge of the wing tip.

Here is the video for its flight.

Burnelli UB-14 Flight
Oct 13, 2013, 01:00 PM
So I'M meant to be in control?
Colonel Blink's Avatar

Well, there you are.....


....by any measure, IT FLIES!!!!

I offer you my most joyful congratulations, Sir!

It flew, and when you think how it was last time, it flew well. An infinite percentage improvement. Possibly overpowered, if the second flight was anything to go by - it seemed to be happier cruising around on low power. What are your plans for mods for the next flight??

Bloody well done, mate!
Oct 13, 2013, 01:43 PM
Registered User
Congrats.. Burnelli would be proud.
Seems a bit sloww going up and definitely speeds up going down.. Weight issues?
Oct 13, 2013, 04:13 PM
Registered User
eagle4's Avatar
Thread OP
Bare, it was slow climbing, cause i wasnt pushing full throttle, with the motors i have on it, it gave such a thump of power even just going half way, i didnt want to turn it into a rocket ship.

Colonel Blink, thanks mate. was really awesome to see it fly. yeah definately over powered, seeing as i chipped these props, the only ones i have left are 6x3's they are a bit flimsy, so it wont be nearly as punchy as this flight. also after assessing the damage, i realised it was actually in rather good condition. (yay for burnelli's designs) it was about 5 minutes with the hot glue gun and it seems ready to fly again we'll see how i'm feeling tomorrow, might take it down to the park again.
Oct 13, 2013, 05:15 PM
TonyS
Congratulations on the improved flight characteristics, eagle4! Changing the center of gravity appears to have vastly improved the handling. You are well on the way to proving your design.

It now appears to me to need some down thrust angle (2-3 deg.) as the nose is pulled up immediately you advance throttle and drops immediately you reduce throttle. Of course this tendency is exaggerated with the excess power you have available. I might also suggest a little left thrust angle (2 deg, or so) on the left motor and zero to 1 right on the right side motor.

Please continue to persevere. I hope to build your UB-14 during the non-flying winter months.

TonyS


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