Flying Wing - Vectored Thrust

[Tom Harper]

Just started design of a new AP model that is based on the one in the photo. At present that model has lost the center fin and the motor is near the wing center line. Good size with acceptable performance.

The goal of the new design is a model that will carry a small camera for 20 minutes at 60 KMH. The model will be controlled by AttoPilot. I picked the flying wing because it is easy to transport, has simple construction and Attopilot users have reported success with it.

Area: 4 sq ft
Span: 56"
Root chord: 12.25"
Tip chord: 8.375"
Aspect Ratio: 5.4:1
Airfoil: Eiffel 401
Motor: Heads Up RC - 2830-10, 1000KV, 27 oz thrust @ 14.5 Amps
Battery: 2 3S1300mah Lipos in parallel
Camera: Olympus FE 360
AUW: 44 ounces

First design question:

The model shown uses a thickened Eiffel 401 section as a center pod to house the camera and electronics. Would there be less drag if a thickened section were used over the entire wing - tapering to the Eiffel 401 coordiantes at the tips. In that case everything would be housed in the wing without the discontinuity of a 'pod'.

Second design question:

The model shown is very responsive to control movement. Is there any advantage to the full span elevons? Would it be more efficient to use elevons near the tips only? If so what percent of span? 20%? 30%?

Third design question:

The fact that the present model is sensitive to control movement and has no center dead zone indicates that the airflow is attached at the TE. So, a clean section could yield benefits in low drag. The obvious thing to clean up is the elevons - get rid of them.

I have done some rigging on the present model with thrust adjustments. It would be possible to gimbal the small electric motor and use it for yaw and pitch control. Might use some of those neat linkages from helicopters. The sweep should act as dihedral for roll. If not dihedral could be added.

Vectored control would require a landing gear. No dead stick landings on a skid.

Whatcha think?

[Tom Harper]

eBay has plenty of swashplates listed. Looks like a good motor mount candidate.

I've found a few examples of thrust vectoring, but only to augment yaw. Haven't seen it used as the primary control force.

Tom

[BMatthews]

I think your motor is too close to the CG to provide decent authourity unless rather gross angles of travel are involved.

But then I'm amazed that it is flying at all with the motor so high on that center fin yet does not have a really radicl amount of upthrust to compensate for the odd placement location.

There's other ways to get a nice big flare in the center section without adding drag or fattening up the airfoil. A "delta" center with a longer chord would provide the thickness needed to house the camera and radio gear and then the normal swept wings could extend from the delta like center section tips. This would provide a smoother transition and avoid the need for thickened airfoils over the center area.

[Tom Harper]

Bruce,

Thanks for the comments.

The model in the photo has been modified so that the motor is close to the center line of the chord.

However, there is a small amount of upthrust in the motor. It requires full down trim to compensate for it. Thats what gave me the idea for using vectored thrust.

Tom

[BMatthews]

Does that elevator trim stay consistent when you get up a bit and throttle back? If you need to reduce the down trim to avoid a dive then the trim is fighting the engine thrust. But if you do not need to alter the trim and the model transitions to a nice glide then the trim is being set by the CG location or some other factor.

Also I'm not seeing any reference to this Eppler 401 airfoil you're using. Can you post up a picture of it? There's a Gottingen 401 and an Eppler 403 but not an Eppler 401 from what I've seen in my resources or from a quick search of the 'net.

Anyhow, on the vector thrust control. Yes vectoring the thrust can provide good control but due to the loads from the gyroscopic effect of the motor and prop you'll find that it requires much larger servos than moving flight surfaces around. How much larger I would not try to guess. You'd need to set up the motor and prop on a gymbal and then with a scale in the pushrod move the thrust line and see what it says. And because it's a gyroscope the force measuring will need to be in both planes at the same time due to precesion. Once you know how much force you need in inch-lbs to move the prop so far and in a time consistent with the fastest transit time of the servo you'll want to pick servos with twice that much torque to avoid operating them anywhere near the stall point. Doing so will pay back in lower battery pack consumption as well as insurance of trouble free controls.

I always suggest this 100% safety factor for regular flight controls and servos as well.

[nmasters]

He's using Eiffel 401 not Eppler. It's a nice looking section but the only published coordinates I could find were in the old NACA format and too coarse for xfoil/profili to do anything with. Even after running "smooth" on it it was still kind of wavy. I played around with it a bit and got a fairly good +CM airfoil out of it but it still wasn't as good as an MH45 or PW51 thickened to the same %.

--Norm

[Tom Harper]

Norm,

I believe the document you attached is the only one ever published with the Eiffel 401.

So what do you suggest?

Tom

[nmasters]

For the thick center section, performance wise, it doesn't make much difference. At 18% thickness, with turbulators, the polars of the Eiffel 401 and the PW51 are so close that the only consideration is the pitching moment. If you want a positive CM the Eiffel is better if CM0=0 is okay then the PW is the one. As you get thinner the PW51 shows better lift and drag both with & w/o turbulators. Turbulators have an interesting affect on the pitching moment at low Reynolds numbers. Without them the CM curve tends to have two big kinks in it but with them it tends to straighten out. The PW51 is also slightly fatter so you would get a bit more volume in your center section.

So my suggestion would be to go with your plan to taper but switch the tip section to PW51 at its stock thickness and use turbulator tape where it's appropriat.

--Norm

[biber]

In this application there'll be not much benefit in thrust vectoring, other than the fun in playing with gadgets in itself.
I'd keep the pod, because further thickening of larger portions of the wing where it's not necessary will do no good.
If control is too twitchy, cut the inner 30 or 40 percent off the elevons and leave them attached fix to the wing.

biber

[BMatthews]

Granted drag is not really our enemy in model flying like it is with commercial or other classes of full sized aviation but the pod, with it's sharp internal corners, is far more draggy than a properley expanded center section would be. For example the smooth increase in thickness with this shape is due to the localized incread in chord rather than adding to the percentage thickness. It also adds wing area which can be benificial to carrying a load.

[nmasters]

There are compromises any way you go here. While thick airfoils will ruin your glide slope a high taper ratio will increase the danger of spinning unless you use a lot of washout and/or some stall delaying devices. The fat center section that Tom is using may be benefiting from propeller scavenging. That would drastically reduce the drag of that part. With the taper I suggested the thickness would drop off fairly quickly so only part of the inner span would be the draggy thick sections. Of course the wing is most efficient with a thin airfoil section over the whole span. This brings use back to the original question which I believe was probably really concerning the interference drag of all those sharp corners around the pod. So here's how to decrease the interference drag and keep the wing thin:

Taper the pod so that it's, say, 1/2" wider at the trailing edge than at the leading edge and fillet the corners with a radius about 1/2 the maximum hight of the step. The gradual expansion will help maintain a favorable pressure gradient in the intersection and avoid separation.

--Norm

[Tom Harper]

Biber, Bruce, Norm,

Thanks for the discussion.

The application here is aerial photography so the drag consideration is reduced power at cruise. I've looked at using increased chord to get a thicker center section (similar to the Hortens), but it takes a lot of chord to get enough thickness to contain a camera.

A limiting factor is balsa building materials and my skill in using them. Molded composites would allow lots of compound curved shapes that hold the contents with a close fit, but I'm just a balsa butcher.

I've been using my own software for airfoil plots. I looked at Profili and was impressed, so I downloaded it and have applied for a registration number.

The Eppler334 section is interesting. I won't have the polars until I get my registration number, but the shape lends itself to easier construction than the Eiffel 401 and PW51.

For this application, I keep the aspect ratio between 5 & 6:1. The taper is modest and I can incorporate some washout.

So, perhaps an Eppler 334 section for the wing and the same section thickened for the POD. The POD will widen a bit toward the TE and fillet or at least taper into the wing section. 5.5:1 Aspect ratio with square tips and 3 degrees or so of washout.

Vectored thrust is probably too novel for this project. I need this platform. I think I'll play with vectored thrust on a more conventional configuration.

Without vectored thrust I can eliminate the landing gear. I'll drop a couple of plywood skids below the wing to function as handholds during launch, place them outside the prop arc.

That leaves the control surfaces. Do full span elevons contribute greatly to drag? Are tip elevons better? Do large tip plates help in spin recovery?

The application is for a reliable, light weight, low drag platform. Don't need aerobatics.

[nmasters]

The Eppler334 section is interesting. I won't have the polars until I get my registration number, but the shape lends itself to easier construction than the Eiffel 401 and PW51.
That is an interesting airfoil. When you get your registration number run a type 4 polar on it at Re =100,000 with and without turbulators. You can use the pressure distribution plot (second from the bottom of the first group on the polars menu) to see where the separation bubbles are


For this application, I keep the aspect ratio between 5 & 6:1. The taper is modest and I can incorporate some washout.
Don't be afraid of using too much washout, it improves circling

Without vectored thrust I can eliminate the landing gear. I'll drop a couple of plywood skids below the wing to function as handholds during launch, place them outside the prop arc.
Got a little bad news (http://www.rcgroups.com/forums/showthread.php?p=11437400&highlight=precession#post11437400) here. A two blade prop will induce a vibration during turns at 1/2 the RPM. You may want to include mounting points for LG just in case it turns out to be enough to affect your pictures

That leaves the control surfaces. Do full span elevons contribute greatly to drag? Are tip elevons better? Do large tip plates help in spin recovery?


I can't say how tip plates affect spin recovery but there are stall devices that definitely do (http://www.rcgroups.com/forums/showthread.php?p=5967881&highlight=vortilon#post5967881).

--Norm

[BMatthews]

Tom, even reducing the sharp center section angles will help a little with power requirements. And if you're willing to give up on the center section lift efficiency but still produce minimal drag then you can still do a lot without making it all swoopy looking.

What about something like this? The idea is that the "lifting" wing transitions from the fat but lower drag center "airfoil" via the tapered transition segments. This could easily be done in sheet balsa or other sheet materials but it avoids the sharp 90 corners and the rolling turbulence that will occur around them. The top option would be grossly thick for a 6 foot span but if you need to face the camera forward it may be the best option. Obviously much of this center pod would be made from clear mylar covering to allow the camera a clear view.

Actually this has me thinking. I was wondering about a camera model of my own.. NAH! It limits the view to down or ahead. I want sideways as well.

[Tom Harper]

Bruce et al,

I think it is moving toward something like your plan.

I ran some numbers using the flight log of the present model. The Re numbers are much higher than I thought - above 300,000. That's good. Then I backed out the Cl at cruise:

Altitude: 4611 ft
Weight: 2.41 pounds
Area: 3.68 sq ft
Speed: 60 KPH
Cl: .214

For an Eiffel 401 that's a L/D of 16:1. The measured glide angle was 9:1 - probably about right when you add in the exposed gear, linkages and some building errors. The curves show that I would be better off at a higher Cl.

I am looking at the MH60 section. It has a wide Cd bucket and is a little easier than the Eiffel to construct (not as easy as Eppler334). The AOA design value for the middle of the drag bucket is +2 degrees. That gives me a Cl of .4 and an L/D of 45:1. For the present weight and selected cruise velocity, the wing area backs out to be 2 sq ft.

I know that I tend to build models that are too big for the motor, but 2 sq ft seems small. However, on the present model, the Cl at take off (43 KPH) is .424. So, I'd have to fly almost at stall speed to get a Cl near .4 with this size airplane (3.68 sq ft).

I briefly considered carrying my Canon A650 (15 oz.). That produces the correct Cl but that's about 20 Watts per pound at cruise. Seems marginal, and I couldn't stuff the Cannon in the POD anyway.

Reduction from 3.68 to 2 sq ft is a multiplier of .74 for the linear dimensions. That would be:

Span: 38.5"
Root: 9"
Tip: 6.25"

Sure would be easy to get into the car!

So, is the answer to go down to a wing area of 2 sq ft?

[Tom Harper]

Combining the above discussion with the items that remain constant (POD size), I get the following:

Area: 2 sq ft
Span: 36"
AR: 4.3:1
AUW: 40 oz.
Wing loading: 20 oz per sq ft

Issues:

The wing may not be thick enough to house the Lipos - but I may be able to use smaller batteries.

Smaller batteries and elimination of the LG will save quite a bit of weight - lower Cl again - moving target.

Washout will load the center section more than the tips - perhaps a bell shaped distribution. That should raise the center Cl to .5 - that's good


Whatcha think?

[Brandano]

I'd add fins inboard of the ailerons to protect the prop, that would allow you a minimum of safety through a power on landing

[Tom Harper]

Maybe I could use the fins as launch handles

[BMatthews]

Tom, you'll need to start again. The old "scale" effect is going to KILL you if you try to fly a 288 sq inch model that weighs 40 oz or anything at all close to that weight. A good max weight for this size wing would be 24 oz and I'd prefer to see it closer to 20. At 40 it'll have to fly at a rediculously fast speed and landings will be done at speeds close to 40 mph. On top of that it'll be a stall and snap roll looking for a place to happen.

Given that you likely want this model to have a pleasantly slow to medium speed cruise the 40 oz weight is just way, way over the top. Especially on a flying wing where the "effective" area is more like 2/3 of the wing's real area. The other 1/3 is washed out to supply the pitch stability needed and due to the angles isn't working at the same lift coefficient as the root area. This forces the load onto the center area and further loads up the part of the wing that is doing the lifting. I think you'll find that it forces the center area Cl up higher than you think.

To get around this your 40 to 48 oz wing should have an overall wing area more in the 500 to 550 sq inch range to get the cruise speed you're likely after.

[Tom Harper]

Good comments. Let me work on it.

[nmasters]

Washout will load the center section more than the tips - perhaps a bell shaped distribution. That should raise the center Cl to .5 -

Any swept wing flying significantly faster than its design speed (or below its design CL) will develop a bell shaped lift distribution. You can verify that with this little java script (http://www.rc-soar.com/tech/winganalysis.htm) I hate to accuse Reimar Horten of making stuff up after the fact but according to people who worked with him there wasn't any theory about a BSLD until the '50s. Anyway, before I go off on that rant, I just want to expand on my comment in favor of shorter elevons that I made in a previous thread. The function of the elevons is, of course, to reduce and even reverse the lift of the section of wing in front of them. By extending them inboard of MAC you're reducing the lifting ability of the inboard panel. This is exactly what you don't want. The lift reduction outboard of MAC is rotating the wing to a higher AoA in order to increase the lift generated by the inboard panel. If the inboard panel is also being de-cambered at the same time it must be rotated to an even higher AoA thus causing more drag than necessary and reducing CLmax.



--Norm

[nmasters]

The old "scale" effect is going to KILL you if you try to fly a 288 sq inch model that weighs 40 oz or anything at all close to that weight...

I agree with that although I would have said it in terms of CL and Re. Basically you want to size the wing according to the expected load and desired landing speed. So what you do is get the Reynolds number at the tip at the landing speed. Then get polars for your airfoil at that Re. The CL at landing should be about 80% of that CLmax. If the polars came from a wind tunnel I'd say that landing CL should be 10% below the clmax on the polars but the computer may be a bit optimistic so it's best to err on the side of caution (same with using the Re at the tip). It sounds like you already have software but this spreadsheet I've been working on, on and off (mostly off) calculates Re and lift at altitude. It's a kludge with a few broken formulae but those parts work and it calculates twist too.

[Tom Harper]

Norm, Bruce,

Thanks for the spreadsheet - really colorful!

Above, you are saying to position the elevons outside of the MAC spanwise. In past comments I was thinking of this only chordwise.

In the spreadsheet you state a Cl of .2 as the normal goal for cruise. That's interesting since the present model cruises at .195. So, is that the best working value? Is it unrealistic to go for those super high L/D ratios at Cl=.5?

The spreadsheet gives a twist value of 1.1. Is that degrees? Seems low.

The sketch in #16 does have unrealistic wing loading.

With a cruise Cl of .2, the present design doesn't need to be any larger. I may trim it a bit.

I'll work on another sketch this afternoon. This morning we are doing some volunteer work for the BLM.

[nmasters]

BTW it's set up to take metric input and the imperial input column is protected because all the ugly bits in the hidden columns reference it. I've included the password in the spreadsheet so that you can use imperial units if you want to. Just be aware that putting numbers in thos cells wipes out the conversion formulae so going back to metric will be kinda' tough unless you save the original.

Norm

[nmasters]

Thanks for the spreadsheet - really colorful!
I'm a bit dislexic and tend to get lost on a black & white page so the colors are to help me navigate around the page. Also when I added the metric columns I had intended to hide the columns that weren't being used


In the spreadsheet you state a Cl of .2 as the normal goal for cruise. That's interesting since the present model cruises at .195. So, is that the best working value? Is it unrealistic to go for those super high L/D ratios at Cl=.5?

Typically minimum drag occurs around cl=0.2. The best glide of the real airplane occurs when induced drag and parasite drag are equal. Since the cd shown on the polars is only profile drag a real CD of a real wing at its best L/D will be double what the polars show. If you build a very clean airplane and are lucky a flying wing may have an L/D 1/3 of the 2D polars. For tailed airplanes it's often 1/4. Since induced drag decreases as CL increases and parasite drag (including profile drag) increases with airspeed minimum drag of an airplane is where the two are equal and best L/D is just a bit slower than that

The spreadsheet gives a twist value of 1.1. Is that degrees? Seems low.

Yes, it's degrees. And it is odd, I think it's because I have a positive number in there for pitching moment. I don't remember what airfoil polar I was looking at last before I zipped it.

The MH60 has zero lift at -0.68 and cm0=-0.002

That part of the spreadsheet is the Helmut Schenk's formula AKA "the Panknin formula" because Panknine popularized it. I used the XB-35 to test the spreadsheet and it produced the right twist and lift numbers so I assume it's generally valid although I'm still trying to get it to work with local temperature instead of global average.


--Norm

[Tom Harper]

Based on the present model and all of the suggestions, I get the model shown in this sketch (quick photo, cause it won't fit in my scanner).

Span: 52 inches
Root Chord: 12.25 Inches
Tip Chord: 8.375 inches
Area: 537 sq inches; 3.7 Sq Ft
AR: 5:1
AUW: 44 ounces
Wing Loading: 11.8 ounces per sq ft

Elevons are outboard of the MAC
Inboard fins serve as prop guards, landing skids and launch handles
Motor center line raised 1.5 inches to enhance prop clearance


For construction considerations I prefer to use the same airfoil section for the root and tip. The wing will be built up and covered with 1/32" balsa with the grain running chordwise. The Eppler 334 section looks like the easiest to build. Not a final decision yet.

Radio antenna will run through a tube in the wing. GPS will be internal. This model will use an IMU AttoPilot so there won't be XYZ sensors on the outside of the airframe. Should be pretty slick.

Tom

[nmasters]

I'll comment on this latter, I've been called away on a "rescue". The short version is that flat bottomed airfoils don't look good for the tips of flying wings

--Norm

[BMatthews]

Tom, do a google for "spreadsheet panknin". It should steer you to a place to download the Excel spreadsheet workup of Dr Panknin's math for calculating how much twist you need for a given design.

And yeah, the elevons are better on your latest design. Remember that in a swept wing planform the outer portions are the "stabilizer" for the design. You gain nothing but drag for elevon area inboard too much.

When you look at the Panknin info it asks for and play with it you'll soon understand why it's best to use a lifting airfoil out to mid span and then smoothly shift to a symetrical section at the tip.

[Tom Harper]

I downloaded the files and will work with them.

However, what's the practical limit. Balsa frame construction will approximate a section shape, but it's not like building a full size aircraft with molded composites. Some laminar flow freaks are concerned with +-.1 mm on a full scale wing. Balsa, silkspan and mylar don't even approach that degree of accuracy.

Careful workmanship will pay off, but the calculations are only a mid point to work toward. It is not realistic to believe that open frame balsa construction will attain and hold .1 degree tolerances.

So, what are the practical limits within which to use these design tools.

[Tom Harper]

This model will use shims to rig the thrust vector, but I'm going try a vector mount on a more conventional configuration. So, I bought a swashplate and it looks like just the thing.

The ball links remove with a 1/16" allen wrench and the pieces are just pressed together. Very nice device. Could never make anything like it. Now I have to figure out how combine it with the motor X mount.

[nmasters]

I downloaded the files and will work with them.

However, what's the practical limit. Balsa frame construction will approximate a section shape, but it's not like building a full size aircraft with molded composites. Some laminar flow freaks are concerned with +-.1 mm on a full scale wing. Balsa, silkspan and mylar don't even approach that degree of accuracy.
True but maintaining laminar flow isn't a problem at Re<500,000. In fact at the Reynolds numbers your plane will be operating at the flow will stay laminar far too long and that's why we get laminar separation bubbles. Balsa and silkspan construction will trip the boundary layer to turbulent at the edge of the balsa sheeting but that's not necessarily the best place for transition. Play with the pressure distribution and trip positions in profili. Transfer what you find there to the type four polar. You'll see that turbulators cause drag but also make the Cm more linear. Since a turbulent BL makes about 3 times as much drag as a laminar one you want to keep it as long as possible.


Careful workmanship will pay off, but the calculations are only a mid point to work toward. It is not realistic to believe that open frame balsa construction will attain and hold .1 degree tolerances.
Again I agree and the real model isn't as critical as the software. It's just that the software requires a very smooth shape or it has fits and draws useless spiky curves because the math doesn't converge on a solution at some Res and AoA.


So, what are the practical limits within which to use these design tools.

The lift and drag info may be a bit optimistic but the overall shape of the curves is good enough if you can get it to stop drawing spikes.

Now to the comment's about that comparison polar. Here's a less cluttered version. The curve labeled EPPLER 334-flap is with an elevon deflected 10degrees. Notice that the straight 334 has a sharp kink in its cl/cd curve and goes to high drag with very little change in the lift. This shows lousy inverted performance. With 10° up elevon it still doesn't go to zero before the rapid drag build up. As mentioned the tip of a swept 'wing is where you hide the tail so you need it to be able to produce some negative lift efficiently. That doesn't necessarily mean a symmetrical airfoil. Notice that the curve for the Eiffel 401 is nearly vertical as it crosses the X axis (0 lift) that's the characteristic you want at the tip for low trim drag

--Norm

[biber]

I don't quite understand why exactly you need a helicopter swashplate for thrust vector.

Just tilt the whole motor and be done with it, I'd say.

biber

[Tom Harper]

Biber,

Shims are good enough for rigging.

I'm considering another model that has only vectored thrust for control.

Tom

[Tom Harper]

Norm,

Interesting - that's a lot of stuff, even for a data freak like me!

I looked at some symmetricals for the tips - NACA and E168. They get ragged when flapped. The Eiffel 401 is a well behaved section. Maybe I should stick with it.

The construction techique I'm considering is to run the balsa sheet over the entire chord with the grain going chordwise. I'll cover it with light weight silkspan. A well sanded coat of sanding sealer will not add much weight. That should give me a very even surface with no waves, dips or discontinuities.

[Tom Harper]

So far, I like the Eiffel 401 for the root section with E168 at the tips.

[biber]

Tom, I mean, just use a gimbal mount and tilt the motor.
Believe me, it has been done, it works. ;)

biber

[Tom Harper]

Biber,

I cut up the swash plate to get a bearing and ball links. Just an easy way to make a gimbal mount.

From your comments, I thought you were against the idea:

"In this application there'll be not much benefit in thrust vectoring, other than the fun in playing with gadgets in itself." - Biber

Have you seen a thrust vector controlled flying wing?

Any details?

Tom

[HugePanic]

i did similar test, but this has never flown....

http://www.rcgroups.com/forums/showthread.php?t=784248&highlight=vector+thrust

the idea was to have vector thrust without the need to tilt the motor.

major problem: the prop blades were cutted koax-blades. so lots of thrust at the workbench, but now speed. after takeof (hand launch) the plane had no thrust.. :D

i didn't install correct blades here...
my idea was to buy a trex tail assy and mount it to a swashplate....
the i could use heli-tail-blades....
with three servos even collective pitch control is possible....

[Tom Harper]

Very clever - thanks.

I think it will be easier to gimbal the motor. Of course, your scheme has the advantage that it would work in a streamlined cowl.

Tom

[nmasters]

Interesting - that's a lot of stuff, even for a data freak like me!

I looked at some symmetricals for the tips - NACA and E168. They get ragged when flapped. If you're considering blending airfoils you would be better off with a thinner section at the tip, 8 to 10% (12% works at this Re but there's more drag and turbs are even more necessary). Something like an E-230 or HN-153s. If the raggedness you're talking about is what's shown in this plot turbulators can smooth it out but, as you can see, at the cost of some drag. It could also be statistical scatter. It's also good to keep in mind that Xfoil doesn't deal well with hinge lines (or any deviation from a smooth curve for that matter). Hay, it's free, you only payed for the nice interface :) XFLR5 also contains Xfoil and some more code that allows it to to analyze a 3D wing but the interface takes a bit more work to figure out

The construction techique I'm considering is to run the balsa sheet over the entire chord with the grain going chordwise. I'll cover it with light weight silkspan.
The sanded silkspan sounds like it should make a nice smooth surface. The grain direction of you sheeting sounds a bit unusual though. Are you saying that the grain will run parallel to the ribs? I always did mine parallel to the leading edge. Seams like it would take a lot of clamps to put it on the other way.

--Norm

[Tom Harper]

Norm,

Thanks.

Long spanwise runs of sheet balsa tend to get dips and waves. I think I can get a stiffer surface by going chordwise over stringers. It will take longer, but that keeps me out of the bars.

Tom

[BMatthews]

Tom, there's a simple way to avoid the dips on your wing sheeting. Presand the sheet before covering then just do the most barely minimal sanding at the glue lines once done. Doing it that way avoids pressure on the sheet that pushes the wood down between the ribs.

Or if you're talking about the sheeting caving in and giving that "starved dog" look then you need to use more ribs to better support the sheeting and covering. For this reason I always use a 2'ish inch spacing on my own designs and cringe if I'm building a model that uses 3 or more inch spacing. And even then the only time I won't modify it for more ribs is if I'm building something that would not be kosher to modify such as an old timer design where everyone would know that I "cheated".

Also using the sheeting with the grain spanwise adds a very significant strength factor that would be totally missing if you go with the grain the other way.

[BMatthews]

Getting back to the thrust vectoring. Your latest sketch shows the motor as being well towards the overall middle of the planform. Thrust vectoring, like aerodynamic controls, work best when they are out at the periphery of the model. In your case the motor is pretty close to the overall neutral point of the planform. This is going to greatly reduce the effectiveness of the thrust vectoring. Although it'll hurt the balance of the model you may want to consider moving the motor back a little more.

For the same reasons moving the fins in to the wing joints between the outer and tapered panels will move the surface area in towards the same point. This means the fins would need to be MUCH larger than they would have to be if they were out at the wingtips and further rearward with respect to the aircraft's neutral point. How much bigger? No idea without doing a bunch of calculations. It would probably be easier to make up a little all sheet test glider and play with the size. Start with them WAY HUGE and cut down with scissors until it starts to show signs of not wanting to be stable in yaw. And to test this you'll want to do test glides where it's launched directly into a sharp bank and deliberatley launched flat but with something like 20 degree yaw angle. If you just launch it straight ahead it may not respond well enough during the short flight time to test for more than just straight ahead stability.

[Tom Harper]

Bruce,

I'll consider more ribs. The rib spacing on the present wing is very sparse.

I've put off the idea of using only thrust vectoring on this wing. I will use it for rigging. The present model is very responsive to thrust offset - even with the motor so close to the CG.

[nmasters]

Long spanwise runs of sheet balsa tend to get dips and waves. I think I can get a stiffer surface by going chordwise over stringers. It will take longer, but that keeps me out of the bars.

Tom

Yep, secondary bending AKA "second order bending" (http://www.homebuiltairplanes.com/forums/design-structures-cutting-edge-technology/2911-foam-plywood-wings-3.html#post18024) is a pain in the butt. I don't think presanding will help because all it would do is make the sheet thinner. Closer rib spacing will help but there is always going to be some "sag" between the ribs because it's actually caused by internal stresses that build up during the bending proses. One way around it would be to soften the resin by soaking in ammonia but that's nasty and slow. A cheat is to just make the skin extra thick and sand it down with a T-bar after the wing is all assembled. Orienting it chordwise may work to some extent but I always thought of the skin of the D-tube as part of the bending structure. Orienting the grain chordwise would take it out of bending but the silkspan would restore some of the skin bending strength. Lots of changes there from one seemingly small change

--Norm

[Tom Harper]

Norm,

I glue it to the LE and let it cure. Then I spray the sheet with ammonia and slowly form it to the ribs and stringers.

BTW: Profili produces distorted ribs for the sparse data of the Eiffel 401 - even after smoothing. So, I'll fall back to the MH60.

Tom

[vintage1]

Bruce,

I'll consider more ribs. The rib spacing on the present wing is very sparse.

I've put off the idea of using only thrust vectoring on this wing. I will use it for rigging. The present model is very responsive to thrust offset - even with the motor so close to the CG.

Indeed. use twiice as many half the thickness. Ribs add little to wing strength, and surprisingly, not that much to weight.. its the sheeting taht absorbs great sheets of tree stuff, and the webs and spars.

Oh, and use a BLOCK to sand with, and sand lightly..with a coarser grit.

[Wendi Smol]

Norm,

I glue it to the LE and let it cure. Then I spray the sheet with ammonia and slowly form it to the ribs and stringers. Tom


Hi

please do yourself a favor and do not do it as above explained.
You ruin your job and waste money.

W.

[Tom Harper]

W,

How come - it's always worked before.

Tom

[nmasters]

Profili produces distorted ribs for the sparse data of the Eiffel 401 - even after smoothing. So, I'll fall back to the MH60.


I may have had better luck smoothing the Eiffel 401 than you did, Tom. I erased points that looked bad in the graphical editor then ran <smooth> again. Anyway after doing that a few times I still wasn't satisfied with it so I played around with changing camber and the other modifications available in profili. After about 15 iterations of that it was still not a great airfoil so I moved it to XFLR5 (with which I only have a few hours experience) and ran the inverse design procedure on it. I don't know what it did, but it did something :p BAM it turned into a whole new airfoil section so I gave it a sequence number and am calling it my own. If you're still looking for a 13% airfoil for your root section look at this. It's pretty good with a trip at 22%. Of course that's just computed

--Norm

[Wendi Smol]

W, How come - it's always worked before. Tom

Hi Tom

allow me to have some doubts about that.

in an earlier post you write that you will consider the construction technique, which is an indication that you have not done this before.

I presume that you plan to use 1/16th balsa for the skinning. You probably also butt- join the pieces before planking to get a continuous panel?
The stringers you will use may be about 2” apart (which BTW add unnecessary weight if they are strong enough to not been deformed by the pressure of the skinning). When you then use ammonia to soften the balsa you will again get the unwanted starved horse effect, only this time the “ridges” will be span-wise.

The proper way to deal with large sheeting panels is to prepare them completely beforehand. If done correctly you will not get S.H. effects between the ribs, even if they are spaced more than 2” apart.
To seal the balsa I use cheap and cheerful wall paper paste, which after drying is sanded very lightly to remove protruding fibres. After drying for at least one day the treated surface is covered with tissue paper or thin fabric using PU lacquer or WBL. One can also use glass fabric (1.5 oz/sqy) and WBL or even better epoxy as a binder. The latter is best done in reverse, i.e. the glass fabric is laid out first on an appropriate surface and the balsa panel put on top. The whole should be weighed down overnight. That is the stuff wings are made of these days. The non sheathed surface is bare wood and easily glues to the ribs and Le / Te. I use Pu glue for the latter.
The wing half in the pictures is made that way and about ten years old by now.

Have fun

W.

[Troy]

On the topic of the vectored thrust, how far in front or behind the neutral point of the wing do you think you need to feel a difference in response? I know it is an issue of leverage so the closer to the NP the more throw you would need for an effective difference. So for a tractor prop set up would the normal motor placement of a tailed aircraft or plank wing be far enough "out" to make an impact?

[Tom Harper]

W,

Thanks, I see the logic. Why do you seal it with wall paper paste? Is that just to keep the balsa from absorbing too much binder?

I have plenty of 3/4 oz glass, but it sets to a rough (egg shell) surface. For a model this small believe silkspan will be lighter and provide sufficient strength. Your opinion?

Tom

[Tom Harper]

Troy,

I don't have good answers to your questions.

On the wing shown above, thrust adjustments are very effective. I speculated that I could use thrust only for control of a similar model. I've postponed the idea. When I get the new model flying, I may try some thrust vectoring experiments with the old one.

Tom

[Wendi Smol]

Hi Tom

wall paper paste is cheap and does not smell; it is also “greener” than solvent based sealers. The water evaporates and leaves a non porous surface.
If you don’t seal the surface it will absorb whatever you put on it afterwards and makes the panel heavier than necessary. If you use the “reverse” glass fabric lay up the porous balsa will suck up so much resin that you end up with millions of micro pores on the outside.
If you are interested in more details I can look up an old article of mine which describes the method like a cooking recipe.

I regard 1.5 oz glass fabric as the best solution for an average model, so in your case I would just cut two pieces of ¾ oz fabric and use them as one.
Silkspan is certainly not lighter than ¾ oz, but the real drawback of all textile materials is their miserable module of elasticity. Glass has a comparatively high E-modulus, so you not only get a stronger material but also a much stiffer one – and that’s the point. The stiffness of the panel when it is bent in one direction (to conform to the ribs) prevents it from sagging between the ribs.

And BTW glass fabric of this sort of weight is made in enormous quantities for the PCB industry and can be had far cheaper than any other fabric.

Have fun

W.

[Tom Harper]

W,

If it is not too much trouble, I'd like to see the article.

Is 1.5 oz necessary? This is a small AP model. No IC engine vibration and no aerobatics.

Tom

[Wendi Smol]

W, If it is not too much trouble, I'd like to see the article. Is 1.5 oz necessary? This is a small AP model. No IC engine vibration and no aerobatics. Tom

Hi

sorry I took my mouth too full, can't find that article at the moment.

To make it short, the principle is very simple:

Take a level and smooth surface large enough to accommodate the panel or even two.
Prepare it for laminating, i.e. wax and put on PVA.
Apply one layer of fast hardening resin and let harden till the finger probe is positive (about one hour at normal room temp.).
(The finger probe is positive when your finger leaves an imprint but the resin does not stick to the finger). When you apply the resin make sure that no bubbles are left.
Have your balsa panels ready prepared.
Laminate one piece of glass fabric (at least 50gr/m^2 or two pieces of 25gr) onto the previously applied resin surface. Use the laminating resin sparingly (for a 50gr glass fabric not more than 80gr resin per square meter) Use a sponge roller for ease of application.
Place the balsa panel with the prepared surface onto the whet laminate and gently roll it down with a rolling pin to remove entrapped air. A good idea is to place a thin plastic foil on top of the balsa to prevent resin to get on the roller.
On top of the plastic foil place piece of MDF or any flat board and weigh the whole down. Let harden overnight (according to resin manuf. instructions).
Gently pull the panel from the laminating surface and carefully wash the PVA from the panel, hey presto - wing skin.

Regards

W.

[Wendi Smol]

N.B.
One piece of 3/4oz (~24gr/m^2)fabric needs as much resin as two layers to be whetted out properly. You add only little weight for a lot more strength.

[Tom Harper]

W.,

Thanks - I've just started cutting ribs.

Tom

[Tom Harper]

The outer panels are complete. LE and TE are 1/32" plywood laminated with epoxy and carbon strips. The root rib of each panel is doubled with a gap for insertion of the launching fins. There is a vertical grain web between the center spars.

The POD ribs are just sitting in place. They are next. The center rib is 3/16" ply for the motor mount. The holes in the outer ribs are for the Lipo bays.

[nmasters]

Ooooo... I just love naked airplanes

[BMatthews]

Ooooo... I just love naked airplanes


PERV! ! ! ! :D Mind you this isn't any worse than Wendi Smol's mention of finger probing..... Reminds me it's pretty much time for a physical..... *shudder* :D

I'll add one little bit to Wendi's excellent write up. If you time it right your composite panels will be cured but still green enough to flex over the wing ribs for glueing down FAR easier than if you wait too long. I made some glass skins in the same manner but without the wood inner layer many years ago and built a test wing using glass skins instead of balsa. It worked well and wasn't TOO much heavier than the wood option despite using 4 oz (about 120 grm/sq meter) cloth. In my case I used epoxy laminating resin which seemed to have a pretty decent "green" working time.

Tom, the frameup looks great. Just be sure your center section has the proper carry through structure to pass the loads from spars in the center out to the skins in the main panels. For the same reason you'll need to glass fillet the joints with glass tape that has a goodly amount of spanwise fibers in it. Such is the results of using what is going to be a stressed skin format.

[BMatthews]

Tom, if you try Wendi's method on THIS size of model I'd suggest you go with two layers of the 3/4 out to a little over 1/2 the span and one layer for the outer portion. Sorry Wendi but there will be a slight weight advantage this way with less cloth to hold the resin. And on a smaller model like this even a few grams helps.

Tom, for the same reason you'll want to pick your sheet stock very carefully. Given the wing area you're going to want to use only the lightest contest grade stock. There's a LOT of sheeting on this and the weight from it will add up quickly if you don't use only the lightest sheet and techniques. In fact the resin is the killer in this method. I'd go so far as to suggest that you lay on the surface coat as mentioned keeping it VERY thin and then do the cloth layup. Then before you add the balsa you wick away as much excess resin as you can using paper towels to blot up some of it. As long as the cloth does pick up a white look to it that won't roller back down and go clear again then you've got enough resin. It'll be a fairly "dry" bond to the wood but if you stop to think that you'll have maybe a few hundred small spots per sq inch that DO bond that's all you need. There's no need to use enough resin to result a full wet, but very heavy, bond layer.

[Tom Harper]

Bruce,

I am leaning toward using Wendi's method up to a point. I'll make and fit the skins and use the light weight filler, but use silkspan/nitrate dope instead of glass epoxy. This is an AP model, not 3D.

Also, I'm considering running the grain at a 50 degree angle to the LE so that the upper and lower panels are crosswise.

Tom

[BMatthews]

Oh Lordy! DO NOT use dope and tissue on only one side of a balsa skin. You think you're avoiding the starved horse look by doing this? Na 'ah.... :D Seriously the dope and tissue will shrink like blazes and suck the skins in like an old guy's cheeks that just took out his dentures. You need to stick to a non shrinking bonding agent such as epoxy, polyester or some other two part resin. And don't even consider something like water based polyurethane. It's got all the stiffening ability of a wet noodle. Nope, if you want to run with that method it's in for a penny, in for a pound.

Crosswise graining the wood will produce lots of other issues. The least of which your construction method is depending on the skins for a spar function. Angling the grain will greatly reduce that ability while at the same time making it hellishly hard to flex the wood around the ribs without inducing some monumental warping. Even if you jig the wing down to allow this sort of method differences in the wood stiffness will guarantee a lot of built in unbalanced stresses that will turn your wing into a pretzel once you unlock it from the jig. The only way that makes sense to use balsa skins is to have the grain run parallel to the airfoil high point line so it curls down to the leading and trailing edges with the least force required.

It's 20-20 hind sight by now or perhaps someone did suggest it earlier but for a model of this sort of size you're far better off with either regular spars and an open frame with some supporting turbulator spars or at most a D tube style wing. Weight at this size is a big issue and full sheeting in itself is overkill unless like I mentioned before that the wood is all selected contest stock. Adding glass skins may well put the project over the top into a higher wing loading than is desireable. Glass and resin skins is typically the province of larger models and high performance gliders.

You've still got options but I'd figure out the fixed gear weights for the RC, camera and drive components. Add in a bit for hardware and then figure out what sort of wing loading you want to fly at a speed you want. Then figure this back to find the desireable final model target weight. Subtract the fixed parts weight and see what's left. Then weigh what you've got for a framework at this point and decide if you've got the room for your planned final steps. Even the skin idea can be tested by making up a single panel set so if it works out you can use them while still fairly green and flexible. But if the skins come off the glass and tip the scale too far then place them aside for some other project and start looking at alternatives.

[Tom Harper]

The present design is:

Area: 3.86 sq ft
Span: 52"
Root chord: 12.25"
Tip chord: 8.375"
Aspect Ratio: 5.4:1
MH-60->E230
Motor: Heads Up RC - 2830-10, 1000KV, 27 oz thrust @ 14.5 Amps
Battery: 2 3S1300mah Lipos in parallel
Camera: Olympus FE 360

Wing as it sits - 6.5 oz
battery 8.0
attopilot 6.0
reciever 1.0
camera 4.6
motor 2.0
esc 2.0
servos 2.0

AUW, at present, is about 32 oz. No gear, just the launching skids discussed above.

The performance required is level cruise at 60 kph.

Running the grain at opposing 50 degree angles should provide torsional rigidity. The present model has a 'D' section, but with a much thicker airfoil.

I could use Polycrylic, but I agree that it is rather flacid. Could set up the silkspan/nitrate under a weighted plate ala Wendi. Could use butyrate dope instead of nitrate. Could plasticize the nitrate with castor oil.

The options are still open for a while.

[BMatthews]

Dope doesn't stop shrinking for years. And you can't clamp it while wet or it won't dry. And I've never seen plasticisers do much other than slow down the shrink. Eventually it catches up with you. I heartily recomend not using a dope and tissue skin over the sheeting. But its your nickel.... :D

If you're able to keep the weight under 40 oz you should have a winner. I'd also think that there's room for the resin and cloth method with that sort of weight.

[Tom Harper]

Is there a special resin for this sort of thing?

[Wendi Smol]

Is there a special resin for this sort of thing?

Hi Tom

yes - it's called EPOXY

[Tom Harper]

LOL - there's as many to choose from as there are hair sprays.

So, it's just anything with a reasonable working time?

Tom

[Tom Harper]

Here we are agonizing over section shapes while this guy just uses steps:

http://diydrones.com/xn/detail/705844:BlogPost:68076

Looks like:

48" span
2.67 sq ft area
15 ounces weight (20 oz max)

At the minimum weight the wing loading is 5.6 oz per square ft - that helps.

[nmasters]

There's not much real data on stepped airfoils. The only wind tunnel work with them that I [know] of was done with a variety of step heights at 50%c. All of them produced higher drag that the comparison airfoil. Some produced more lift and a slightly higher L/D in a very narrow range. The stepped airfoils that are being used on some fomies these days are deffinatly better than the slab-wings that are so common on that class of model but I haven’t seen any convincing evidence that they’re a good replacement [for "real airfoils]" on low powered endurance models

--Norm

[Tom Harper]

At that wing loading, anything will fly if you get the CG in the right place.

[Tom Harper]

Built the boxspars to hold the Lipos and covered the inner panels with 3/32" balsa. I need to build the camera mount, POD hatch and elevons before I tackle sheeting the outer panels.

Tom

[Tom Harper]

Camera mount and framework for hatch. I need to add the pitot tube mount and status LED before I forget them.

[Brandano]

Hmm, I'd glue a sanded EPP fairing to the nose cover.. :)

[Tom Harper]

Oh, you mean stuff to keep the wind out?

[BMatthews]

Yep, EPOXY is the best resin. I'll just add that you're going to want to get the laminating resin epoxy rather than using any of the "glue" epoxies. Laminating epoxy is a lot thinner and runny than the epoxies that are formulated to work as glues. This thinner makeup allows it to spread and saturate the cloth much more easily and completely.

Since you'll be clamping the sheets onto the glass with a barrier and weights do not go the route of using a glue style and thinning it with alchohal as you may have read. The non stick barrier between the skin and weights will prevent the "thinner" from evaporating and it'll never harden or will harden with lots of issues.

Any boat repair outlet that caters to glassed strip canoe and kayak building or "stitch and glue" boat building will have this sort of epoxy. One commonly known version is the West System brand but there's lots of others. For mail order any of the companies selling kits for those style of wooden boats will also have the correct sort of resin kits.

I have not been following the advances in small digital storage movie cameras for the past couple of years. I was shocked when I wandered over to the camera area in the local "toy store" the other day and saw the range of sub $400 options that were not much larger than a candy bar. Some of them were actually under $300. These may not seem like good prices to the US folks but for up here this is really CHEAP! I may well be joining you in making up some sort of photo/movie carrier design sooner than I thought.

[Tom Harper]

I'm tempted, but the prices are still beyond what I am willing to pay.

BTW: I did get the wall paper paste filler. Mixes 1 cup of water to 1/2 oz od wheat powder (250ml to 15 gr) for what may be a lifetime supply. I tried it on some scraps and it seems to work as a sealer.

I'm still not convinced that a model of this size and weight warrants glass epoxy. I am leaning toward silkspan and polycryllic.

There is an added benefit to Norm's suggestion of a tapered POD. The hatch is nicely aligned by the taper. I can hold it in place with a single screw.

[BMatthews]

Go ahead and give it a try for something like a half sheet of 1/16 x 4. Once it's dried I think you'll agree that it has added about as much stiffness and strength as a sheet of overdone lasagna pasta. But at least with the polycrylic on an electric model you'll have a nicely prefinished sheet of wood for the skin. The only issue will be if you choose to try the glass plate and weight method that it won't dry for literally days and days all the way to the center since the polycrylic needs exposure to air to dry. Really as a finish at that point you may as well just sheet the wings and then apply the tissue and poly' to the model as a finishing medium rather than as a structural skin since that's really all it'll be. And if you were to use colored Jap tissue you'd be adding a color scheme at the same time. You'll want to use honest to gosh tissue from Pecks or FAI Model Supplies for this since the stuff they sell has wet strength when wet from water. Although since there isn't MUCH water in the Polycrylic you may get away with art store tissue. You'd have to try it.

[Tom Harper]

I've done the polycrylic experiment and agree that it does not add much stiffness. I also used it to bond the carbon strip on the LE. It is not as good as the epoxy I used on the TE, but it is adequate. That's the issue. The optimum solution is one that is just adequate to the task.

I'm using 1/32 x 3 sheet and GM silkspan. I think the advantage of bonding the panels before assembly is to add stability. So, I'll air dry the polycrylic before I glue the panels to the wing.

[Tom Harper]

MicroDan motor arrived today. Like the mount inside of the bell diameter.

The POD nose is roughed in and started on the rear hatches. Need to cap strip the hatch seam. Motor mount is complete but not glued.

That's it fo today - picnic tonight at the bat caves.

Tom

[BMatthews]

I'm starting to feel like I'm right there at the bench with you and I'm getting all excited to hear about how it flies.

Spent a bit of time today looking at some of the SD chip and micro hard drive camcorders. Still not sure which way I'll run with this but the price of the SD cheapies is sure attractive given that I'm much more a still picture sort than a movie sort so whatever I buy will be purely a toy for model and other action hobby giggles.

[Tom Harper]

The POD is roomier than I had planned - not all bad. Routed the battery cables. I'm working on the motor drive wires. Finished the stringers on the bottom of the outer wing panels. They don't add much strength at this point but they make the structure easier to handle.

Need to work on the elevons and servo mounts.

[Tom Harper]

Finished the elevons and servo mounts. Soda straws are for antenna and servo wires. I think I'll finish the sub-fins before sheeting the outer panels.

Weight of the airframe is now just under 10 ounces. The estimate/budget is 15 ounces. Should be close.

[Tom Harper]

Finished the edges and glued the 1/32" sheets together. Came out OK but the glue joints are more rigid than the sheet wood.

Then I brushed on the wall paper paste sealer. That caused it to roll up like a mailing tube - almost. I took it outside, in the shade of the portale. New Mexico is 100 degrees and 8% humidity at present. After a couple of hours with the edges weighted it was dry, but with a definite curl. So, I pressed it between flat boards overnight. That worked.

Early this AM, I brushed on the polycrylic and laid a sheet of ironed GM silkspan over it. Then worked in the polycrylic with the sponge brush and weighted the edges with scrap strips. Initially I put a strip across the center of the sheet - bad idea. The edge weights seem to be sufficient.

Per instructions on the can, sanded surface after 3 hours and applied second coat. Will repeat for a third coat.

Looks like about 1.75 ounces per panel, finished.
Tom

[Tom Harper]

The panel set up flat. I'm pleased with the result. Now to build 3 more panels.

[Tom Harper]

Finished paneling the right wing. Not a bad result, some low waves, but the best I have ever managed. It is extremely strong and torsionally rigid. Especially considering that it's 1/32" balsa.

I came up with a modified Wendi procedure:

Edge glue the panels
Apply the wheat flour sealer and sand
Apply the silkspan and three coats of Polycrylic
Flatten with weighted boards as it cures

I time it so that I apply the sealer in the evening. In the morning I apply the silkspan and then the Polycrylic coats 3 hours apart. Three hours after the last coat the Polycrylic is dry but not cured so it can be weighted overnight. Perhaps not as good as epoxy, but it sets up flat.

Tom

[Tom Harper]

Should get the last panel on by tomorrow.

[BMatthews]

Your procedure sounds like a nice compromise if it works out long term. I hadn't thought about letting it dry and then weight it once dry but still flexible.

The model is looking great and I'm looking forward to the flight reports.

[Tom Harper]

It's looking pretty good. My weight budget was 15 oz for the airframe. I'll probably hit 20 with the tip plates. Still not bad.

Did you get your video camera?

Tom

[BMatthews]

No, I did a bit of looking around but then other things drew me away. Far too many hobbies. I'll get back to it now that I know they are out there and cheap but for now I'm trying to get caught up on a bunch of dragging on projects.

[Tom Harper]

Finally got the last panel finished and on the wing. On part 0f that panel I tried the 50 degree scheme - it really works great!! That's the way I'll do it next time.

[Tom Harper]

Working on leading edge and elevons.

Weight of basic structure is 17 1/2 ounces.

[airmcn_3]

Working on leading edge and elevons.

Weight of basic structure is 17 1/2 ounces.


Big surfaces!! Tom you are a great builder! Looks good. Can’t wait to see Atto in it and flying.

Good luck! You seem to have done your homework.

Chris

[Tom Harper]

Thanks Chris - this one gets the Atto IMU.

Tom

[Tom Harper]

Got some time to work on it. Installed the sub-rudder skids. Finished the 3/32" balsa sheeting of the POD and inner panels. Covered the exposed balsa with silkspan then gave the POD one coat of sanding sealer.

Weight is now 19 ounces.

[Tom Harper]

Photo 0 shows everything in place with main hatch open. The hatch cover is tape hinged on the LE. Lot's of room.

Photo 1: Rcvr bay. The retainer screw for the main hatch is inside the rcvr bay. You can see it just below the rcvr case. The receiver is an old Airtronics with it's PC board modified to take Z and Futaba servo connectors. Also, you can see the tip of the soda straw that holds the antenna wire.

Photo 2: All buttoned up.

Photo 3: Aileron with tape hinge.

Photo 4: Ready to go.

With the camera in place the CG is at the rear most position. I will use lead ballast for first flights and move the CG further forward. Will probably always require an ounce or so of ballast.

AUW, as shown, is 38.25 ounces.

[Brandano]

That would be a shame, IIRC one of the reasons to chose a flying wing was to reduce drag and improve endurance. Ballast weight might negate part of the benefits of the planform. Any plans on making one with a bigger sweep if this one turns out well?

[Tom Harper]

First - I'll move the motor forward and down a bit before the first flight. The ballast required should be small.

Second - I'm starting to sketch the next model. Considering lighter Lipos. A little more sweep may be necessary. However, CG prediction is iffy for balsa construction of a new model. I thought I did rather well on this one.