Cactus.

My Feedback: (1)

i remember seeing a NASA type article about a remote spyplane, it was designed for efficency and had the volume of the plane controled from nose to tail. the pic of the plane had a graph next to it, bottom line was plane nose to tail, side line was volume, over all the shape ended up being damn close to a wing section, ie, smooth and bigger in the middle. on that graph it had coloured ares that showed what part of the plane was causeign that volume, this meant they had the jet engine intake just behind the wing on top of the plane to bulk out the FUZ before it got to the tail to keep the graph smooth on top. quite how this works as not all of the plane flys though the same patch of air i dunno. but NASA like it.
also some plylon planes have a bit that bulges behind the wing, i think is to get to the fuz width rules, but behind the wing seams to be the fave place to put it, maybe acts like wing fairngs or relates to the stuff above??
PS, saw a pile-in racer lastnight online that had a swept forward V tailplane, more efficent apparently

just saw your reply Ollie
sounds like the plane is comming from AKMacs render, swapping the taper on the LE to the TE with more or less straight LE and less taper on the wing.

just a note on sommit i've got floating in my head, i might be doing a delta soon, was gonna be DF, but the engine we have isnt sounding DF after all, so a mid mounted pusher is plan B, most of the engine is hidden in the fuz, so the nose can be sharper and smooth.

Oille while your looking,... does it matter if the engine is mounted forward or aft of the CofG? aft sounds more pitch and yaw sensitive to me...

banktoturn

Ollie,

I'm not much of a builder, so I could probably not reproduce an airfoil faithfully enough to get the benefits of a laminar section. I have been thinking about the things I could do crudely but effectively. Since all these blistering speed designs seem to be launched rather than taken off, I would be inclined to go with the smallest wing that would sustain level flight at low angle of attack, with sweep for landing, if possible. The picture in my head is closer to an arrow or missile than any kind of scale plane. Picture AKMac'c beautiful, clean, mid-wing design, but with about a third of the wing area, and possibly a radical leading edge sweep, if you decide there is any hope of doing a traditional landing. If you give up on landing, then I would consider just cutting off AKMac's wing 1/3 of the way out from the root, and adjusting the stabilizer accordingly. I am fond of the symmetric tail feathers of a missile, just to let people know at a glance what your intent is.

Of course, if you could do all this and use the best section as well, that would be ideal. If you keep the angle of attack low in level flight, even a low aspect ratio wing might get the benefit of the efficient section, but I don't know about sweep.

It would be fun to build, but I couldn't fly it, so I may never know.

Thanks,

banktoturn

Cactus.

My Feedback: (1)

then picture ACMacks beautiful, clean design going straight in to the ground as it torque rolls on take off. those wings are small enough, you wont be winning any climb and glide, trust me. i went that route before.
apart from the tapers... ( i'm thinking cheat the tip vortices here by having swept forward TE's ) (( they cant move forwards right?)) AKmacs idea is pretty spot on.
i also think large long spinner to get engine as far back as possible into thicker bits of fuz, and somehow i keep think back to sommit i heard about square fuzs with shape corners are more efficent than round ones, at least for fitting the gear in.
flat bottom laminar for low AoA ( dragqueen had that ) i could scan the templates i found the other day.
on smooth surfaces... i covered the wing in 1/16" balsa, stronger than Obdeci and you can sand it hiding any contors in the foam wing. foam was blue for strenght, so little of it it didnt matter about the weight.
umm what else, engine, rear induction rear exhaust, hide that in the fuz or canopy.... torque rods to move surfaces, no showing controls, hide fuel up point behind hatch.. i know i'll think of more

scan of the templates, the size of the pic is a full lenght of A4, thats how small the wing was.

tuliosb

Could someone explain to me if it is a normal aerodynamic characteristic the Delta Wing model have a pitch up after a steep turn in a low speed?

I was flying my Hot Spoth and during the approach maneuvre to land I had this problem.

Thanks,

Tulio

BMatthews

Lots of good discussion here about the airplane design...........

But you are forgetting that for a prop driven model the engine and, in particular, the prop is largley the controling factor for overall speed assuming a fairly clean design of average proportion and low camber airfoil.

Dusteater, if you want to go fast then just get yourself a formula one pylon racer and a Nelson engine. I can't remember what their top speed is but it's way up there. Since you're not actually racing it you can leave off the canopy and arrange for a takeoff dolly and gain yourself another 10 mph or so. That should get you up close to 180 or so.

But having seen a formula one racer fly I think the biggest problem you're going to have is keeping it in sight for very long. The flight path ends up being one long string of reversals in order to keep the model close enough to you to see what it's doing.

Ollie

For all practical purposes, at maximum speed, thrust is equal to drag. Any increase in thrust will increase speed until thrust is again equal to drag. Any decrease in drag will increase speed until thrust is again equal to drag. Where speed is concerned, thrust increase is as important as drag decrease. To say that the prop is controlling from a design for speed point of view, is to say that you have given up or reached the limit of drag decrease.

banktoturn

From a practical perspective, it seems that drag reduction and prop selection are about the most critical variables in the search for maximum speed, right up there with engine selection. The speed fans here do seem to have given up on drag reduction, in a sense, since they are flying 'stock' Whiplashes, Diamond Dusts, etc., and choosing engine/prop combinations to maximize thrust. I would say there is quite a bit of room for drag reduction, if you're willing to spend some time fairing the engine and burying the servos.

banktoturn

BMatthews

Ollie and Bank to Turn. You are right up to a point. I'm not arguing with the basics of the drag to thrust balance. That's a given.

But the prop can only spin so fast and the pitch can only do so much.

The current world speed record is 215 mph. I seem to recall the latest ducted fan fun fly models doing speed passes close to that speed or even faster. Certainly they exceed that speed by a good factor in a dive. I seem to recall seeing reports of 240 mph speeds from ducted fan scale jet models. Without getting into a lot of research I will suggest that these models can do this argely because they don't have a prop to slow them down.

A few years back the world speed record was held by a glider because it didn't have the props of the day to slow it down.

Obviously a clean airframe is all part of the equation but to ignore the aerodynamics and efficiencies of the prop is to only look at half the picture.

None of this takes away from the interesting discussion in this thread though. There's lots of good info here. But it just seemed to me that Dusteater is looking for a "quick" speed fix without doing the homework first. So I offered a suitable option along with reminding you folks about one of the main speed limiting factors in any propellor driven airplane.

Cactus.

My Feedback: (1)

DF has an advantage or two....
first is a sleaker profile witn no engine at the front. second is the ducting acts like winglets for the *prop* this allows it to spin much faster giving a greater thrust than equlivent size 2 blade prop. the thrust is concentrated from the outlet and pushes better than the bigger wash from a prop. like having a shower head on centre feed compaired to lots of little holes at the edge.
* as i see it anyway *

banktoturn

BMatthews,

I must confess I don't know much about the ultimate effiency or speed limit for a prop driven plane, apart from compressibility issues at the tip. My intuition is that there is no fundamental advantage to a ducted fan, unless you decelerate the flow before it passes through the blades. Generally, my guess would be that there is more drag involved in pumping a bunch of air through the duct with an engine in the way. It does seem that the typical pitch for ducted fans is much higher than for traditional props, with a corresponding smaller diameter. I am not aware of any specific reason that traditional props could not be run at higher pitch and smaller diameter, although there might be some benefit to using a shaft extension to get the prop a little further ahead of the blunt object which is the engine. I will try to do some reading on this topic, and see whether I can learn something more.

Thanks ,

banktoturn

banktoturn

phillybaby,

Actually, until you are power-limited, the opposite is true; it is better to have a larger diameter prop than a smaller diameter. Given a fixed engine power and the goal of max. speed, though, we are forced to use the center of the shower head.

banktoturn

Cactus.

My Feedback: (1)

dia = thrust, yep.
but higher pitch = speed and for that the poor engine has to turn a smaller prop to cope. thats where turbines take over
like a screw with a coarse thread gos into wood on less turns for those who haven't got to grips with pitch. * not insinuating anything those who are posting *

Ollie

Each prop tip has a trailing vortex just like a wing tip. That trailing vortex produces a substantial portion of the total drag on the prop when the prop is thrusting hard.

The duct of the ducted fan cuts the tip vortex at each blade tip which drastically reduces the induced drag and allows more of the engine power to be used to accelerate the flow through the duct. A well designed duct actually increases the efficiency of the system compared to an unducted setup.

Cactus.

My Feedback: (1)

props and wings are basicaly the same thing, a aerofoil moving though air causing lift... so.. why on props is the thrust, or the amount of air being passed though considered important, when with wings, any downdraft is argued to be unimportant and that the lift is caused by the high pressure below VS the low pressure above is what creates the lift. shouldnt it be the same for both?, or is the air blown by a prop proof of the lift caused by the low and high pressure created by the section. after all ,we dont measure the planes lift by the amount of air it throws down as it flys.
ollie?

banktoturn

phillybaby,

Funtionally, wings and props are different things, even though they do their jobs in a similar way, and thus are measured a little differently, but not that differently, after all. In point of fact, though, thrust is not the amount of air that a prop moves, it is the amount of force it provides to move the plane. In this regard, the measures are similar: props make thrust, wings make lift. Both of these desired effects are balanced against negatives, or costs: the power consumed to get a given amount of thrust from a prop, the drag produced by a wing for a given amount of lift.

banktoturn

BMatthews

Ah, the old grey cells are starting to churn now........

I'm going on distant memory of control line speed model articles but I seem to remember that the slippage factor of the tiny, high pitch toothpicks that were/are being turned on the class C .60's suffer from a higher slippage % than do the normal props that we use. This would also act as a limit since there's decreasing returns for increased rpms.

AQ500

Not to belabor the point……

It is my understanding that the ‘profile’ drag is the drag due to the pressure drag of the airfoil summed with the skin friction drag. It is a coefficient that is multiplied by the wing planform area as well as dynamic pressure to calculate the total profile drag. The differing Reynold’s numbers due to the chord difference will cause the profile Cd to change slightly at low angles of attack, but usually not enough to make a huge difference. So you will be flying at nearly the same angle of attack. You are right that at high speeds, induced drag is fairly low and you don’t need much of a wing. The drag you are trying to decrease is the skin friction drag from the extra wing you do not need.

So I’m a little confused when it is said the higher aspect ratio wing with the same planform area, frontal area, and airfoil has a large increase in 'profile' drag. Ollie, maybe you could enlighten me on the subject. I would appreciate it.

I’m not saying to make a paper-thin wing U2 type wings, I’m just saying don’t make a super stubby wing. You will get a better flying, controllable plane. Structurally, it is not hard to make a very strong light wing for our models. When you get to 50-pound planes, then yes. You can decrease the wing area and AR so much that the plane will be impossible to land well and handle at low speeds. You can always bungee launch the plane and then have a parachute recovery. That way you wouldn’t need much of a wing at all.

The benefits of a 'higher' aspect ratio for a more efficient wing diminish when the Reynold’s number gets below 70,000, I believe. If you are going to fly a fast model airplane, you will be higher than that. A higher efficiency means less drag and slightly higher speeds.

Ollie

Reynolds number is useful in comparing flows around similar shaped objects. If two objects have different sizes but similar shapes and orientations to the flow, the pattern of air flow will be the same in the two cases if the reynolds numbers are the same. This is a handy concept when using lift and drag coefficient measurements from a wind tunnel model or computer simulation of an airfoil. If the air density is the same and the product of chord times velocity are the same, the reynolds numbers will be the same and the data can be applied with the assurance that the results will be valid. For example, if the wind tunnel used a 12 inch chord, at an angle of attack of zero degrees and a velocity of 100 feet per second, then the data could be applied to a model whose chord was 6 inches flying at a speed of 200 feet per second at an angle of attack of zero degrees.

When comparing two wings of the same area and airfoil but different aspect ratios, the frontal area of the two cases will be exactly the same. In this example, as the aspect ratio ratio increases, the thickness decreases such that the frontal area remains the same. If the two wings are operating at the same speed then the reynolds numbers will be different because of the difference in chords.

As the reynolds number decreases the profile drag increases slowly because the boundry layer thickens some with decreasing reynolds number. In the case of the S6063 airfoil at zero degrees angle of attack, the coefficient of profile drag increases from 0.006 to about 0.0068 when the reynolds number decreases from 300,000 to 200,000. At an angle of attack of two degrees the profile drag coefficient increase is from 0.0065 to 0.008 when the renolds numbers decrease from 300,000 to 200,000.

The detailed behavior of the boundry layer is quite complex with changes in reynolds number or angle of attack because of changes in the location on the airfoil surface where turbulence sets in and where laminar seperation bubbles occur when the angle of attack or reynolds number changes. The airfoil polars reveal the effects of this complex behavior.

AQ500

Thanks Ollie,

You will be slightly faster with stubby wing and a light plane at fast speeds 'if you pick the right airfoil.' Just make sure you don't turn. You'll lose that small gain in no time. The cd versus alpha charts I have show a lower profile drag with a higer Re. The difference is very small. So, I guess it goes back to airfoil selection.

AQ500

One more request Ollie...

Do you know of a good source for charts on airfoils at lower Re around where we fly. I've been looking for a while and haven't found anything too useful. I picked up one of Eppler's books a while back and the only airfoils it had data on at lower RE were the high lift low RE aifoils.

Thanks.

jettstarblue

You may want to try Barnaby Wainfans' website- He's the guy who came up with the Facetmobile. Also I have built several delta's of every desription. I have built some that even had a negative aspect ratio. Some were VERY THICK- up to 30 percent thickness to chord ratio.
Ieven built one that was a flat plate with all the radio stuff hot glued on top, and believe it or not, it had one of the better glide ratios of them all!
Feel free to e-mail me and we can discuss this delta stuff at length.

BernieG

make a search on the web for Martin Hepperle. he had to close down his site, but still you should find some stuff.

Bernard

Ollie

Here are some sources of airfoil polar information:
http://soaring.cnde.iastate.edu/calcs/frames.shtml
http://www.nasg.com/afdb/index-e.phtml
Soartech 8
Summary of Low-Speed Airfoil Data, Vols. 1, 2, and 3; by M. Selig, etal.,
Soartech Publications
1504 North Horseshoe Circle
Virginia Beach,VA 23451
The hard copy publications are also available from the publishers of Flying Models magazine (I forget the name).

You can also download a free copy of X-foil by Dr. Mark Drela at:
http://raphael.mit.edu/xfoil/
You can use the X-foil program as a vitrual windtunnel to generate the polars from the airfoil coordinates that you input to the program. This is the most powerful tool of all when you learn how to use it.

UsedBits-RCU

There is a bit of technical stuff about the delta wing toward the end of the DeltaVortex web site. Also a little about the NASA lifting-body re-entrant vehicle.

UsedBits-RCU

There is a bit of technical stuff about the delta wing toward the end of the DeltaVortex web site. Also a little about the NASA lifting-body re-entrant vehicle.