My club has a duration contest and I was wanting to get some input on what engine and airplane to use The rules are when the plane leaves the ground till it lands any number of pilots can fly the plane no limit on fuel weigh last year a os 32 4 stroke flew 8hr
and another club in the area flew for 14hr any ideas.

What an interesting concept. No rules? Nothing about how much fuel?

This is a first off sort of concept idea but I'd start with a large sailplane like wing running a higher lift airfoil that still offers reasonably low drag. Some of the high lift Selig offerings like the 1223 would be good. Worth looking into the drag thing though. You want to use as small an engine as you can get away with so it's probably worth trading off some lift for lower drag.

The model for my concept would be loosely based on something like a 100 inch wingspan polyhedral glider with a big fat guppy shaped fuselage to hold the fuel. The main pod of the fuselage should be shaped using a proper airfoil to offer lower drag. And to gild the lilly the fuselage would be "cambered" so the mean camber line followed the airflow entering the leading edge and the downwash off the trailing edge.

Why polyhedral? Simple. You can then trim the model to fly as a free flight with turn trimmed into the flight path. This would reduce the pilot's workload to using minor throttle changes to trim for altitude and minor rudder and elevator trim changes to trim the model to keep it "on the step" as the fuel burns off. The pilot would only disturb the circling flight to return upwind occasionally and would then release the controls to let the model return to circling flight.

Engine mods. Change the carb to one off a smaller displacement engine. How small? You still need enough to get the model airborne with a full load but only just. Full power should offer a very lazy climb but it should be enough that you can deal with wind turbulence. Change over to gasoline and spark ignition. Gasoline has a lower stoichiometric mixture ratio and more BTU's per pound than glow fuel so you use less gas per minute to achieve the same power. It's been good enough for Maynard Hill to keep the duration record so it should be good enough for you if the rules allow gas. 4 strokes have overlap between the intake and exhaust functions so you loose less fresh fuel out the exhaust. So go for a 4 stroke spark conversion with the carb off the next size down engine.

Interesting. A long time ago, one of my flying buddies said his father flew in distance competitions in the early seventies. The airplanes were very heavy with fuel.

I have a couple thoughts:
If you plan on flying all day, you need to think about ease of flight. I agree that a polyhedral design, although not the ultimate in efficiency, will suit you much better for 8+ hours of flying. You should be able to adjust the trims and let it circle hands-off.

Engine choice is tough. Most model engines are designed for peak power. What you want is good part throttle efficiency. A smaller carb will be effective in this regard. I wouldn't write off two strokes. Some of them are very efficient. Most modern four strokes have very aggressive cam duration and lap. Not so good for part throttle efficiency. Also, propeller choice is critical. I think you want as much pitch as you can stand. That way, when you throttle back, you still have adequate pitch speed.

Your flying technique will be very important. I believe you want to fly at maximum L/D. This is not an easy task. Most sailplanes, for example, fly faster at max L/D than they do at minimum sink. You usually want a moderately quick pace but not fast. If you're near stall, your flying too slow.

Obviously, carry as much fuel as possible and use little or no nitro if burning glow fuel. Gasoline would be the ultimate.

Good luck.

I like what I read as for the gas conversion for a 4 stroke is their a company that makes this? also I have to look at batts receiver and transmitter. as for a plane I agree on the polyhedral my thoughts are a old timer style cabin for the fuel and batts. also most are very easy for anyone to take the controls. Maybe a buzzard bomeshell, playboy? Another thought I have is start early before light have lights and a batt. on board that I can drop at day break so I can get the most flight time as for I live in Ohio and only for a very short part of the summer do we have that much light.I need all the help I can get as you can see it sounds easy till you really get into it.

DUMB THUM,

This is a cool idea. Generally, I think there is a lot to learn from the sailplane folks, since they have had to design everything around budgeting limited energy to stay aloft. The main difference is that, early in the flight, you need a lot of lift to carry your fuel. I will take a look at Model Aircraft Aerodynamics tonight, to get some ideas. Off the top of my head I would definitely suggest:

1) high aspect ratio wing
2) flaperons or long span flaps to allow higher camber while the plane is heavy, and lower camber when the fuel load goes down
3) maybe span load the wings by putting some of the fuel in there?
4) and engine with conservative intake & exhaust timing
5) large diameter prop

banktoturn

Just a thought for you, don't ignore the humble old diesel as a power plant.

For low end power they can not be beat. One thing I would suggest is to have the fuel tank wrapped in polystyrene if you are flying extended hours in high temperature. This to stop too much of the ether (igniter) evaporating from the fuel.

YES ! ! ! !

I'd forgotten about the diesel option. And I even used to fly diesel combat!!!! DOH!!!

Even running at a high rev and power setting I'd get over 6 minutes on about 2oz of fuel. And I understand you can get much more time per oz by using an engine that is designed more for sport flying. The fuel can be a pain to find though.

Not sure where to find the spark conversions. I know you can find them for some of the older 2 stroke loop scavenged OS engines as they were approved for SAM competitions.

I have an older 40 Surpass and run it for 10 minutes at a time on 4 oz of fuel and land with lots of reserve. Not much power but it's miserly. Fits in with the NON overlapped 4 stroke design. Probably a good option if you want or need to stick with glow fuel.

The old timer model is a good starting point. It wouldn't be as good as a purpose built design but you'd have something to fly afterwards that has character. The Bombshell is quite a lumberyard so its going to be heavier. Go for one with a basic stip built fuselage to save a little weight. One of those old Texaco models would be terrific. Or that Lanzo model with the wire cabane mounted wing would be a killer design. Big box of a fuselage and a HUGE wing. The 40 Surpass would be just enough power to lift the model and a couple of quarts of fuel. Or run the gas option and one of the repo spark engines. A Super Cyke would be nice but the front bearing might give problems as it's so short. Find a sparky that has a decent length front bearing and you're set.

But either way if you want to build an old timer design and don't plan on entering SAM events with it then you can always change the wing section to something better. Some of the new high lift stuff from Selig won't work as you need composites to build the super thin trailing edges accuratley but there's lots of high camber "balsa friendly" sectoins you can use that are better than the Grant's and stuff that many old timers came with.

For the record I've got a Roger Hammer Flamingo that I fly using an older OS 35 loop scavenged engine. 84 inch span and a super wide center chord with something over 1000 sq inches. 5 1/2 lbs and it climbs like a homesick angel getting to 500 or so feet in about 90 seconds when proped for a climb. For an even like this I'd probably try using a 25 and see it it could carry 2 quarts of fuel.

Speaking of fuel feeding, you'll probably need a holding tank set up with a float valve with the tank mounted higher than the engine and holding tank. There's no way the engine will suck 2 quarts evenly from full to empty of course. Maynard Hill did this in one of the record accounts from the 70's. A one gallon high mounted gas tank running to a float shutoff equipped 2 oz engine run tank. If you can't use gravity you'd need a fuel pump. And more batteries of course. Also he make his own tank equipped with numerous baffles to prevent the fuel sloshing around. You can imagine what a quart of fuel in a 2 quart tank will do to model trim if it's slapping around from front to rear. I know I wouldn't want to be the pilot.....

Starts to make the purpose built model look better, doesn't it. In any event I think it's safe to say the engine and it's fuel per minute is as important as the model. I hope this is a team event for planning and building as well as flying.

AS for the diesal and the best he could get was almost 3 hrs he used a os 40 with a brown head on it he had the crank break at 2hrs. then the next flight at 2 1/2 hrs it just quit no reason at all the 3rd try broke the crank . he has the fuel economy but reliability is the down fall of this combo maybe a engine made for diesal. he had to order fuel in and boy did it smell the place up. as for a group to work on this it me my self and all of you who send any input I already have the names on a list to but on the plan

Here is a picture that might help. The plane in the picture was designed to compete in the SAE Aerodesign competition. It took third place. It is pretty much a powered glider. It is powered by an OS 61 FX that is completely stock. The wing span is 110 inches. The root chord is 12 inches and the tip chord is 5 inches. There are a couple of degrees of washout built in to make it stable. The airfoil is a modified Eppler 423. The plane weighed in at 9 pounds. The plane had the most gentle stall I had ever seen. It would never drop a tip.

The plane was able to take off in 200 feet (which was the hard part) with a payload of 25.2 pounds. Gross weight was over 34 pounds (with a little .61). The plane would fly loaded at half throttle and one quarter throttle with a gross weight of 15 pounds. I'm sure it would carry more weight if the take off run would be extended or used a more powerful engine.

Now imagine a diesel engine around .40-.61 cubic inches with 25+ pounds of fuel (now imagine if it crashed and burned). That would easily fly well over 10 hours, maybe close to 15. I'm sure it would be a lot closer for a glow engine. You can always get a gas conversion. They are pricy though.

Just an idea. There is not much to a plane like this. The only hard part of the construction would be the wing. In this case the D-tube and center section of the wing was foam core and the rest built up.

I have a 104 inch prototype wing that is sitting and collecting dust. I'm wondering what I should do with it.............

The Universtities in Ohio took 1st and 2nd places. You may want to check out there stuff. They probably have their plane on display.

The wing before covering it. We figured that is was good for about 4 G's of loading with a fully loaded plane, which was plenty.

This is great! all this info and ideas the guys don't have a chance.keep the input coming

AQ500 we have a 500ft grass run way the airfoil 423 with something around a .32 4 stroke is it possible

I would believe so. I'd imagine the grass would make it difficult.

There might be better airfoils out there. You might want to go with an airfoil with less camber for less drag.

At the competition many of the teams did use flaps and I believe one team tried slats. Sure they are great designs and look good on paper, but they didn't work well in pratical application. Infact, I beleive none of the planes in the top 5 used any type of flap or slat. I noticed two serious problems with the flaps ans slats. First of all as the competition got going and payloads increased the flaps came down. A lot of times the flaps probably created enough drag that the plane was unable to reach take off speed, which is probably the most important factor in this competition. The other problem was sever instability. If a loaded flapped plane did make it off of the ground it wouldn't make it too far. We decided to go simple and use a high lift low speed airfoil with no flaps or slats. We did install flaperons but they were pretty much useless on the airfoil we selected. They created way too much drag. The plane flew better without them. I'm sure if we had more power then we could of used them.

It is an exciting competition and I would recommend going. Make sure you take a video camera to get the carnage on tape. Most planes end up crashing and some stray into the pit area. I'm going to probably go next year to watch. They also have some giant open class planes that carry 100 plus pounds using less than 2 cubic inches of displacement.

This sounds like it was a glow conversion. If so it doesn't surprise me at all that there were problems. I have about 4 or 5 diesels in my collection and they are much more robust than their glow cousins. And yes the fuel is SUPER SMELLY and the residue stays in your clothes for wash, after wash, after wash, after..... Not to mention your hands that stink for a week worth of showers...... :stupid:

And to add to the tank and holding cell issue Banktoturn has a very good idea in the span loading fuel cell. It's hard to get the fuel all up above the engine so gravity can do it's work. A wing cell draining into a fuselage tank may just help out the design to a "T". Never mind the spar loading issues

Nice call B2T...... :thumbup:

PS: guess you guys can tell that this "mission" has hit a chord with me. There's just no one around here that seems to be interested in stuff like this. Please forgive me if I sound too enthusiastic but I LIKE this sort of stretching of the mind.

Ah fame...... such a fleeting mayfly it is.........

Glad to help DT, glad to help. We can live vicariously through your efforts.........

AQ500,

I don't know for sure that flaps make sense for the duration mission, but there is one key difference between this problem and the maximum lift problem. If you assume that the fuel is a significant fraction of the weight at takeoff, then the weight of the plane will be significantly lower as the flight proceeds. This means that the ideal camber of the wing for minimum drag will be lower and lower. If you choose a high lift airfoil to get off the ground, it will not be ideal after a lot of the fuel has been used. In a perfect world ( or at least a pretty good world ), the wing section with flaps deployed would not be much higher in drag than a highly cambered high lift airfoil section. The advantage is that the camber can be reduced by retracting the flaps, to reduce drag as the lift requirement goes down.

banktoturn

It's a trade off for sure. As for the flaps and slats it doesn't surprise me that there was not much to be gained from there use. Flaps and slats versus a "flap built in secton" like the S1233 is only a method of extending lift into a wider speed range. But in our model cases we don't need a wide speed range for most applications. Especially in this one. A single camber foild that will work over a relativley narrow range is just fine. What about fuel load changes you ask? Simple, as the load goes down you throttle back to extend the run time. This slows the model speed and thus the lift coeficient remain fairly constant and the airfoild keep working at it's best L/D.

In Using a smaller engine for less load than the SAE comps call for suggests you probably don't want ALL lift at the expense of a heavy drag load. And add to that the realities of building an accurate airfoild as I suggested in one of my (probably too many) posts above and it starts to look like the "porridge that's just right" may be a better option for this situation. Using Profili I see the Eppler 423 has 7% camber. That's a lot for a model that's going to be expected to fly with minimal power at a much more moderate loading than the SAE models. I'd probably think it would be better to cut back to 4 to 5 % camber for less drag as AQ500 is suggesting as a happy medium.

I'm sure there's a formula that can help. I know there are some lift coefficient calculators out there on the web. A few realisitc numbers to punch in and find out what the Cl will be should point to some suitable airfoils.

So just how much does 2 1/2 quarts of fuel weigh anyway?.....

On paper this is the case but if you look at the airflow pressure gradient's across a properley designed high lift section that uses no flaps and a lower lift section with flaps then you'll find there is lots of room for serious separation bubbles thanks to the discontinuities of the flap hinges. Remember that flaps are a real world band aid to extend the low speed capabilities of high speed airfoils. As such they don't really apply to a model or real aircraft that's being designed to fly at a constant lift coeficient in a narrowly defined flight envelope.

I mentioned in one of my other posts that DT needs enough lift to get off the ground. After that he can reduce power as the fuel burns off and thus the requirement for max lift is reduced. By the end of the flight he might even be able to thermal for an hour or so.......


Hmmmmm......... thermals............ DT, have you thought about timing your flight so the fuel is being used up at the hotter part of the afternoon? With some thermal help and the glider type wing we are suggesting it would be well worth allowing your flight path to venture out from the beaten path and look for some help. An idling engine uses less fuel........

It would mean a longer "light's on" time but if the rules allow an earlier start...... ????

I agree.

The problem is with the design. At our small scale it is hard to get the same benefits of a moderate cambered airfoil with flaps due to building problems. You'll get sharp edges and corners where a cambered airfoil would be smooth. There will be a difference. How big? Depends on the geometry. If it can be done it would work wonderfully. I'm sure that an airfoil with camber would do fairly well in this type of competition.

I read about a competition years ago. The planes were electric and they had to fly a course. The winner did use flaps for takeoff and landing.

I think one of the major things overlooked by the teams with flaps underestimated the pitching they would encounter with a high flap displacement. Pitch up, loose speed, stall and crash. I'm sure that they never loaded the plane to it's capacity before the competition.

Sometimes simplicity is the best policy. You might have enough drag with the flaps deployed to counteract the drag from the cambered airfoil later in flight. There are so many factors that it could be a Doctoral thesis to build this plane.

Thumbs, if you are crazy and want to try the airfoil, I have an Excel spreadsheet with plots to make the templates out of.

Plot, close to Re in which the plane will fly.

AQ500 & BMatthews,

It's a little harsh and misleading to refer to flaps as a band-aid for high speed airfoils. Actually, they are a clever way to allow a small, efficient wing to generate more lift than is needed in cruise. They definitely apply here, as the CL will be far from constant. While we naturally associate flaps with take-off and landing, they are more generally applied than that. In fact, flaps are indeed used successfully on some model sailplanes and pylon planes, specifically to give higher CL when needed, and not just for take-off and landing. The case of a pylon plane, which needs very low CL most of the time and moderate CL for turns, is fairly similar to the duration problem we are discussing.

I think I would characterize this decision as a tradeoff between imperfectly implemented variable camber and well implemented ( smooth, lower drag ) constant camber. One is an attempt to tune camber for the varying weight, and pays a small drag penalty, while the other is an attempt to choose a camber which is OK for the whole flight, and also pays a drag penalty for having too much camber late in the flight, when CL is lower. I can't prove which is better, but the fact that some sailplanes & pylon planes are managing the implementation problems suggests to me that a careful implementation of "camber flaps" would give superior performance.

It might even be feasible to arrange the fuel tanks to counter the pitching moment caused by the flaps, by having the first fuel tanks to empty be behind the CG. Ideally, we could avoid huge elevator inputs to deal with the changing pitching moment of the wing.

banktoturn

Another thing to consider. The sailpane and pylon racer would use the flaps momentarily in flight versus for large portions of the flight. The extra drag due to the flaps would be regained quickly after the turn. Even then the flap deflection would have to be very small to make it worth the trouble. I fly Quickees and all of the people I know do not use flap-elevator coupling, 500's or Q40's.

I gave up slope soaring after a near miss with a hang glider.

Also another thing to consider.....a lighter airplane does not mean that the Cl has to decrease. Lift is a function of the Cl and the velocity squared.

B2T. It was a little harsh and was sort of meant to be a little tongue in cheek but it doesn't change the facts as they pertain to this mission. As AQ said it's very hard to build the flaps at our model sizes so that there is a smoothly directed airflow at any sort of decent deflection. It may be possible but it's much more likely that the flaps would generate a disproportionate amount of drag for the extra lift they would offer.

And our model sailplanes only use flaps during the launch where there is mucho extra energy available thanks to the winch. Or during landing approaches where we want to dump the lift and add drag to control the diving speed. Yes I know there has been some work done at using flaps to enhance the lift coefficients on low camber airfoils but I'd guess that it would not be valid on airfoils that are already using a high % camber. I'm thinking that for this mission DT would want to start with something more than the 2.5 to 3 % camber stuff that fast cruising sailplanes use. And in any even without the windtunnel info to support the flaps on a given airfoil it's hard to make a sound decision in any event. I do remember reading an article by Dr Selig about his 1233 group of airfoils that stated that it was very important to build the rear secton very accuratley as the design was stretching the airflow to it's critical maximum. Any little discontiniuty or distortion would result in a collapse of the flow and a large drag bubble as well as loss of maximum lift. Soooo I fall back to my claim that the model would be better off using a moderately high cambered airfoil of about 5 or 6 % camber that has a shape that can be built using the techniques comfortable to DT. And if that means he can't build the swoopy, curvy shapes required by the Selig high lift stuff then a more "classic" shape would be better.

Actually in proof reading this over I will offer one option that works wtih your idea of decambering the wing to lower drag when not needed. All the above assumes that it's very hard to deflect a plain flap in a positive direction (droop) without risking a big pressure point and possible resulting separation bubble. But there is a very greatly reduced risk of bubbles and drag if we RAISE the flap to reduce the camber. So the idea would be to start with a high camber airfoil and then reflex the trailing edge to reduce the lift and thus the drag........ Now this I like. It wouldn't be much, perhaps 3 or 4 degrees. Just enough to decamber the median line without making it into an "S" curve.

Whadda you guys think?

You might try to ask the guys on this site : http://tam.plannet21.com/

They are trying to get 4 models over the Atlantic ocean.