altbob
New member
Here's my best guess, based on a conservative AUW of 380g (hoping to get that number down a bit, but I think I am hitting the end of that particular path...)
Using http://www.ef-uk.net/data/wcl.htm -
Chitty Chitty Bang Bang!
- Thread starter altbob
- Start date
Here's my best guess, based on a conservative AUW of 380g (hoping to get that number down a bit, but I think I am hitting the end of that particular path...)
Using http://www.ef-uk.net/data/wcl.htm -
ClearSkiesCalmWinds
Active member
Keep in mind you can't have 233 sq in and 233 dm2 in both entries. If it's 233dm2 it's roughly 36 sq in.
rcspaceflight
creator of virtual planes
6.5 sounds a bit heavy for how slow you probably want it. But that is a very flyable wing cube loading.
altbob
New member
Yeah, that's what I was thinking. If I can strip some weight off when I build the next one I will. This one is just a prototype. I know for sure on the final model I will be using a carbon fiber rod instead of a fiberglass arrowshaft to hold the airframe straight and add strength. That will strip a few grams for sure. And there's still about 15g of Elmer's foamboard paper that could be stripped off if it proves unnecessary - right now I'm leaving it on for strength (and the red paper stripes on the wings, just because they look cool!). I will also be a lot more careful with the hot glue for the final model - I probably went a little overboard making this one strong. Not much more I can remove (and with covering added, much of what I'd remove will be added back anyway). I could stretch the wingspan another inch or so in Sketchup if I really had to, but it's already stretched beyond "scale" and I don't want to distort it any more than I absolutely have to.
I wonder what the weight savings would be using 3mm depron over foam board with the paper removed? I suspect they are pretty close in weight and the extra cost of the depron wouldn't be worth it.
I wonder what the weight savings would be using 3mm depron over foam board with the paper removed? I suspect they are pretty close in weight and the extra cost of the depron wouldn't be worth it.
Foam Addict
Squirrel member
Hey Bob, as promised!
altbob
New member
Thank you so much Foam Addict! I note that you are testing a 7x3.5 prop - I was going to use the 7x3.8 because it's well regarded for slow flying planes. Not much of a difference there, so probably not worth the trouble to recalculate.
Bigger concern is that I need 100g +/- in the nose to balance the GC and you have this motor/power setup shown with a 51g Kokam. Is there any problem using a bigger battery? I would think not (and could get a longer flight time as a result).
Also, you show a 25A ESC. HobbyKing says you can run this on a 10A ESC. I would use a Plush 12A, which I already have. Any problem with that?
Thanks again!
Bigger concern is that I need 100g +/- in the nose to balance the GC and you have this motor/power setup shown with a 51g Kokam. Is there any problem using a bigger battery? I would think not (and could get a longer flight time as a result).
Also, you show a 25A ESC. HobbyKing says you can run this on a 10A ESC. I would use a Plush 12A, which I already have. Any problem with that?
Thanks again!
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Foam Addict
Squirrel member
You can use what ever battery you would like up to about a 1300 3s.
I will warn you that the 7x3.8 will destroy the motor on a 1700ish kv blue wonder or similar 24g motor on 3s and might not have enough power on 2s.
I will warn you that the 7x3.8 will destroy the motor on a 1700ish kv blue wonder or similar 24g motor on 3s and might not have enough power on 2s.
altbob
New member
Now you're making me nervous
I suppose I can try out a 6" prop - might be a good compromise, but I can't find a good 6" slow fly (preferably light carbon) prop at HobbyKing. I shall have to try a wider search. If you know of a good place to buy props, please pass it along (unless you think this isn't the way to go).
I suppose I can try out a 6" prop - might be a good compromise, but I can't find a good 6" slow fly (preferably light carbon) prop at HobbyKing. I shall have to try a wider search. If you know of a good place to buy props, please pass it along (unless you think this isn't the way to go).
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altbob
New member
Just a quick update:
It took a while but I finally found the time to get Chitty all wired up with motor, servos and receiver and ready to fly. I added a carbon spar system to the rear end to hold the motor in place (see photo) and keep the control surfaces level, although I could have done a better job with this (and will in my next build). I tried flying the model this morning and crashed it repeatedly until it couldn't be fixed anymore. The first flights seemed nose heavy, so I moved the CG back a bit. The crash damage made it hard to tell, but I think the later flights were then tail heavy. And the AUW was a bit too much for the motor too, on 2 cells. I think a smaller/lighter battery can solve most of that (and I can probably build the next one lighter, with less hot glue and more foam tac, and a few other changes). So it's back to the drawing board and the workbench!
Changes I will work on:
1) Increase the wingspan. I might be able to squeeze another inch of wingspan without ruining the look. I don't want it to end up looking like a plane. I want it to look like a flying car...and that's the problem. Planes have been pretty much figured out and all share certain basic design similarities for a good reason. I'm bending the rules about as far as I can with this one. Hopefully there is a compromise I can find...
2) Add ailerons As feared, it has a tendency to be unstable in roll even with all that dihedral I gave it. This was not the case with my test glides up to 15 feet - the model righted itself naturally. I suspect it didn't with the motor running because of motor torque. Without ailerons I don't think I can correct for this. Until I get it moving it will have no rudder authority. I might also add some veritcal height to it by including a cutout of the crew (aka the Potts family. I know some people wanted to see this anyway (although in the movie Chitty can fly by herself).
3) Overall more stiffness to the back of the fuselage The torque of the motor tended to bend the fuse, even with the CF spars that I added - I can work on this by adding another longeron. At least this will be easy to solve
4) Lighten it somehow / move the CG back I knew going in that my next build could shed a few grams. More significantly by moving the CG back I also don't need nearly as heavy a battery, which helps keep the AUW down, and therefore I can hopefully avoid a bigger motor. The current CG location worked well on the 7.5" test glider, but that doesn't necessarily mean it's the best CG location. The wing is very unconventional so unfortunately there's a fair bit of guesswork involved (as the math on finding CG on a wing this shape is sadly well beyond my ability).
5) Verify whether the down thrust on the motor is helping or hurting I'm going to make a motor mount that can be easily adjusted with a screw to I can run some tests. I thought 1 or 2 degrees of downthrust would be a good idea to push the nose up, but now I'm not sure. A better solution may be to increase the span of the horizontal stabilizer by an inch or two, which shouldn't change the appearance very much.
I have a coupon for 50% off foam board...that's going to come in handy...
It took a while but I finally found the time to get Chitty all wired up with motor, servos and receiver and ready to fly. I added a carbon spar system to the rear end to hold the motor in place (see photo) and keep the control surfaces level, although I could have done a better job with this (and will in my next build). I tried flying the model this morning and crashed it repeatedly until it couldn't be fixed anymore. The first flights seemed nose heavy, so I moved the CG back a bit. The crash damage made it hard to tell, but I think the later flights were then tail heavy. And the AUW was a bit too much for the motor too, on 2 cells. I think a smaller/lighter battery can solve most of that (and I can probably build the next one lighter, with less hot glue and more foam tac, and a few other changes). So it's back to the drawing board and the workbench!
Changes I will work on:
1) Increase the wingspan. I might be able to squeeze another inch of wingspan without ruining the look. I don't want it to end up looking like a plane. I want it to look like a flying car...and that's the problem. Planes have been pretty much figured out and all share certain basic design similarities for a good reason. I'm bending the rules about as far as I can with this one. Hopefully there is a compromise I can find...
2) Add ailerons As feared, it has a tendency to be unstable in roll even with all that dihedral I gave it. This was not the case with my test glides up to 15 feet - the model righted itself naturally. I suspect it didn't with the motor running because of motor torque. Without ailerons I don't think I can correct for this. Until I get it moving it will have no rudder authority. I might also add some veritcal height to it by including a cutout of the crew (aka the Potts family. I know some people wanted to see this anyway (although in the movie Chitty can fly by herself).
3) Overall more stiffness to the back of the fuselage The torque of the motor tended to bend the fuse, even with the CF spars that I added - I can work on this by adding another longeron. At least this will be easy to solve
4) Lighten it somehow / move the CG back I knew going in that my next build could shed a few grams. More significantly by moving the CG back I also don't need nearly as heavy a battery, which helps keep the AUW down, and therefore I can hopefully avoid a bigger motor. The current CG location worked well on the 7.5" test glider, but that doesn't necessarily mean it's the best CG location. The wing is very unconventional so unfortunately there's a fair bit of guesswork involved (as the math on finding CG on a wing this shape is sadly well beyond my ability).
5) Verify whether the down thrust on the motor is helping or hurting I'm going to make a motor mount that can be easily adjusted with a screw to I can run some tests. I thought 1 or 2 degrees of downthrust would be a good idea to push the nose up, but now I'm not sure. A better solution may be to increase the span of the horizontal stabilizer by an inch or two, which shouldn't change the appearance very much.
I have a coupon for 50% off foam board...that's going to come in handy...
Attachments
rcspaceflight
creator of virtual planes
I think you're going to have to move the motor from the back to the front. Maybe not to the nose, but at least make it a prop and slot with the motor more in the middle. It will solve your CG issue without adding any weight.
I may be wrong, but I think trackers fly better. Maybe this analog doesn't apply, but I think of it as a train. It doesn't really matter if the engine is pushing the cars or pulling them because the track keeps them straight, and yet it's better to pull those cars, not push them. If that makes any sense and my logic may be flawed.
I may be wrong, but I think trackers fly better. Maybe this analog doesn't apply, but I think of it as a train. It doesn't really matter if the engine is pushing the cars or pulling them because the track keeps them straight, and yet it's better to pull those cars, not push them. If that makes any sense and my logic may be flawed.
altbob
New member
I'd like to poll you guys on what you think looks best. Below are two photos of the new Chitty prototype - one with the regular sized rear horizontal stab and the other with an enlarged version. Having moved the elevator to the front canard, I'm not sure that there is much need for a rear horizontal stab, so this is probably just a question of which looks better. Do I need to go with the bigger one top match the bigger wings? The bigger one does weigh more, which is a disadvantage.
Please vote, thanks!
Please vote, thanks!
Attachments
I actually think the bigger one looks better. It will also help with stability (I believe the canard will create some pitch instability).
Regarding the motor angle, for it to be pitch neutral it needs to point towards the COG. You need to find out where the COG is both on the length axis, and the height axis, and point the motor towards that point. You might want to point it a little above, since those wings probably will give more lift with higher speed, and that will prevent the plane pitching up when speeding up.
Regarding the motor angle, for it to be pitch neutral it needs to point towards the COG. You need to find out where the COG is both on the length axis, and the height axis, and point the motor towards that point. You might want to point it a little above, since those wings probably will give more lift with higher speed, and that will prevent the plane pitching up when speeding up.
altbob
New member
altbob
New member
Here's a video of where I am with the build, what I think is working and what might be some problems I still hope to solve:
https://www.youtube.com/watch?v=j34aS-kxyS8
https://www.youtube.com/watch?v=j34aS-kxyS8
altbob
New member
I'm downloading the poor quality video that I have today and I'll see what I can compile and upload to YouTube (stay tuned). Meanwhile I have one photo to show (note crumpled foam board front canard, which made an excellent crash bumper!).
The vectored thrust is very effective so no need for a rudder. There really wasn't enough room indoors for me to confidently bank and yank once airborne. The arena dome I flight-tested in was 3 basketball courts laid out side to side, so about 150 feet of effective flightline. You don't want to be turning on the sidelines and hitting walls, so this didn't leave me much time to experiment with turns. By the time she is off the ground, she's going real fast and the wall is only about 30 or 40 feet away. It's entirely possible that I am just being a wimp, but I'd really like to take her through a few patterns outside to know the flight envelope...
AUW was 443 grams, with 28 grams of lead in the nose to get the CG right.
Motor is a Turnigy 2730 Brusless 1500kv, with 7x3.8 prop (max recommended is 7x3.5), 12A ESC and 3S battery. Presumably this will max out at 410g thrust (I think I got a touch more with the 0738 prop), but my thrust ratio was 1:1 at best. I need to shed some weight. And add some more lift?
The front canard can be reshaped into an airfoil from a flat piece of foam board, and perhaps made 10% bigger. I can then hide my lead weights way up front and not only make them invisible, but also, perhaps, use fewer of them. Certainly use fewer if I swap out the indoor training wheels for more robust gear and wheels that I can use on grass.
I can also swap out the paper covering for tissue, and swap out the monokote on the tail for a lighter covering material.
My rough estimate of the wing area is 360 sq. in., although actual "usable" camber is probably less than that, given the shape of these wings. Glide slope ratio is probably about 3/2. So gotta keep the motor running to fly... And because of the vectored pusher prop, I have to remember to keep the motor running to have rudder control!
The vectored thrust is very effective so no need for a rudder. There really wasn't enough room indoors for me to confidently bank and yank once airborne. The arena dome I flight-tested in was 3 basketball courts laid out side to side, so about 150 feet of effective flightline. You don't want to be turning on the sidelines and hitting walls, so this didn't leave me much time to experiment with turns. By the time she is off the ground, she's going real fast and the wall is only about 30 or 40 feet away. It's entirely possible that I am just being a wimp, but I'd really like to take her through a few patterns outside to know the flight envelope...
AUW was 443 grams, with 28 grams of lead in the nose to get the CG right.
Motor is a Turnigy 2730 Brusless 1500kv, with 7x3.8 prop (max recommended is 7x3.5), 12A ESC and 3S battery. Presumably this will max out at 410g thrust (I think I got a touch more with the 0738 prop), but my thrust ratio was 1:1 at best. I need to shed some weight. And add some more lift?
The front canard can be reshaped into an airfoil from a flat piece of foam board, and perhaps made 10% bigger. I can then hide my lead weights way up front and not only make them invisible, but also, perhaps, use fewer of them. Certainly use fewer if I swap out the indoor training wheels for more robust gear and wheels that I can use on grass.
I can also swap out the paper covering for tissue, and swap out the monokote on the tail for a lighter covering material.
My rough estimate of the wing area is 360 sq. in., although actual "usable" camber is probably less than that, given the shape of these wings. Glide slope ratio is probably about 3/2. So gotta keep the motor running to fly... And because of the vectored pusher prop, I have to remember to keep the motor running to have rudder control!
Attachments
SnowRocker88
Amateur pilot and builder
Can't wait to see the video of her in the air!
altbob
New member
Thanks for your patience - it was busy at work and I only just had the time to edit this video in a way that I think makes it useful and mildly entertaining
Keep in mind that this was never intended to be a flight test, but just a ROG test. I allowed myself to be egged on a bit too much... The problem, in a nut shell, is that once the bird flew 75 feet or so and got airborne I saw the wall and didn't believe I could turn it fast enough, so cut throttle and tried to land almost immediately every time I got off the ground.
One thing that I notice in the video is how it seems to porpoise as it's getting up to speed to take off. I think this means that I need more downthrust angle on the pusher, correct? Or is my wing incidence off? Or is this a sign that the CG is wrong?
Right now I am thinking about raising the canard above the main wing a bit more, based on some reading I've been doing. MAYBE also adding some positive incidence to the canard as well (as my helpful "friend" kept repeating while I was trying to concentrate on controlling the plane...do you folks think he was right?).
Currently in the process of lightening it a bit by stripping off some paper and changing from paper covering to tissue, so I'll be busy for a while before I try flying it again. In the meantime, anyone have any other comments or thoughts? I'd love to read them! Thanks!!
Here's the link:
Keep in mind that this was never intended to be a flight test, but just a ROG test. I allowed myself to be egged on a bit too much... The problem, in a nut shell, is that once the bird flew 75 feet or so and got airborne I saw the wall and didn't believe I could turn it fast enough, so cut throttle and tried to land almost immediately every time I got off the ground.
One thing that I notice in the video is how it seems to porpoise as it's getting up to speed to take off. I think this means that I need more downthrust angle on the pusher, correct? Or is my wing incidence off? Or is this a sign that the CG is wrong?
Right now I am thinking about raising the canard above the main wing a bit more, based on some reading I've been doing. MAYBE also adding some positive incidence to the canard as well (as my helpful "friend" kept repeating while I was trying to concentrate on controlling the plane...do you folks think he was right?).
Currently in the process of lightening it a bit by stripping off some paper and changing from paper covering to tissue, so I'll be busy for a while before I try flying it again. In the meantime, anyone have any other comments or thoughts? I'd love to read them! Thanks!!
Here's the link:
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Foam Addict
Squirrel member
Very nice! She'll fly! I do agree, some upward incidence would help, but you might need an excessive amount on the canard, maybe a little bit on each surface might be better.
