Well, I'm checking out on the technical end of things. It sounds like we have all the puzzle pieces and need to assign project tasks and responsibly. Something like:
* Operations and aircraft handling
* Powerplant
* Structures
* Fuselage design
* Wing design

It would be a shame for all this discussion to amount to a debate. I would like to see a design.

Ilikeplanes,
I like your suggestion but don't know enough about engines and their fuel consumption. If someone can start me out with a suggested engine and prop, its fuel consumption at various throttle settings and the associated static thrust at various throttle settings, I can put up a straw man aerodynamic design for anyone to comment on. Then it will be necessary to design structures to meet the weight and strength criteria.

That reminds me that a camel is just a horse that was engineered by a committee.

Bang on ma' man!!!!

We need some fuel consumption versus power info before we can go ahead. I know the old bit about static versus flying thrust but to get something to work with I think we could use some thrust at oz per minute figures. With this we can figure out the maximum takeoff weight of the model and then we can decide if there's enough weight difference between full to empty to determine if the model can run long enough to have a stab at the prize.

I think DT's been absent because he's running engines........ I hope

yes i am I have a new sato 72 4 stroker on the bench now still in the break in stages but will get some starting #s soon I cant run it with the wife home my workshops in the basement sure makes lot of smoke . ant shes not very happy about that .
It will have a 14x6 prop to start I am trying to figure out how to get the thrust #s ?any ideas

I would try a 13 x 6. The 14 might be a bit big for a 72 and lug it down too much, a lot of people run the 14 x 6 on their 90's.

I have run some static tests with a digital fish scale on some enignes. The way to do it is suspend the engine such that it is free to move backwards and forwards. Then hold the engine back with the fish scale and thus you will have measured the static thrust. You can get the RPM from a tach.

You do have to be careful. The motor will try and torque off to the side. I do have a NIB 72 I am going to put on a Kyosho Gee Bee. The plane is not mine but I'm building it and going to fly it.

I might as well run some static tests on it. I'll post them as soon as I can. I donated the last test mechanism (minust the digital fish scale) so I will have to build another one. Give me a couple of days and I will have some results. The only problem is that I live at 4300 feet so you will probably get a little more thrust and be able to fly a little slower at lower altitudes. I can also run some fuel consumption tests as well. I usually run omega 15% fuel.

I'll see if I can get a hold of some different props to try. I already have the 13 x 6. How about a 14 x 5, 12 x 7...... what else. We could test very low pitch props. The problem I see is that with static tests the lower pitch props will win, but at speed and flight altitude the engine will have to run at a higher RPM to fly at the most efficient speed and probably burn more fuel. I do remember reading about another duration event where the prop had nearly the same pitch ad diameter. The plane was very sluggish on climbout, but held speed at a lower throttle setting.

Let's hear some testing suggestions, and no......I think it would look a little crazy to drive down the road with the motor, digital fish scale and tach mounted to the top of the truck to get some dynamic results.

Ahhh......more variables......give me a week and let's see what I can accomplish.

You might mount it on a rolling platform like a skate board. Then secure the skate board with a length of sturdy line so it can't roll off the bench but can roll a short distance. Use a spring scale, like a fish scale, to pull the board against the thrust enough for the safety line to go slack and read the spring scale. Take the corresponding RPM with a tachometer and record the time to run out a full tank at that throttle setting. Repeat the three readings for various throtle settings including full idle.

BTW, I suspect that the engine will work best for duration with a model that spans around 14 feet.

The "skateboard" was my first thought too. But from my running an older OS 75 I'd say you want to build it up yourself. There's enough torque off these engines that I'd be worried about the loose skateboard flipping sideways.

I was thinking of a plate of plywood or chipboard about 16 x 16 with hard wheels about 3 inch in diameter. You'll be running this a lot getting info so some sort of metal channel screwed down to another slightly larger board to hold the wheels wouldn't be a bad idea to prevent the truck from slipping around. And to add some safety I'd bend the channel up at the ends or add 2 inch high bumpers of more wood so the truck can only move about 2 inches back and forth. Or less if the digital fish scale doesn't move like a spring scale does.

While you're at it why not get a couple of saw horse brackets and make the whole lot up so you can break it down. This way you can get out to some industrial area where they don't mind a little noise on a weekend and keep the house clean. I love my models but I'd draw the line on MYSELF if I thought about smoking up the house That smoke is largely oil fumes and mist that will settle into a sticky film on everthing around the house..... especially the amount of fuel that 74 is going to use.

And yeah, the model is going to be a lot bigger with that engine. Going back to Maynard Hill's article he did a lot of experimenting with oversized props. I think the idea was to get the engine working at it's torque peak rather than it's horsepower peak. It's all about trading off power for fuel consumption. RPM's are certaily worth recording but the bottom line in this case would seem to be the thrust versus fuel per minute. If one prop can offer more thrust for the same fuel consumption then that's the winner. Get a bunch of figures on the best prop for some given thrusts and then later, when the model is being tested, I'd start with the highest thrust prop and if the model, ballasted to full take off weight, can take off then it's on to the next one. Keep going down the list until the model is doggy and you're worried about how it'll do. This would be another reason for an early start. Our models, the engines and the props just work better when it's cooler thanks to the denser air. Give the model a chance to burn off some of the fuel while it's operating at it's highest efficiency on a prop one step beyond what it would fly with later in the day when it's warmer.

DT, obviously you have a duration in mind. If it isn't better than the last winner's then you're a non starter. And the target would probably be 15 to 20% longer for a safety margin I'd gather? So how long are you going to try for?


I had a thought the other day about this....... I seem to remember seeing some info that twins can offer more performance than an equivalent single in our model sizes. Something to do with the efficiency of two prop discs rather than one large one. If so then this could offer a whole range of new possibilities. A few that popped into mind were a trimotor and a four engine. The tri motor would use a larger center engine with helpers in the wing pods. All three would run for the first thir or so of the flight. At some point where the single engine could handle the flight then the pilot could cut off the singles. Folding props on these would cut down the extra drag. The four would use roughly the same strategy but in that case the inner two would carry on. The selection of a triple of quad would be based on the fuel versus thrust of the larger single versus using two smaller engines.

Out there or feasible option? Whadda ya think? Could you imagine the looks in the eyes of the others if you showed up with a big C130'ish looking model ?!?!?!?!

Oh, and another thing. Most of the past duration models of this type relied on gravity feed off the large tank because it's just not feasible to use a fuel pump and carry the extra batteries. But what about a CO2 capsule and regulator? 3 or 4 psi into 2 liter pop bottles would deliver the fuel in a fine manner. And I know from people doing other silly things with pop bottles that they are good for almost 100 psi.

What fuel are you using? Nitro helps with power but it's not "fuel" in the proper sense so it actually increases the richness you need as you replace the volume taken up by the nitro with a richer setting to restore the alchohal mixture. Not sure how the engine would run with it but if it can run on straight meth/oil you'd be better off. Similarly if you could run less oil without ruining the engine you'd get more time for the same fuel.

That's about all I can think off for now.

I THINK I AM GOING TO USE GRAVITY FEED WITH EXH. PRESSURE AND A CHECK VALVE ON THE EX.LINE IN ? I HAVE TO PUT ON BOARD GLOW ON TO KEEP THE PLUG CLEAN AT THE LOW RPMS I HOPE TO BE ABLE TO MAINTAIN FOR LEVEL FLIGHT.
10 FT WING SPAN

Exhaust pressure? I didn't think exhaust pressure was that much. You'd probably do better with crankcase pressure and a one way valve. At least it would be better if the fuel have very far to lift.

A pressure guage and one way valve will tell you soon enough.

I am hoping on gravity to do the work and exh pressure to keep it flowing at the lower rpm range. but I have a Friend who has a pump that works on crankcase pressure to fall back on for I am using kiss as my motto at this time?
I have good news and bad news I ran the 72 on 10 oz of fuel set on the rich side at 4000 rpm with a 14x6 prop for 45 min. not bad for a new engine I think we can get more in time (don't want to burn it up just yet needs more break in)
The bad news is I don't know the thrust. I have a hand full of parts to build a test stand but on afternoon shift and the noise in the shop will wake the kids and my wife! will get it just not as fast as I would like.
Is there a way to do this by prop size and pitch and get a number to shot for?

If we need a "first parameter" to start the design, I vote for the total amount of fuel carried. Let's say one gallon. For a 12 hour flight, our consumption will be .18 oz/min. I think this sort of calculation narrows the engine selection quite a bit. I have a Saito 72, and it probably burns close to this at idle. I would opt for a smaller engine. The OS four strokes seem more mild than the Saitos. It seems to me that they get better fuel economy too. Remember, BSFC and reliability will be more important than power/weight for this design.

With 6 pounds of fuel, our total airplane weight should be around 10-12 pounds. Can we get a 12 pound airplane off the ground and only burn .18 oz/min at cruise? It almost seems possible. Engine selection will be critical.

Going back to the very first post in this thread you mentioned 14 hours as a previous flight. Based on that I would say you need to target 20 hours to have a shot at the trophy.

Sounds like the engine will get down to about 10 oz per hour with not very much work. And that's a full throttle. Factoring in the part throttle operation later in the flight you can probably get down to about 6 or 7 oz per hour. Hell, lets say 8 oz per hour average just as a ballpark. For 20 hours that's 160 oz or about 1 1/4 US gallons. Hmmmm, this sounds a little too good to be true to me. But if you can achieve 6 minute per oz at full throttle then it's a go). Hey, add another 1/4 to make it 1 1/2 gallons and go for the full 24 hours...... So we have 9 lbs of fuel and a 120 (?) inch span model. I think that by using a lot of "air" in the construction we can keep the model weight down to 7 or less lbs. So takeoff weight is targetting at 16 lbs. Certainly not too heavy for the Saito by any means. My 84 inch 5 1/2 lb Old Timer goes uphill on about 3 clicks off idle so I don't think we are off base at all here.

Airfoil update.......
I've loaded up the old David Fraser program and I'll be damned if the plain jane old reliable Clark Y isn't almost as good as anything I've tried so far. The 7032 comes out slightly ahead but only by a few %. The 2091 is better than the 7032 in some areas and worse in others and the Clark Y curve touches the so called better stuff at just off the minimum sink point. I want to try loading up some polar info for a few other newer airfoils. But frankly I'm stunned and shamed to admit that for all around building simplicity and allmost state of the art performance the Clark Y would not be a bad choice. We'll have to nail down some model size parameters for wing aspect and fuselage size and stuff but at least now we know how much fuel is involved........ sort of. I'd like to see you nail down the hard numbers before this goes much further.

More to come.......

BMatthews,

Try running the S4233 on David Fraser's program. I think you will find it a contender.

DT,

You mentioned limiting the wing span to 10 feet. I doubt that this is optimum but, it is probably workable. Using BMatthews estimates of weight and an aspect ratio of 10, the wing area would be 10 square feet, with a wing loading at takeoff of 25.6 ounces per square foot. Using an estimated coefficient of lift of 1.0 for the wing, the takeoff airspeed would be about 37 feet per second or, about 25 miles per hour at sea level. This seems reasonable. The thrust required would be somewhat above above 1 pound. If you can deliver about 2 or 3 pounds of thrust at full throttle, a reasonable take off run and climb out are assured. The full load cruise will only require about one pound of thrust. It will be interesting to see what your fuel consumption will be at that power level. The above guestimates will need refining as more data becomes available.

Ollie, your one lb of thrust seems about right considering that my old timer "glides uphill' at settings just off idle.

And I tried the 4233. It's one of my favourites thanks to the thickness being advantagious from structural considerations. Youre right, it's up there with the best (and I forgot to mention that in the last post) but that stupid Clark Y is snapping right at the heels of high tech pack........ I'll have to get it running with some numbers more representative of this new direction and I'll see if I can get some charts printed or at least report on the numbers.

I should be able to get some thrust numbers tonight. I have a 14 x 6 MA somewhere that I originally had on my 91. I almost think that a 13 x 8 would be more efficient once in the air allowing a lower throttle setting. I'll see if I can get the tests run tonight. I'm sure the engine will put out 6+ pounds of thrust at full throttle for an easy take off.

A "baffeled box" 4x5x18 inches will hold 1.5 gallons of fuel. It could be made of epoxy soaked lite ply. The sides, top and bottoms could be part of the fuselage structure with the baffles and end plates acting as formers. It could extend from near the firewall to near the trailing edge of the wing with its center near 25% of the wing chord. In this way, the tank wouldn't result in much, if any, weight or frontal area penalty.

Ollie, I'd make the tank a lot shorter and fatter and live with the frontal area penalty. Baffles will stop the fuel sloshing but it won't prevent it moving to the rear or the front and causing a large CoG shift. When it's full it won't matter but the dangerous time would be when it's 2/3 full and the model gets into an attitude that promotes the fuel moving to the front or rear. With DT's wish for a gentle handling model there won't be a lot of elevator authourity to combat such a state.

Someone a while back mentioned wing tanks. I think this would be a good thing. DT could put a spanwise tank in the wing that is about 2/3 of a gallon and it would just drain down into the fuselage tank that could then be a lot shorter. Keep the tank mounted just under the wing and gravity would handle the fuel delivery duties totally. This way the fuel is centered around the CoG much more tightly. I may be worrying about this for nothing but if it did turn out to be a problem then it would be a real pain once the model is built and being tested. I'm thinking the wing tanks and fuselage tanks would be independent but there would be a fuel line that would be hooked up as the model is being put together and then the tanks would be filled as one. There would need to be two lines actually. One for fuel and one vent to let air out of the lower and into the upper as they are being filled. The fuel line feeding down to the base of teh lower tank and the air vent coming off the top of the lower tank and up to the air space in the wing tank.

BMatthews,
I like the wing tank approach the best. If one were to use the S4233 airfoil at 13.6 % thick, then the whole gallon and a half could be put into four wing tanks about 1-1/2 x 2 x 36 inches long, one in front of the spar and one behind on each wing half.

DT,

It seems to me that the Saito 74 is way too much engine for a model whose span is limited to 10 feet. If you want to keep the 10 foot wing span limitation, you should go to an engine about half the displacement. If you want to stay with the Saito 74, you should consider at least a 14 foot wing span.

Ten foot span option:
Use a Saito 35 or 40. Fuel requirements are cut roughly in half. Empty weight goes to about four pounds, or about 8.5 pounds with a full load of 3/4 of a gallon. With a span of 10 feet and an area of 6-2/3 square feet, the aspect ratio will be 15. The wing loading will be 20.5 ounces per square foot, loaded with fuel. The take off speed is lowered by about 10% and the thrust requirement is decreased by roughly 50%. Twenty % of the thrust requirement decrease comes from a lower wing loading and 25% comes from increased aspect ratio. With the smaller engine, the thrust required for take off is only about one and a half pounds.

Saito 74 option:
To get fuel duration improvement similar to the ten-foot-span option above you would have to go to a 14 foot span. The extra cost to build an airframe of this size would offset most of the cost of buying a smaller engine.

I got busy last night. I also have a Saito 30. I'd have to say it is the cutest engine ever. Full throttle will burn about 1/2 an ounce a minute. It will turn a 9 x 6 or 10 x 5 prop around 10,500 RPM. From flying the plane I know it has less than 3 pounds of thrust. I replaced a .25 BB 2 stroke with it. It has considerably less power and I now run 30% in it. I would go up to the next step and try the 50 or 56 if the 72 is too much. OS has a 4 stroke in the 40 range.

I like the wing tank idea except for the construction difficulties. The tank ends up having a lot of surface area for the enclosed volume. One option may be a bladder system. Where you get the bladders I don't know. Or, just plop a regular gallon jug of glow fuel on it's side in a big fat fuselage. The cap would need an improved seal though.

What about the propeller? It seems to me like a prop with more than normal pitch would be desired. This should result in a reasonable pitch speed at 5k-6k rpm. Obviously, the take-off thrust will be down, but the rest of the flight should be more efficient. It also seems like a prop which maximizes full throttle thrust will be way under-propped at half throttle. I'm thinking 8 to 10 inch pitch. I've herd that the propellers used in SAE competition, pylon racing, etc. are jealously kept secrets.

If we want to use a Saito, it seems to me like the 45 or 56 is just about right. Not a novelty in miniaturization like the 30, and more moderate than the 72.

Although the Clark Y seems "old school", it would be very easy to build accurately. Especially if an open-frame structure is used. Any 'foil with a concave surface will be difficult to build unless fully sheeted. Doesn't the Clark Y have a very forgiving stall? That will be important when the pilots start to fade.

Yes that is a very good point and you do get very tired of watching big circles for hours at a time and you will get tired and fade that is a given. as for 10ft. wing it can be bigger 14 is OK. at this point we need to do whats going to work.
Fuel in the tank a piece of clear 1" inside plastic hose 15ft long will hold 1 gal. of fuel give or take a oz or two. so if we have some dihedral and lay this pipe on the cg. and let it feed a tank in the cabin what ya think?

I woud use eight 3' lengths of 1" ID plastic tubing, two ahead of the wing spar and two behind the wing spar to keep the weight as centralized as possible laterally. They would carry 1.5 gallons of fuel. If less fuel is needed then the eight pieces of tubing could be shorter. That way a bank with half the fuel consumed would cause less of a lateral CG shift. With about 10 or 12 degrees of dihedral per side and a generous rudder area, control could be maintained even in a bank of a little more than 10 degrees. The drag penalty of that much dihedral is not a big consideration. It would take 7 Y-connectors and two check valves to bring the eight tank feeds down to one piece of fuel tubing. The check valves would be set up to prevent one side from feeding the other.

Throttle, elevator and rudder control would be the minimum and another channel for needle valve adjustment might be desirable.

A weight without fuel of 7 pounds seems easily attainable and 16 pounds with maximum fuel load isn't excessive. Less weight would be desirable.
With a span of 14 feet and an average chord of 12 inches, the wing loading would be 18.3 ounces per square foot. With the fuel tanks in the wing, the and a minimum fuselage crossection, the parasitic drag of the fuselage would only be one or two percent of the total drag budget. With a tail moment arm of 4" the horizontal talil area would only need to be about 12 % of the wing area and the vertical tail area about 9% of the wing area. the parasitic drag (tail, fuselage and landing gear would only be about 14 or 15 % of the low speed drag budget. A lift to drag ratio of about 12 or 15 to one would be easily attainable. The thrust requirement in level flight would only be about 1-1/3 pounds and any thing over 2-2/3 pounds of thrust would make takeoff and landing easy. Crusing speed would be about 31 feet per second fully loaded and aboiut 21 feet per second empty. Stall speed would be about 90% of crusing speed.

I've got just the thing. Aquarium stores carry a stiff but sqeezable(as opposed to the acrylic tubing they also have) light wall tubing in 1/2 a dozen sizes up to about 1 1/8. It comes in 3 foot lengths and it's thin and light. It's tough enough to use as explosion tubes for rubber models so you know it's tough. If you know someone that could make up some plastic press fit ends you could use something like 4 or 5 x 16 inch lengths on either side of the center line and plumb them together at the root and drain down into the main fuselage tank. And if you don't have anyone locally to help with this option I'll even volunteer to make up the wing tanks and mail them to you. Hands across the border and all that.....

But that means you would want to make the wing with full on polyhedral so there's a dip at the wing center to let the wing tanks flow to the center.

We are also leaning to a slightly thicker wing section to hold the tanks in that case. This would point to the the Selig 4233 as a good choice to get the room........... so far. I still haven't found the time to run the David Fraser software on the proposed numbers yet. I also want to input the lift-drag polar info for some likely looking prospects since the DF software uses the still valid but hardly current results from Soartech 8.

I think I can build a 14 ft wing may have to make it two piece . as for the wing tanks I need to but a regulator of float in the lower tank I am starting to worry about it going lean out ? any ideas
Should I use a valve and servo to fill the lower tank? Put some type of float and valve in the lower tank . Or scrap the wing tanks
and use something else?

I suggest a three piece wing with a four foot center section and 15 inch chord and two 5 foot span plug in tips tapering from 15 to 7 inches in chord. Making the center section one piece will simplify the plumbing and the plumbing hook up. A fuel pump should take care of the lean question.

A wing spar to handle up to about four positive G's would only require a top spruce spar cap 1/4 x 1/2 and a bottom spar cap 1/8 x 1/2inches for the center panel. The tip panel top spar cap of 1/4 square spruce and the bottom spar cap of 1/8 x 1/4 spruce would do the job. The shear web at the root would require 3/16 inch vertical grain, medium balsa tapering to 1/32 balsa at the wing tip.