Chris's Webbit

[Biggles' Elder Brother - Moderator]

The support for the flap servo is out of line with the flap?

 

BEB

[Chris Bott - Moderator]

Lol no I think I'll put you out of yr misery before everyone points out lots more imperfections!

 

I thought I'd bring it up just in case anyone was using any of my pics as a reference for theirs.

 

The centre section sheeting on the top of the wing should have its grain running spanwise. As I've done it, it sags a little between ribs. So I've added a few supports underneath, between ribs.

[Chris Bott - Moderator]

Not much achieved this morning, I've mislaid some hinge material so the ailerons cant be attached yet. Here's the almost complete wing.

 

What to use the disks for? Dummy engine anyone?

Off to do some household chores and make sure the kids have remembered Mother's Day.

While I'm doing that more thought will go into the fus. I'm struggling with wanting to be able to use either LiPos or A123's. They are such different shapes I can't decide on the battery area.

[Chris Bott - Moderator]

Domestic duties done, (for now ) so on with the Fus.

Decision was made to go with balsa as the ply had twisted out of shape. My fault as I'd left it out in a very hot conservatory for a week.

 

To leave the battery area open I've added some of the ply bits in the wrong places and then made up a couple of formers from 1/8" by 1/2" balsa. I can add more bracing between the sides if it proves necessary.

Firewall is drilled for a motor mount and a hole for wires added.

 

And just to make sure it looks a bit Webbity

 

[Kozmyk]

What is this device clamped to your bench?

 

[Tim Mackey]

Small vice?

[Chris Bott - Moderator]

Yep that's it exactly its a very small vice, mounted on a ball swivel so it can be at any odd angle you like. Small jaws, bot quite a large throat as you can see. No idea where they can be obtained from but it's very useful.

 

Right, a few more pictures. Fus servos are in, as is the motor. Most of the bottom is sheeted too.

 

The 4 cell A123 is feeling far too heavy now, maybe I'll stick to a 3S lipo.

[Chris Bott - Moderator]

The vice by the way looks similar to a combination of these two parts.

 

and

These are on the RS website but are £62 and £45 respectively.

Must look after mine!

 

 

[Chris Bott - Moderator]

One for our resident aerodynamicist now.

 

I've deviated from the plan again a little.

Is there an optimum angle for a V tail? and how much should I have increased areas by?

I tried 45deg each side and it looked far too steep. So I've reduced it now.

I'm assuming I need more of a component in the tailplane direction and less in fin? Hence the lower angles. Does that make any sense at all?

 

 

[martin Rowley 1]

Blimey Chris, you get on at a fair rate of knots. Looks good, as usual with your planes.

Martin

[Biggles' Elder Brother - Moderator]

Re the PM Chris - well I'm very flattered that you think I have such obscure aerodynamics knowledge to hand . I must admit that I had to go to a couple of textbooks to look this one up!

 

Anyway the long and short of it is I found two bits of info that should be useful to you.

 

1. If your going down the V tail route then the way these are designed in pratice would seem to be that the designer first works out a conventional tail arrangement and then modifies it!

 

If the area of vertical stabiliser (fin plus rudder) is Av and the area of the horizontal stabiliser (tailplane plus elevator) is Ah, then your V-tail has to have an area of (Ah+Av). In other words the total area of the V-tail must be same as the total "tail feather" area in the conventional set up. Only this will ensure that the tail will have enough power to stabilise the plane in normal flight. Note the area of the "ruddervators" must also be the same as the total area of the rudder plus elavator. All this means you must make a new tailplane, you can't just take the existing conventional one and tilt it!

 

2. To your actual question about the angle. It appears there are two different solutions to this depending on whether you want to have the same stability as the conventional layout or the same control authority!

 

To have the same control authority the formula for the V-tail dihedral is:

 

Angle=arctan(Av/Ah)

 

To have the same stability the formula is:

 

Angle=arctan((Av/Ah)^2) [meaning you have to square the ratio then take the arctan].

 

Which to use? Conventional wisdom seems to favour the "same control authority" method. But you could work out both and pick an angle between the two for example.

 

Bet you're sorry you started this aren't you!

 

BEB

[Danny Fenton]

Fascinating stuff, I look forward to seeing numbers in that formula Chris

 

I do like your V tail might have to borrow that idea once you have done the sums of course.......

 

Cheers

Danny

[Big Bandit]

Chris,

 

That's inspirational, and I don't know how you build so fast. Brilliant.

 

Chris.

[Mowerman]

Chris

looking through plans of models with V tails they generaly seem to go for 110degrees.

(no science, just follow like sheep )

Lightening holes look good - after all that work you are really stuck with transparant covering rather than hide all the effort. ( see the Femto fun 400 in my album, that was done with a compass cutter and took ages)

[Pat (rActive) Harbord]

my bonanza v35 tail angle is nearer 120º and yes the control surfaces are more like rudders than elevators in size (over 1/3 of total area by the look of it)

[Chris Bott - Moderator]

Blimey, wish I'd switched the PC on sooner this morning!

 

Martin, cheers, hows your's coming along?

 

BEB many thanks (glad I asked now, knew you'd have an answer). I've already made bigger surfaces so will apply the maths and see if they come close.

On the other hand, I wonder how much maths went into the original design? I expect it was more of a "that looks about right".

 

Chris the build speed, all down to Pink Zap. Where I've used aliphatic resin, it's been planned beforehand and left while I get on with other parts.

 

Mower and Ra, thanks for the info, that will help too.

I agree about translucent covering. Except, I have quite a stock of covering, none of which is translucent . So may cover it all up yet.

 

Danny, if yr goung to borrow ideas, then you really will have to prove it can be built in a day like you promised . I'm sure it can, I've done a bit at lunchtime then long evening on Friday. Then didn't start till lunchtime yesterday, and I've been faffing around with extras/changes.

 

Not sure I'll get much done today, family commitments at the in laws etc..

I did weigh it. With all the radio, motor, ESC, and a lipo chucked in, its 1lb 11oz.

At the moment it balances correctly without the LiPo in place. More tail sheeting and covering should require the battery to correct the C/G again.

 

Better go find a dark corner and a calculator I guess..

[Chris Bott - Moderator]

Area sums complete, looks like I still need to add 7025 sq mm to my areas. That equates to an extra 16mm in chord. I'll see if I can glue some triangles on the fronts I think.

 

Now to look at angles.... hey ho!

 

[Chris Bott - Moderator]

Actually the ruddervators need to be 3350 sq mm bigger. It works out to 8mm on the chord of the tailplanes and 8mm on the chord of the ruddervators.

 

Is it worth it??

 

Can probably be done with triangles making the root chord 16mm bigger tapering to nothing at the tip. 

Edited By Chris Bott on 03/04/2011 12:01:33

[Biggles' Elder Brother - Moderator]

Chris,

 

I think the answer to your question is "no, its not worth it" Its not worth the fuss to add 8mm.

 

The problem is you might find the stability isn't quite as good and/or the elevator/rudder authority is slightly impared. But there are things you could do to help this situation.

 

1. Add a little bit more negative incidence to the tailplane - only a degree or two - via a thin shim under the trailing edge of the V-tail. This will help the tail to better counteract the lift moment with a smaller area.

 

2. Simply increase the control throws a bit - that will help with the control authority. You might even experiment with some "opposite expo" on them. By that I mean a little expo (say 10-15%) in the opposite sense than we would usually use it - ie more movement near the centre. That should help - but only add it bit by bit or she might become too twitchy on the pitch.

 

BEB

[Chris Bott - Moderator]

Thanks BEB I'll bear all that in mind. I usually use some expo so I guess for that last part I can juts reduce the expo I normally use.

I'm keeping the model light too. Intuition tells me that this will help with agility/control authority but I have no real reason why I think so. Maybe a bit less inertia?

[Steve W-O]

Posted by Chris Bott on 03/04/2011 11:12:42:

Blimey, wish I'd switched the PC on sooner this morning!

 

Martin, cheers, hows your's coming along?

 

BEB many thanks (glad I asked now, knew you'd have an answer). I've already made bigger surfaces so will apply the maths and see if they come close.

On the other hand, I wonder how much maths went into the original design? I expect it was more of a "that looks about right".

 

Chris the build speed, all down to Pink Zap. Where I've used aliphatic resin, it's been planned beforehand and left while I get on with other parts.

 

Mower and Ra, thanks for the info, that will help too.

I agree about translucent covering. Except, I have quite a stock of covering, none of which is translucent . So may cover it all up yet.

 

Danny, if yr goung to borrow ideas, then you really will have to prove it can be built in a day like you promised . I'm sure it can, I've done a bit at lunchtime then long evening on Friday. Then didn't start till lunchtime yesterday, and I've been faffing around with extras/changes.

 

Not sure I'll get much done today, family commitments at the in laws etc..

I did weigh it. With all the radio, motor, ESC, and a lipo chucked in, its 1lb 11oz.

At the moment it balances correctly without the LiPo in place. More tail sheeting and covering should require the battery to correct the C/G again.

 

Better go find a dark corner and a calculator I guess..

 

 

My weights.....................

 

 

Total weight of the plane without motor, prop, ESC and battery is 30.3oz, and the motor, prop, ESC and battery adds another 13.4oz, total of 43.7oz. Heavier than I had imagined, but still a fairly light wing loading

 

 

What did I cover mine with? Didn't look like lead sheeting!

 

Just gone and checked it again, it has the wings on, motor in and prop on, but no receiver, battery or esc or wheels on, and it is 34oz.

 

Looks like mine is very heavy, or yours very light

[Biggles' Elder Brother - Moderator]

Hi Chris,

 

just tried to reply to your email. I've checked out the calcs and we agree with 2% - not at all bad. I've also added the angle calculation to your spreadsheet.

 

Unfortunately, your email system doesn't seem to like me and it bounced back! Maybe you could send me an alternative address and I could send you the full calculation. In the meantime, and to cut to the chase, the figures for the V-tail dihedral come out as:

 

For the "same stability" model: 12 degrees.

For the "same control authority model: 25 degrees.

 

These seem small to me but there you go! I suppose the 25 degrees isn't that far away from figures cited earlier from other existing examples - it would give an included angle of 130 degrees, reasonably close to the 110 and 120 degrees given above. I will double check the calculations to make sure. If I can get the spreadsheet to you, then you will be able to independently check them as well.

 

As I say most people seem to prefer the "same control authority" model. So you could go entirely with that and make it 25 degrees. Or you could go somewhere between the two. If you do that I would suggest that you bias it somewhat towards the "same control authority figure" - say 20-22 degress.

 

BEB

[Biggles' Elder Brother - Moderator]

OK I've done some more calculations on the webBit. They are too long and complex for here but the bottom line is this:

 

The webBit's tailplane is much larger than it needs to be aerodynamically. How big a tailplane has to be depends on the moment arm length (ie fuselage length) and wings lift moment. Doing this calculation for the WebBit tells us its tail could be 30-40% smaller and it would still be pitch stable!

 

What is happening is that Ah is much larger, compared to Av, than it needs to be. Our method for calculating the V-angle is intended to give us V-tail with the same properties as the convential tail. So we end up with a large V-tail at a shallow angle to recreate the original WebBit's over stability in pitch.

 

What to do? Well first, this means the fact that your V-tail area is a little lower than the threory suggests is absolutely no problem! In fact it might be positively desirable.

 

Secondly, I think we should increase the angle. My reasoning is: if the angle is too shallow, and the area smaller (which it is) we run the risk of reduced relative directional (yaw) stability - the dreaded Dutch roll again! If the webBit is short of anything its rudder area relative to the tailplane area. I'm not saying the rudder is too small - its not - its just small compared to the tail plane. So, having done these additional calculations I'm pretty confident that your existing reduced area V-Tail, but at an increased angle of say 30-35 degrees, would be pretty well spot on! It might even be better than the original because the yaw and pitch stability would be better balanced!

 

BEB

Edited By Biggles' Elder Brother - Moderator on 03/04/2011 17:46:38

[Chris Bott - Moderator]

BEB - fantastic, I owe you a pint or a sticky bun or something at the fly in.

 

That post coming in has meant I can delete a long post that I was writing.

Not sure why emails aren't working. I'm getting yours so you shouldn't be being told I'm not!

 

I will go ahead now with what I have. I was really worried about the 12 to 25deg, precisely because it would give me a small projected fin area.

 

30 to 35 deg it is. Just need to make some sort of jig to lean em on while I sand em to the right angle.

 

Cheers

Chris

[Chris Bott - Moderator]

Right, 30deg has been chosen. For obvious reasons see photo.

 

A bit precarious but it did the trick.