Hello:

I am building a sport scale version of a caproni ca.42. IT is a triplane bomber with three engines, the middle one being a pusher and the others tractors. THe wing chord of all three wings is 5.5" and the spans are all 72". The scale tail size is about 4" chord and 28" long. On the original the tail had a similar airfoil to the wing. I am using a simple flat bottom airfoil for the wings. Should the tail have a similar foil or should I just make it flat like most sport planes. Also, is it large enough to stabilize the plane?

SHould the wings all have the same incidence? I was thinking of making the middle wing, bottom wing and tail zero incidence and the upper a degree or so of negative. THoughts?

Jeff

That's a flying drag farm if there ever was one!
All those wires!
A slab horizontal won't be a handicap in that size. You could add some chord to make it more efficient
Setting all the wings at zero won't hurt anything.
The upper wing will create a noticeable requirement to hold down-elevator in flight due to the drag above the c.g.
How much power are you planning?

I'm no so sure the lower wing drag won't cancel out the upper wing drag. The overall shape of the "stuff" seems to put the center of drag for the wing stack at the middle wing or pretty close to it.

To best evaluate the ability of the tail to contol all that "stuff" you need to do a tail volume coefficient calculation but I'm not sure how to put all that area into the figureing since the area AND chord are part of the calculation. I'm going to guess that you need to treat it as a short chord wing with a HUGE span to get the area and ignore the fact that the area is folded over each other in that triplane configuration.

There is some tail volume coefficient calculators out there that use Xcel spreadsheet or you can do it manually at a lot of other sites. I had some links but they went away a couple of days ago when my hard drive died. Do a google search for tail volume coefficient calculation or calculator to see if you can find them. Most of the sites with the TVC info also include what the desireable range of the value should be.

I agree with the above and would suggest that since the real airplane was basically a slow flier that scaled areas would be OK. If you can find the full scale CG location I would feel comfortable in using it.

In doing stabililty calculations I think you would consider it to be a wing with 3 times the lift of a regular wing with the span and chord of one wing. The existing horizontal tail is about 3 times the area needed to stabilize a single wing especially with that moment arm. You have 3 verticals of a size that would be adequate for one wing, fuselage and small horizontal.

It looks like the original designers just multilpied everything by three!.

That rigging job would take the patience of Job! The drag was probably measured in tons.

Yep, if this thing is built reasonably light I doubt it could achieve a high enough terminal velocity to hurt itself in a crash with all that drag....

a "flying drag farm". I like it

The tail is about 9% of the total wing area but it is 4 chords back so it has a bit of leverage.
I would go for all 3 wings thin and slightly cambered at +1 degree and a flat plate or thin symmetrical tail at zero, and a CG about 20% to 25% (my formula gives 23%) of the wing chord.
and the best of luck keeping it all lined up and warp free.
Alasdair

THanks everybody.

I expect I can build it somewhere in the neighborhood of 7 1/2 to 8 lbs. I am building it as lightly as I can, especially in the back end. I am also stretching the nose a bit to get the battery as far forward as I can. I have several options for power. I hope it will fly with three saito 30s. One of them flys my DH2 nicely. Here are its specs:

Wingspan - 44"
Chord - 8"
weight: 4lbs, 1 ounce.

I figured the triplane has less than twice the wing area, twice the weight, more drag. Three Saito 30s should fly it (in a scale like manner!)

If not, I can put an RCV .58 in the middle or even a Saito .72.

My next hurdle is to figure out how to engineer the three full flying rudders. Any suggestions?

Working out the tail volume coefficient from the top view, it's .06. This is a typical value for an early airplane where most attention went to the wings; lots of area, short tail moments, tiny horizontals.. like my Camel. Nowadays it's so far from the minimum recommended value of .25 that we aero-pundits would throw up our hands in horror!
(and that's using only a single wing's area.)
As WWI stuff does fly, it shows the limits aren't hard-wired and are only practical suggestions.

So far it's looking pretty good. You've got 1188 sq inches of wing area but it's not THAT large a model. If you can keep the weight closer to the 7 lb mark or even less then I think you'll have a nice slow flying model that 3 30's will fly in fine form. The engines don't have much moment arm to help balance the model but then you've probably already figured out that it'll be VERY important to keep the tail light.

I thought there used to be an online Tail Volume Calculator out there but I can't find it. But I found this hand and calculator method.



I've edited part of the above to include the idea of the reference points being the 1/4 chord points as seen elsewhere. This version actually used the leading edges which is wrong by other accounts.

That gives you a TVC = (112/1188)x(20.5/5.5) = (.094)x(3.73) = .35

This is actually a LOT better than I thought it would be. I think a large part of this has to do with the fact that the average wing chord is so short. A ratio of wing to stab area similar to what you have is USUALLY the kiss of death. But the short effective chord as a result of the stacked wings works in your favour in this case. .35 is right in the ball park for "normal" airplanes.

Subbing in to find the CG placement you get 16 + (36 X .35) = 28.6% back from the leading edge. A slightly forward CG but given the small stabilizer quite respectable. To keep it safe for your first flights I would recomend 25 to 26%. One thing for sure, if you try the "usual" 1/3 back the model WILL bite you. There is no way of avoiding the fact that it has a very small tail area for all that wing. Knowing that the CG will be this far forward you'll want to take extra efforts to keep the tail super light.

This means your hinging system for the verticals will need to be done with things like thin wall carbon tubing and aluminium tube slip bearings. No steel wire or brass tubing for you ! ! ! And since there are three of them I would consider things like strip laminated outlines with ribs. And pull-pull controls made from Spectra line or similar will save you a lot of weight over pushrod systems. As complex as the controls would be in this case I really don't think you want to try putting servos in the tail for this one. Putting the elevator servo just behind the engine on one outboard boom and the rudder servo just behind the engine of the other boom would be best IMHO.

Even if you are careful with your design and control system work I suspect you'll still be adding a lump of ballast to that short nose pod.

Doing some quick figuring and knowing how I like my old timer RC assist models to fly I get a few numbers for you to consider....

You've got 8.25 sq feet of area total. At a light loading that will fly like an old timer of 12oz/sqft you want the model to only weigh just a bit over 6 lbs. At 16 oz, which IMHO will still fly slowly but not look like it's floating, you need to keep it to 8.5 lbs total. At wing loadings over 16 oz/sq ft you'll loose a lot of the "look" in the air since it'll tend to fly faster than a WW1 model of this sort should. Of course all this is subjective but in my case I like my WW1 stuff to fly slowly for a scale look.

Best of luck with the rest of the build. As you can see from all my figuring and time to type this above you got me all enthused with this one. A truly unique project and I wish you the best of luck with it's outcome.

PS: In thinking about your power situation and recognizing that this is not actually that large a model I wonder if you could not get by with just the two outboard engines and just put a free wheel prop on the center rear engine position. That would certainly help to control the overall weight in general and the nose ballast issue in particular. To power a lightly loaded 600 sq inch model with one Saito 30 is not unresonable at all and that is what you would basically have with only two engines in this. Loosing a pound or more worth of engine and fuel tank and the perhaps another pound or more of pod ballast to balance these rear mounted items would go a long way towards helping with the flying speed and overall weight issues. The resulting lighter model should easily be able to fly with the 2 engines.

That CG formula you quoted
CG [in % back from the wing's LE] = 16 + (36 X Tail Volume)
is for regular monoplanes. So is the even better one I use
CG [in % back from the wing's LE] = 10 + (40 X Tail Volume)
(That one is better because it gives better CG's for a wider range of aircraft, especially WW2 scale types).

This is far from a regular aeroplane. It has 3 wings and lots of rigging wires so the assumptions used in making up the formula are no longer valid. This aeroplane will have more downwash over the tail and there will be a greater loss of dynamic pressure (airspeed) over the tail.

Therefore the CG has to come forward to 20% to 25% of chord for in itial test flights at least.

Alasdair

Alasdair: THere won't be many wires. Only the very minimum necessary. With the flat bottom air foil and struts that passfrom bottom, through middle into top wing, it should be fairly stiff. I think I will need wires to brace the landing gear and maybe a single set of landing and flying wires.

Bmathews: Wow!

IT looks like it might fly afterall. Since you are obvioulsy enthused, here is a bit of background. I am building this for a new event called the Dawn Patrol Games at Charles County RC Club in Waldorf MD. I have built three profile fighters and the guys responsible for putting it together wanted some bombers. One guy is making a Handly Page 0-400. THere are designs for a Gotha as well. I always like to be a bit different so I am building this glorious thing. In the process, I have gotten really interested and picked up some good reference books and made contact with Gregory Alegi, a Caproni scholar. I am seriously considering making a 1/8 scale (detailed, all the wires, fittings etc) version with a 12 foot span and scale airfoil. It must have been an amazing thing to see rumbling down the runway and floating into the sky. They were not terribly standardized and even though not that many were made, there were many combinations of engine, tail shape, armament etc. THey used Liberty, Fiat, Isotta Fraschini and even rotary engines.

Just built the third wing.

Yeah, the Caproni should blow their minds away...

In my early days of modelling I read a story in, I believe, American Modeller. They used to have the odd historical story and the one in question was an account of a pilot flying a Caproni on a raid. It's always stuck in my mind because the airplane was almost described as a floating castle of wings and wires. The sketches with the story didn't do much to shake that belief with the huge size of the Caproni. Of course a lot of the WW1 bombers were very large. I remember reading another account of the Illya Mourometz (sp?) Russian bomber where mechanics would go out for a stroll on the wing to check the engines as the aircraft blazed through the air at a heady 50 mph.

Be sure to keep this thread updated with your progress and flight report. Or open a new one up in the Scale forum and post a link to this thread. I'm sure there's many of us that will get a kick out of your efforts with this one.

And do consider the twin engine option. I honestly think you'll gain more through the lighter weight than you loose through chopping the center rear engine. If nothing else at least you won't need to stick your hand in there.

Yep. I've been running the numbers in my head. The twin engine option saves me about 10 ounces that would be centered about four or five inches behind the cg. Also a small servo plus linkage, about a half ounce. A lighter structure with no firewall, at least another ounce. Fuel system - maybe 2 ounces empty (mostly at the cg). Probably at least 12 ounces out of the nose. If two engines will fly it I don't see how I can justify the third engine (I would love to hear three Saito 30s humming along slightly out of phase!).

The Sikorsly Ilya Mourometz was another one that I considered but I don't think I could possibly balance it. In fact, I don't know how it flew. They had crewmen way back in the aft part of the fuse. THe tail area is huge and the engines sit just about on the cg. Very short nose. Must have hired pilots from teh Sumo Wrestling corps!

THanks for all the help.

Jeff

Bruce & alasdair, wouldn't you want to reduce the total wing area figure used in the T.V.C. computation due to the inefficiencies of the triplane wings?

I naively/arbitrarily/experimentally/successfully used a figure of 85% on the last biplane I cobbled up and it flew great. Granted, it was small & light (250 sq. in/8 oz.) but the theory sounded right to me at the time and the resulting flying characteristics sure seemed to back up my possibly flawed thinking.

Well, in a way yes.
Maybe I haven't reduced it enough.
I make the Tail Volume Ratio 0.36 (a tail 9% of total wing area on a tail arm of 4 wing chords)
But when I put it into a CG formula I reduce it, by an arbitrary amount. Three wings cause almost three times as much drag and downwash as one, so to be on the safe side I could take only a third of TVR into account.
That gies a CG at 16% chord, so I would like to revise my CG recommendation to a range of 16% to 20% of wing chord.
Alasdair

Downwash on this model?
I know that the old boys like to use that as part of the equasion - but on a scale model such as this -- how can one accuratly figure "downwash???????

If I do use a third engine mounted as a pusher blowing directly across the tail, would that mitigate any downwash effect?

I thought of that too but then I thought that if the total wing area was kept at the max then it would better represent the worst case scenario. If the "equivalent" wing area was used at the wing area variable then the TVC would have been larger and suggested a more rearward CG location. So

Alasdair, reducing the TVC arbitrailiy by 1/3 AFTER calculating it would represent a much more harsh safety factor than the poor wing area to stab area would indicate it seems to me. I know that you're considering the downwash stuff but I'm not sure that needs to apply the way you're doing it. The idea of the TVC, I thought, was to keep the Cmo of the wing in control. And the Cmo effect of the wing is related to the leverage provided by the wing chord moment. So in effect the TVC calculation IS taking into account the downwash, or Cmo, effect. That is also why I considered this model to have an equivalent "one wing" chord of 5.5 inches but an 18 foot span wing. This would keep the effective forces of the Cmo the same but still relate it to the overall wing area. This was made easier because the wing stack is pretty much centered on the thrust line. If there are stacking effects that change this I am hoping that by using the total wing area rather than a fudged effective area the results are still conservative enough for safe flight. If there are other factors for multi winged models I can't seem to find them on the 'net.

However in amongst all this concern for jwc's model I think there is a sound message that the balance point should be much further forward at least for the first flights than standard doctrine indicates. It's a crap shoot with my original finding suggesting 28% and your fudged factor suggesting as forward as 16%. A nose heavy model can always lumber around the circuit and land for re- adjustment but a tail heavy/unstable model often doesn't make it to the first turn. So in light of that a 20 to 23% initial balance sounds safer but I'll be the first to admit that this is an educated guess.

I know it's common to consider equivalent wing areas for biplanes in wing loading considerations I'm not sure that this applies to TVC calculations. If it does then where does this fudging take place? Like I said before, if you do the fudging in the first equation it produces a more rearward CG which tells us we are being safer than need be. If you use it on the TVC value itself then it indicates we need a more forward CG. If we just use the numbers as is we end up in the middle ground. Granted I'm guessing on this but in doing a quick search for info on biplane and tail volume coefficients using a few different terms I turned up a big goose egg on the net.

So I toss my own coin and suggest that a starting balance of 23%.

Once flying an evaluation of hands off dive recovery will indicate how much to the rear you can move the CG. A more rearward CG that does not compromise stability will enhance the lower speed flying as the wings will be supporting the weight more efficiently and the power on to power off pitch up effect will be more tame. A nice profile to shoot for on a model of this type is a mid throttle 45 degree dive that recovers to level (and then noses up again) within 100 to 150 feet of altitude loss. Any signs of a speed related nose tuck (requireing up elevator to recover naturally ) or an initial flat and steady dive recovery that sharpens up as the speed bleeds off is a sign that the CG is probably as far back as you want. The recovery path should be a fairly smooth arc once the controls are neutralized if the balance point is at a safe spot. Keep retrimming the balance back and elevators for level mid throttle flight and then do the dive test again until you notice some of the bad things creeping into play. At that point you're as far back as you want to be.

Fortunetley this will be a slow flying model and slow flying models don't respond in a super harsh way to CG's being back a smidge too far. Many fun fly models are actually flying with CG's behind the neutral point which is usually a no-no in any higher speed models. The pilots need to be constantly on the controls to fly these but if it was a faster model it would probably be uncontrollable. So there's a safety factor in the slow speed as well.

Having said all this the narrow chord and long span will be sensitive to tip stalling in tighter turns at low speed. JCW, you WILL want to have a fair amount of washout in those wing tips. I'd say, given the short chord, that 1/8 to 3/16 trailing edge up at the tips would be good. Even then practice your slow speed close to stall tight turning about 4 mistakes high at first.

Bruce,
I agree with all your practical advice, but I'd still go for a furthest aft CG at 20%.

Just to clear up the theory, when we calculate a CG what we are really doing is trying to calculate the Neutral Point and then placing the CG a suitable Static Margin (Stability Margin) ahead of it, right?
The formula for NP contains the term Tail Volume Coefficient TVC (as V-bar usually) and also the term (1 - DWF)
where DWF, Down Wash Fraction, (the differential de-epsilon/de-alpha) is how much the downwash is compared to AoA (measured from zero lift).
So (1-DWF) is what is left of a notional angle of attack increase by the time it gets to the tail. i.e. if the tail gets an addition 1 degree AoA the tail gets 0.3 or 0.5 or 0.7 degrees or whatever.

When we calculate pitching moments, lift etc. we have to use the whole of the wing area. We also have to use the whole of the tail volume. You are quite right.

When I said I reduced the TVC, what I am really doing is reducing the (1-DWF) term because on a triplane I believe it will be about a third of what it would be on an equivalent monoplane. So I use the monoplane CG equation, but divide by 3. The (1-DWF) is an invisible built in factor, so I said I was dividing the TVC by 3 instead.
What I should maybe have said is that for a monoplane I use
CG = 10 + (40*TVC)
for a biplane CG = 10 + (40/2)*TVC
The biplane one works well
so for a triplane I would use CG = 10 + (40/3)*TVC
but I have never built a triplane or even checked it on a triplane.
Those figures are designed to give a 15% Static Margin.
On this triplane I would maybe reduce that to 10% or 11% to reduce the tail download, hence my original 16% CG position modified to 20%.
Alasdair

Now that I'm following your thoughts I tend to agree with your 20% starting point. We are both sort of making it up as we go along but I like your reasoning and it's always better safe than sorry. And at only 9% tail area and not that long a moment arm it IS short and small coupled. I was acutally expecting something more like you are suggesting in the first place but reasoned that the short chord distance was overly compensating for the small tail volume.

Once flying SAFELY the balance can always be set using the dive testing method in any event. Or if the model does not have the elevator authourity to actually get airborne then that is another clue that the CG is being set too far forward...

Don't laugh, I saw just that situation one time. A trainer with minimal elevator throw and an overly cautious forward CG based on control line practice. At least it was only funny and the model flew well once the balance point was re-located.

Thanks all:

I am moving this conversation to the scratch building forum. Please feel free to continue there or here.

I built the tail surfaces last night. I increased the horizontal stab area to 10.2% of the wing up from 9%. The rudders are more lightly built than they look and use carbon fiber rods, aluminum tubes and nylon bearings. They will be installed just above the stab surface (they are sticking up in the photo because the tubes are too long.

I am going with two Saito 30s for now. I can always build a second center pod if I want to try three engines.

One last question: If I use a freewheeling prop in the center, what happens if I dead stick? Won't it act like a parachute?

Jeff

You've got 3 wings, two other dead stick props, struts galore, three rudders, gobs of wheels, a suitcase hanging from your lower wing and three fuselages...... and you're worried about the drag of a free wheeling prop??????



Use a similar diameter but very high pitch and the drag won't be that bad. Or only put it on for static looks and remove it for flight.

So what your saying is, I should nose it over and gain as much speed as possible and hope that it makes it to the ground while I am young enough to enjoy it?

Well.......... YES

ROFLMAO

Er, you aren't a senior citizen are you?????

Do you know where the full scale airplane balanced at or is that one of those things lost in the dark ages. Alasdair, does your equation work on the Fokker Triplane??