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Old Jul 11, 2012, 02:33 PM
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Build Log
2.4m UAV build log for AP, GIS, Photogrammetry

Hi forum, thought i would take the plunge and post my build on here, I am in the early stages of planning/building my twin boom electric uav aircraft. It will be controlled by the DIYDRONES Ardupilot Mega 2 board. I initially had my build log over on DIYD, but thought here would be a more appropriate place to get feedback on the build etc. So far I have sourced a number of materials, done some equations, drawn a lot of plans - to get to this point...

Here are the important bits of information...

Ideally i would like the plane to be able to fly the following speeds:-

12 meters per second (26.8 mph) minimum - for no wind days for good photo runs
27 meters per second (60.39mph) max for photo runs in the hills where the wind can be quite strong ~40 miles per hour sometimes. (giving around 20mph ground speed into strong wind)
Anything in this range would be good, so ideally 19.5 meters per second (43.6 mph).

Reynolds Number:
399934.08 = Chord 30cm (0.98ft) x 19.5 meters per second (43.6 mph) x 9360
RE: Chord:- The 30cm Chord & wingspan 2400mm gives an aspect ratio of 1:8. I did some basic research, and it seemed that any aircraft with an aspect ratio in the range of 1:6 - 1:10 was okay for this type of plane, so i went in the middle. Would you advise I increase it to 1:10, giving a chord of 24cm?

Wing Loading:
The wing area is 684000mm sq (7.36 sq ft)
Total Weight is going to be around 4.5kg (158.7 oz)
Wing Loading = 20.88oz per sq ft
Calculation - 158.7oz (total AUW) / 7.36sq ft (Total wing area)

From the above numbers, I guess i'd like to know how much speed i need to maintain level flight, and how much power i need to maintain level flight. The motor I have for this build is going to change. Initially i was going to use the Ultrafly F/18/10 1000KV, but i would like to calculate what i need from the motor, ie how much thrust is needed in order to buy a motor suitable for the plane.

I have the following information below..
  • a 3 view showing the dimensions
  • Photos of materials aquired so far
  • A table of weights

I have the following questions:
1. What motor should i get to power the thing, I have noticed other aircraft in this size range use props in the 14 x 7 / 14 x 10 range. But not sure how to select it
2. Is the Carbon strip top and bottom of 1mm x 4mm going to be strong enough to keep the wings for snapping?
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Old Jul 16, 2012, 05:19 AM
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Should i move this thread?

Is this the best place for airframe design?
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Old Jul 16, 2012, 01:10 PM
B for Bruce
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The 'Wack, BC, Canada
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I found this thread from a link to it in the Modeling Science forum.

IMO 1x4mm carbon is not big enough for something of this size. In particular you will also want a web between the top and bottom caps of something that is more stiff than simple foam. I'd suggest that you want a stepped set of spar caps with vertical grain balsa in between as the webbing. I'd look at 1x10mm caps for full span top and bottom. And the same 1x10mm strip as "doublers" glued to the full span caps for a half span out to each side of center.

It's hyper important to provide a good distortion free web to join the top and bottom caps as well. This requires something far more crush resistant than the foam core. You'll want to use something like 3/8 balsa with the grain VERTICAL as the spacer webbing. This means that the foam cores will need to be split for a front and rear half on either side of the spar assembly.

Everyone seems to want to use thin carbon tube booms but for you. I see you're showing aluminium tube booms. But even carbon and aluminium is flexible. And the drag from the high mounted tail can produce enough flex in the booms this long to bend them down to where you lose some or all of your up elevator ability. Trust me on this. I almost lost a pod and boom 2 meter glider from this very thing by trying to get away with a small "sexy looking" boom which turned out to be too flexible.

The cure to avoid any risk of this is to make the booms either from a larger diameter tube or to build up the booms so they have more vertical depth. If you want to stick with tubing I'd suggest nothing smaller in diameter than thin wall 1 inch OD. The other option would be to stick with something like 1/2 inch OD but to use two on each side with a spacer between them to produce a boom which is about 2 inches deep at the wing's trailing edge and tapered down to the two tubes simply joined at the tail. Anything smaller in size than these two options on something as big as your design is going to be too flexible. By building up a vertical section you can achieve the stiffness needed to resist vertical bending easily. But you still want a good amount of stiffness in the horizontal direction as well so the tail section can't oscillate or whip side to side. Tubing is perfect for this aspect. But it needs to be stiff enough to handle the job. And "stiff" implies a larger diameter than your 96 gms per tube would indicate.

Note that "stiff" or "rigid" and "strong" are two totally different things. "Strong" implies a high load before final failure. "Stiff" means the tube or final structure has a low amount of deflection for small loads. In effect any structure is a spring. The key to locking the tail section alignment to the wing is to use a very "stiff spring" so that the flight loads do not cause the wing and tail to flex out of their designed angular relationship by more than a minimal and tolerable amount.

For a model of the size you're considering I would suggest that if you secure a meter or yard long tube to your table and hang a 2 lb weight at the end that the tube should not flex down more than about 1/4 inch. Ideally it should be less. If it flexes more than 1/4 inch over that length with that much weight I'd suggest to you that it is not suitable as a structural tail boom for a model of this sort of size.
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Old Jul 16, 2012, 01:23 PM
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One further point. I would suggest that you want to resize and reangle the tails so that you obtain a Vertical Tail Volume Coefficient that is on the high side of acceptable. A long nose pod and boom style construction of this sort has a long and large nose with virtually no compensating fuselage side area behind the CG. It thus becomes part of the vertical tail area's job to overcome the destabilizing forces created by the generous side area of the pod which is located in front of the CG. My TLAR senses tell me that you do not have enough effective fin area.
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Old Jul 16, 2012, 01:23 PM
A man with too many toys
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I will be interested in how you mount everything. Be sure and post plenty of photos.


Do you realize that the APM2 has to have exactly 5v regulates power so make sure that you have a 5v BEC to power the receiver and APM2.



.
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Old Jul 17, 2012, 08:19 AM
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Cheers for the advice...

Quote:
Originally Posted by BMatthews View Post
I found this thread from a link to it in the Modeling Science forum.

IMO 1x4mm carbon is not big enough for something of this size. In particular you will also want a web between the top and bottom caps of something that is more stiff than simple foam. I'd suggest that you want a stepped set of spar caps with vertical grain balsa in between as the webbing. I'd look at 1x10mm caps for full span top and bottom. And the same 1x10mm strip as "doublers" glued to the full span caps for a half span out to each side of center.

It's hyper important to provide a good distortion free web to join the top and bottom caps as well. This requires something far more crush resistant than the foam core. You'll want to use something like 3/8 balsa with the grain VERTICAL as the spacer webbing. This means that the foam cores will need to be split for a front and rear half on either side of the spar assembly.

Everyone seems to want to use thin carbon tube booms but for you. I see you're showing aluminium tube booms. But even carbon and aluminium is flexible. And the drag from the high mounted tail can produce enough flex in the booms this long to bend them down to where you lose some or all of your up elevator ability. Trust me on this. I almost lost a pod and boom 2 meter glider from this very thing by trying to get away with a small "sexy looking" boom which turned out to be too flexible.

The cure to avoid any risk of this is to make the booms either from a larger diameter tube or to build up the booms so they have more vertical depth. If you want to stick with tubing I'd suggest nothing smaller in diameter than thin wall 1 inch OD. The other option would be to stick with something like 1/2 inch OD but to use two on each side with a spacer between them to produce a boom which is about 2 inches deep at the wing's trailing edge and tapered down to the two tubes simply joined at the tail. Anything smaller in size than these two options on something as big as your design is going to be too flexible. By building up a vertical section you can achieve the stiffness needed to resist vertical bending easily. But you still want a good amount of stiffness in the horizontal direction as well so the tail section can't oscillate or whip side to side. Tubing is perfect for this aspect. But it needs to be stiff enough to handle the job. And "stiff" implies a larger diameter than your 96 gms per tube would indicate.

Note that "stiff" or "rigid" and "strong" are two totally different things. "Strong" implies a high load before final failure. "Stiff" means the tube or final structure has a low amount of deflection for small loads. In effect any structure is a spring. The key to locking the tail section alignment to the wing is to use a very "stiff spring" so that the flight loads do not cause the wing and tail to flex out of their designed angular relationship by more than a minimal and tolerable amount.

For a model of the size you're considering I would suggest that if you secure a meter or yard long tube to your table and hang a 2 lb weight at the end that the tube should not flex down more than about 1/4 inch. Ideally it should be less. If it flexes more than 1/4 inch over that length with that much weight I'd suggest to you that it is not suitable as a structural tail boom for a model of this sort of size.
Hi BMatthews - I have just read your response (didnt get an email notification for some reason). I understand most of what you are saying, and will need to have a bit of a rethink regarding the booms, wing reinforcement, and tail.

Wing
I am not sure exactly what you mean by 'webbing' between the top and bottom reinfocement balsa pieces. If I slice the wing down the fat part (CG line). And glue in a thick piece of spruce stripwood the entire length of the wing, that should provide enought strength. I have already done this on a similar sized wing that was a one piece 2400mm wing. I will post pictures later showing the wing, and maybe i can use that. The joins between the outer wing panels and center wing are the problem area for me - how far should the joining tube go into the outer wing panel?

Another question regarding the booms, I guess the tail is potentially going to be 400g (ish including booms, surfaces and servos). Therefore When the aircraft is standing on the floor, the tail will drop and twist the wing towards the rear. How do you suggest I reinforce the center wing section so this doesn't happen?

I bought 10mm x 10mm alloy square tubing yesterday to lay into the wing center section at chord and 1/3rd from trailing edge, I would also lay this tube 30cm into the outer panels - then put a carbon tube of 1m length through the center section at chord and 1/3rd from trailing edge. I will post a picture tonight as what i have described sounds a bit difficult to visualise!

Booms
I take on board about what you are saying regarding the strength of the booms - I may change them for these 16mm booms. I did try bending them in the shop and they seemed to be very strong. I thought the 12mm dia tubes were okay, but the last thing I want is poor elevator response!

Tail surface
If the tail surface is going to cause yaw problems in its current size, I may reduce the height of the fuselage, from 150mm, to 120mm. As i dont need all that height in the fuse. would that be okay? It would reduce the fuse side by 800 x 300 = 240 sq cm.
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Old Jul 17, 2012, 08:22 AM
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Quote:
Originally Posted by RC Man View Post
I will be interested in how you mount everything. Be sure and post plenty of photos.


Do you realize that the APM2 has to have exactly 5v regulates power so make sure that you have a 5v BEC to power the receiver and APM2.



.
I will certainly post photos as and when I have something interesting to show. I have already made 10 successful flights with APM2 in my wing. I don't think i gave it enough power before and was getting brown-outs. which led to the demise of my skywalker and ursus!! - This aircraft will be equipped with a parachute recovery system and 'o s**t' button !
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Old Jul 17, 2012, 02:30 PM
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Update - some pictures

Hi all - Some pics - comments on pics.. easier that way....

One thing i didnt mention is that I will be running cables to each wing tip, including
  • GPS
  • Video Transmitter
  • Telemetry Radio
  • Nav Lights
  • Wing Tip Camera

I have got lightweight U shape plastic ducting to embed in the wing. I will leave the exposed side on the underside, and use duct tape to cover cabling once inserted.

Wing is in pretty beat up condition, it is from a previous project.

Next steps - get large diameter booms - source 2 x 8mm ext dia 1000mm Carbon tube for wings.
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Old Jul 18, 2012, 07:16 AM
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Wow, it's beautiful.

Following your project worklog.

Good luck!
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Old Jul 19, 2012, 05:26 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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I'll try to comment on how I understand what you're doing following each part from your post above. My comments being in blue;

Wing
I am not sure exactly what you mean by 'webbing' between the top and bottom reinfocement balsa pieces. If I slice the wing down the fat part (CG line). And glue in a thick piece of spruce stripwood the entire length of the wing, that should provide enought strength. I have already done this on a similar sized wing that was a one piece 2400mm wing. I will post pictures later showing the wing, and maybe i can use that. The joins between the outer wing panels and center wing are the problem area for me - how far should the joining tube go into the outer wing panel?

First off the CG is not always located at the point of maximum thickness. It's simply a non related coincidence that this often occurs. To best find where you should place the CG it's wise to use a CG calculator. There's a couple of them online which are a google away if you use the terms "cg calculator".

If you look at a steel I beam there's a top and bottom cap and a vertical WEB in between. The strongest and lightest spars we can build for our models typically try to model the classic steel I beam but in wood or carbon. So what is often done in high load situations is to use a FLAT top and bottom cap and then add a separating web in between so the top and bottom can't "slide" relative to each other or allow the top cap to collapse inward toward the lower cap. The loads in an I beam are that the top is heavily in compression, the bottom heavily in tension and the web inbetween is supporting a diagonal shear load as well as being under compression as the top cap wants to collapse inward and meet the bottom cap. We size the top and bottom and webbing, often called the "shear webbing" for reasons you can now appreciate, to best resist these loads and the webbing to fully support the top and bottom caps. That help out?

With this in mind you should now be able to see that your vertical slab spruce spar is not too bad but it's not putting the wood where it can do the most good. On a rectuangular blade like this you still have a top and bottom cap and a web. But due to the shape the caps are not very wide. The upper and lower 15'ish% of the vertical spar you added become the compressed and streched load bearing caps. The part in the middle becomes the shear web. The caps do not have a lot of cross section area in this case while the shear web portion is quite over done for the amount it has to handle. That's why we typically go for wider flat oriented upper and lower caps. It puts more of the material out at the edges where it can do the most good. Then we support the material in the caps with a web.

An excellent spar system is the one used on Mark Drela's Allegro Lite. It's well worth checking it out to read about the wing development in terms of structure.

http://www.charlesriverrc.org/articl...egrolite2m.htm

It uses a full width vertical balsa web to fully support stout carbon caps. This is a wing which can tolerate the strongest winch launch producing a line pull of likely up around 120 to 150 lbs of tension. Yeah, it's overkill for your needs. But don't sell short the sort of loads your spar may need to carry. For example, your target weight is 4.5Kg. That's hardly a lot for a model of this size. But what happens when you go into a 45 degree banked turn? The apparent weight doubles to 9Kg. And what if you get into a dive and sudden pullup to recover? The G load for that could be as high as 5 or 6. So suddenly your spar is being told to hold itself against a 22.5 to 27Kg apparent weight. See why it's wise to over design your wing spar?

The spruce spar you used is great for the outer panels and while I would be a LITTLE worried on the inner panels the alloy tubes you're planning on inletting will easily take care of that. It's a bit overkill in terms of weight but it will certainly be strong enough.

Now if you had gone with spruce or rectangular carbon caps with a vertical web between similar to your blade style spar so it forms a spar similar in look to a steel I beam then you would not need the aluminium tubes through the center.


Another question regarding the booms, I guess the tail is potentially going to be 400g (ish including booms, surfaces and servos). Therefore When the aircraft is standing on the floor, the tail will drop and twist the wing towards the rear. How do you suggest I reinforce the center wing section so this doesn't happen?

I bought 10mm x 10mm alloy square tubing yesterday to lay into the wing center section at chord and 1/3rd from trailing edge, I would also lay this tube 30cm into the outer panels - then put a carbon tube of 1m length through the center section at chord and 1/3rd from trailing edge. I will post a picture tonight as what i have described sounds a bit difficult to visualise!

I'm not sure why you're using the two items. Is this to allow cutting through the aluminium to allow the outer panels to be removed for transport? Otherwise there's no need to use both.

Since you're adding sub spars if you arrange the tail booms so that they are secured to BOTH the front and rear sub spars the sub spars should support your tail boom weight and avoid the wing twisting as you're worried about.

If you didn't have the sub spars but you had a really stout I beam pattern main spar I'd suggest epoxy glassing the center out to the booms using some 4 oz cloth and epoxy laminaint resin with the cloth weave biased to a 45 degree diagonal. This would convert the center area of the wing where the glassing was done into a rather strong and stiff but still reasonably light airfoil shaped torsion box design. It would easily withstand the bending loads from the overhung weight or any flight loads. But with your current plans it's not needed. Now if you DO try it on some future model be sure to build in a couple of plywood ribs at the boom locations to aid in transferring the high point loads of the boom attachement out into more of the glass skin and underlying foam. Glass and foam can be very strong but only when it deals with the loads over a large area. So you need to ensure you have "hard points" that take point loads and spread them out. Think of these hard points as snowshoes. They are intended to take the load of the "feet" and spread them out over more snow or foam so the man or attached part doesn't crush the snow or foam.


Booms
I take on board about what you are saying regarding the strength of the booms - I may change them for these 16mm booms. I did try bending them in the shop and they seemed to be very strong. I thought the 12mm dia tubes were okay, but the last thing I want is poor elevator response!

The tubing you linked to appears to be the typical soft home handyman type stuff. If I'm correct I'd seriously suggest you find a source of aircraft grade aluminium tubing for this or switch to a big section thinwall carbon tubing. The typical home hardware store stuff is not able to spring far without taking on a permanent bend. And that's the last thing you want. On the other hand a 6961-T4 or 6061-T6 aircraft alloy tube will easily withstand being bent a good 4 to 5 inches to the side and still spring back. And it's stiffer to resist the bending loads as well.

I outlined what I thought was a reasonable test based on my experience at almost losing my own model due to tail boom flex. The tubes you're looking at may or may not pass this test. But even if they are STIFF enough I would worry about them actually being flexed beyond their permanent bend point and not springing back. You're putting too much time and effort and money worth of equipment into this to risk disaster due to improper metalurgy.


Tail surface
If the tail surface is going to cause yaw problems in its current size, I may reduce the height of the fuselage, from 150mm, to 120mm. As i dont need all that height in the fuse. would that be okay? It would reduce the fuse side by 800 x 300 = 240 sq cm.

You're only looking at cutting down the side area by a small %. There's an amount of guess work in this based on my past but my "Spidey" senses are telling me that the vertical area needs to just about double to deal with the long and deep nose area even with a nose reduction.

To aid in getting a feel for this on your own look up "handley page halifax images". The early Halifax used swept back leading edge fins. It had a hunting tendency. They cured this by switching to a larger area simple rectangular fin. And to my eyes your model looks much worse than the early small fin Halifax. ESPECIALLY when you figure that your design has essentially zero side area to the fuselage behind the CG. If you look around at other designs you'll get a similar feel to all this. For example look at the fin area used on an "edgley optica images" in google.

Hopefully I haven't rained on your parade TOO heavily.... I thought it best to pull the rug out from under you now rather than wait to read about some disaster at the flying field.

There's an option that a lot of new designers seem to not want to use. And that is a small scale test glider. For example if you were to make up a simple small cut foam free flying test glider of around 24 inch span from your scrap foam and angle the outer wings up for about 1 inch of dihedral per side you could use such a glider to see if it has yaw issues. Also you could experiement with the suggested CG location provided by the online calculators to see how stable or unstable it is. But mostly it would give you more of a feel for the size of the vertical tail area to deal with the big forward side area of the pod. With a glider that has the sort of dihedral I suggeseted you should be able to toss it out into a 45 degree banked turn and the return to level flight should be smooth, take a while and not have ANY signs of whipping around to turn in the other direction. From a shoulder height launch with no stall I would want to just see it return to level or nearly so from a banked launch with the wings at 45 degrees bank. If you can tune your design to do this it should prove out to be a delight to fly the full size version.

If it tend to not recover to almost level from such a launch or winds in without even trying then you have too MUCH vertical tail area. If it recovers fast and whips past level or even suggests that it wants to snap roll before touchdown then you have too little. If it's a violent sort of response then you have WAY too little.
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Old Jul 20, 2012, 08:08 AM
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Now that's what i call feedback... thanks!

Hi BMatthews.. Cheers for the info, haven't quoted it as I think the post would be rather large!

Wing and COG
I have certainly learnt a lot from what you have said, and will do my very best to include it into my current build. Regarding COG and thickest point of wing. this wing came from a previous aircraft i built a few years ago, and i remember the COG being at the thickest part of the wing, and it flew quite well....Is the COG always in the same spot for the wing, or does it take into account the nose and tail to determine its location?

I now understand what webbing is and get the concept, I would probably do it as you have suggested with a large 'I beem' and thinner webbing in future to reduce weight. I also understand what yo are saying regarding diagonal shear load. Hopefully the spruce will be okay, just a bit heavy. and should hold up in high G manouvers! 27kg is a lot - I may fiber glass over the top of the alloy rods and certainly connect them to the spruce in the outer wing panels, otherwise I can see the wings snapping at the end of the Alloy square tubes.

Booms
To clarify, I am using the alloy square tubes to maintain a clean channel in the wings, so that I may easily remove the outer wing panels for transport. The Carbon spa will go through the length of the alloy tubes for flight. the 2 carbon spas inside the alloy tubes are 8mm dia tubes, 2mm wall thickness, these seem pretty strong. My only concern is transferring the load between the alloy tubes and the spruce in the alloy wing panels.

I will ensure I secure the tail to the two alloy booms to avoid center wing section twist - I have attached a visual of how i plan to build the boom joiner boxes, basically two wing profiles with two thick blocks of wood holdings them together, with a large dia tube glued in (not on visual) to slide the booms into.

I think I will have to invest in some decent woven carbon tubes - 16mm dia for the booms, as i don't want to risk the alloy bending -Found them for ~£15 each here. It says they are pullwound. Does that mean pulltruded or woven? Any ideas?

Tail surface
I have looked at the reference photos you have supplied, and they do indeed show a considerably larger amount of tail surface. I may extend the tail surfaces length ways, and keep the angle the same (110 degrees)

Is there a rule of thumb for v tail surface area? I have read that the elev surface should be no less than 15 percent of total wing area.

Other
I am certainly glad I am discovering this stuff out now, and would much rather change the direction of my design slightly in favour of having a mid air break up! I have actually thought of making a small flying rc scale model of this to test it. my main issue is the wing profiles, I need to get them sized and cut by a chap in Scotland - which is quite a timely and costly process.

i may not have commented on all your feedback, but i will certainly read it over and over until i understand everything!

I have also attached some refined measurements - excluding tail change,

The tail will need some careful consideration, as Yaw stability is certainly an advantage!

I will post an update after the weekends progress - I am taking delivery of a scroll saw today to help cut the wing boxes - can't use a conventional coping saw!

Rich
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Old Jul 20, 2012, 01:12 PM
B for Bruce
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If you're at all worried about the spar there is a possible fix you could do on this model. It would involve sanding or routering out a groove along the top and bottom following the spar. Then simply glue in some carbon spar caps. I don't know who sells such things over your way but what you'd be looking for is something like 1 or 1.5mm x 8 to 10mm wide flat strip.

The CG location is not determined by the wing. Or rather the wing is only part of it.

Look at http://adamone.rchomepage.com/cg_calc.htm . You'll see all the factors such as the wing area, chord, tail length stab area and chord and other factors that go into calculating the stability of any given model.

Foam is very poor at carrying hard structural loads. For this reason it would be best if your front joiner tube were to be located so it gets glued directly to the spar on one side. For the rear it would be nice if the wing joint faces were hardened up with 3mm aircraft plywood. The plywood would spread out the point loads produced by the tube trying to flex within the foam. Your one hand drawn sketch makes it appear that you're already intending to do something of the sort and incorporate the tail boom mounts along with this. If I'm seeing that right then nicely done.

Large diameter woven carbon tubes can push the cost of a project like this up there fast. One option to look at is to check golf shops in your area that do up custom clubs. You may find that a pair of used carbon driver shafts will provide you with the sort of strong and stiff tail booms that you're after. I've got a shaft of this sort here and it's more than adequitely stiff and definetly strong enough and, of course, has a proven resistance to frequent short term shock loads....

All you need to do for changing the vertical area is to alter the V angle of the tail. There is nothing magical about the 110 degree angle so often used. It's just that 110 is what works out well enough for a regular style of glider fuselage and for many of the regular type of power planes. But your big pod with two skinny booms is simply not a "regular" sort of fuselage side area distribution. So the side area you do have is pretty much 95% a destabilizing area instead of being about 40% destabilizing in front of the CG and 60% stabilizing behind the CG such as found on normal style fuselages. To compensate for this you need to get a lot more vertical tail area.

But by the same token you don't really want to move to a V angle of tighter than about 85 degrees or you'll get more drag from the acute angled joint. Personally if it were my project I'd look at simply giving up on the inverted V tail and go with dual fins and a flat stabilizer. It would prove to be both easier to produce a stronger stabilizer since there's no center joint and you can size up the fins and rudders to provide the sort of vertical tail area you need.

Yes, 15% is a nice general tail size. In particular when it's joined with a fairly long tail moment such as you're using. But if you insist on using an inverted V tail than I would suggest you need to first alter the angle to 90 and then increase the span and chord of the tail up to a projected area from above of 20% of the wing area. This will increase the vertical area to what should be enough to groove well.

Again, I'd strongly suggest that a quick and dirty foam and sheet balsa test glider will soon tell you about all this for not a lot of effort. For this sort of testing the small test model does not need all the trimmings. No need for landing gear or a motor mount. All you need is a block of foam sanded roughly to the shape of the big pod, wings cut from foam or sheet balsa, dowels or some other straight but reasonably strong and stiff sticks and depron or sheet balsa tail surfaces. You can learn much about your design with such a simple glider. And when you're all done it's sort of slick to have it hanging on display over your work area....
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Old Jul 21, 2012, 03:39 PM
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Cardiff, UK
Joined Aug 2008
223 Posts
Update

Hi Forum,

first of all .. @BMatthews - thanks for the second round of feedback, I am taking it all in and factoring it into the design process

The Center of gravity calculator looks fairly complex - I understand most of the number inputs, and i believe they are as follows (attached is the drawing with measurements attached)

Wing Root Chord (A): 315mm
Wing Tip Chord (B): 315mm
Wing Sweep Distance (S): 0
Wing Half Span (Y): 1100mm
Stabiliser Root Chord (AA): 112mm
Stabiliser Tip Chord (BB): 112mm
Stabiliser Sweep Distance (SS): 0
Stabiliser Half Span (YY): 255mm
Distance between both LE's (D): 1048mm
Stabiliser Efficiency*: ??? unsure on this one

When cutting the grooves for the two alloy tubes (before starting this thread), I did wonder if i should have stuck an alloy tube right next to the spa. I may gouge out the polystyrene between and put some spruce in there to transfer the load.

I have already tried the local golf shops for carbon shafts but to no avail. Ordered the 16mm o/d tubes 1.5mm wall from woolmer composites.

I am going to take your advice and alter angle of V tail to 90 degrees - and maybe double the length. The maths is the part I struggle with, so will probably pick back up on it once i have completed the fuse and wings!

As I wont be able to get wings for a small quick and dirty glider, I may make one out of balsa and traditional construction. The scroll saw is amazing and reduces building cuttign time massively!

A few pictures...
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Old Jul 23, 2012, 12:17 AM
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The 'Wack, BC, Canada
Joined Oct 2002
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I didn't really notice from the drawings but your ailerons are MASSIVE for the sort of performance that such a model would expect. Go real easy on the amount of aileron deflection angle or you'll have something that handles more like a fun fly model...

There's no need to go overboard and double anything. All you need is to shift to a 90 degree angle and MAYBE increase the chord by an additional 10 to 15 %. If you did that then I don't forsee any issues at all with either the horizontal or vertical tail area equivalency.

For a quick and dirty test glider flat plate balsa or something equally bend resistant is fine.

The surface of the wings is pretty rough looking. Also judging by the red stripe on the leading edge I gather you're considering not using any outer covering. If so I have a couple of suggestions.

First off fill in the waves and grooves with some light weight wall filler compound. You'll know you have the right product when you lift up the plastic tub it comes in and the container feels empty. Trowel it on to fill the wire cut grooves and then sand down to smoothen out the airfoil shape. You can also use the same filler in the joint rib gaps seen in the second picture of your last post. The stuff I'm talking about isn't all that much heavier than the foam you're replacing.

Then when you have smooth wings you will want to find some clean newsprint to varnish onto the foam to act as a stiffening skin. Movers use this sort of newsprint to wrap up dishes and glasses with when packing for a move. Check around for a moving supply outlet which has this same newsprint paper. To "dope" it down to the foam I suggest you use some of the now common water based emulsion varnish. To skin the wings cut the paper slightly oversize and then soak it in water. Varnish the foam with the Water Based Poly Urethane (WBPU) varnish and while it's still "milky" looking lay on the newsprint. Brush a little more over the paper and work it into the paper well. Do BOTH sides of each panel within a short time and stand up where air can freely reach both sides to dry. Otherwise it can shrink a warp into the panel. But if both sides dry evenly you'll be fine. Overlap at the leading and trailing edges by about an inch

Even this single layer will add a huge amount of torsional stiffness to the foam. And a wing can never be TOO stiff at resisting twisting in flight. And given the weight expectation of your project you definetly want a torsionally stiff wing. Not to mention that it'll greatly improve the surface finish of your model as well as reducing the drag of the final airframe over what the surface of the raw foam provides.
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Old Jul 23, 2012, 03:51 AM
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Cardiff, UK
Joined Aug 2008
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The Ailerons are pretty big, they seemed to work ok when this wing was used for a previous plane - Video of prev plane at the end of this post...

I may glue in the current ailerons and re-cut them to allow for a more efficient wing shape.

Understood about tail size and angle - I will re-draw on plans and order the tail section from my wing cutter - as well as a wing for the smaller test glider.

Great minds think alike - I have already been using Polyfilla Onefill ultra lightweight filler for the wings - I have managed to smooth it over a small portion of the inner wing to test - before using on the main wing surface. It seems to fill small gaps well - however i did try using it to fill in the large damaged area on the damaged root of the outer wing panel, and it seems to crumble - So i am going to cut out a foam block and stick in and cut to shape.

I am going to the store today to try and find some of that paper you speak of - One question though, I would like this airframe to be heavy rain proof. I have though about how to waterproof most of the components yet still allow airflow to the motor and esc and batteries. Is water based varnish water proof? If not i was thinking of using oracover or monokote.

I would like to add some torsional stiffness to the wing so option A would be best using the varnish and paper.

Currently focusing on the wing/boom joiner boxes - I replaced the 12mm dia booms with larger 16mm dia booms.I am finding it difficult to source 16mm internal dia tube for the 16mm booms to slot into, thinking of getting some slightly larger plastic pipe, cutting a section out, and using a jubilee clip on it to secure the booms in place. Once the wing/boom joiner boxes are complete, I will move onto finishing the wings. Will post pics as and when...

Cheers - Rich

Video of wing being used on a plane i built a few years ago...

Space Glider Mid Air Test Launch plane - first flight (0 min 49 sec)
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