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duration flight help - 11/14/2002 9:05:55 AM   
DUMB THUM



Posts: 40
Joined: 11/4/2002
From: POWHATAN PT., OH, USA
Status: offline 		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.
       Post #: 1

duration flight help - 11/14/2002 11:54:18 PM   
BMatthews



Posts: 9005
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		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.

_____________________________

Witty saying to be plagarized shortly.....

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duration flight help - 11/15/2002 1:10:52 AM   
ilikeplanes


 

Posts: 1058
Joined: 3/30/2002
From: portland, OR,
Status: offline 		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.

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       Post #: 3

duration flight help - 11/15/2002 3:37:57 AM   
DUMB THUM



Posts: 40
Joined: 11/4/2002
From: POWHATAN PT., OH, USA
Status: offline 		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.

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duration flight help - 11/15/2002 4:10:31 AM   
banktoturn



Posts: 713
Joined: 8/22/2002
From: Bloomington, MN,
Status: offline 		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

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       Post #: 5

duration flight help - 11/15/2002 4:37:56 AM   
probligo



Posts: 354
Joined: 5/13/2002
From: opononi, NEW ZEALAND
Status: offline 		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.

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       Post #: 6

duration flight help - 11/15/2002 6:01:00 AM   
BMatthews



Posts: 9005
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		[QUOTE]Originally posted by probligo
Just a thought for you, don't ignore the humble old diesel as a power plant.

... [/QUOTE]

[SIZE=6]YES ! ! ! ![/SIZE]

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.

_____________________________

Witty saying to be plagarized shortly.....

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       Post #: 7

duration flight help - 11/15/2002 6:28:55 AM   
DUMB THUM



Posts: 40
Joined: 11/4/2002
From: POWHATAN PT., OH, USA
Status: offline 		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

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       Post #: 8

duration flight help - 11/15/2002 7:13:24 AM   
AQ500



Posts: 692
Joined: 5/22/2002
From: Sandy, UT, USA
Status: offline 		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.

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duration flight help - 11/15/2002 7:16:37 AM   
AQ500



Posts: 692
Joined: 5/22/2002
From: Sandy, UT, USA
Status: offline 		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.

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< Message edited by AQ500 -- Nov 15 2002 3:22AM >

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duration flight help - 11/15/2002 7:26:03 AM   
DUMB THUM



Posts: 40
Joined: 11/4/2002
From: POWHATAN PT., OH, USA
Status: offline 		This is great! all this info and ideas the guys don't have a chance.keep the input coming

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duration flight help - 11/15/2002 7:35:58 AM   
DUMB THUM



Posts: 40
Joined: 11/4/2002
From: POWHATAN PT., OH, USA
Status: offline 		AQ500 we have a 500ft grass run way the airfoil 423 with something around a .32 4 stroke is it possible

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duration flight help - 11/15/2002 7:41:14 AM   
AQ500



Posts: 692
Joined: 5/22/2002
From: Sandy, UT, USA
Status: offline 		I would believe so. I'd imagine the grass would make it difficult.

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       Post #: 13