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Old Nov 08, 2012, 04:38 AM
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High speed, high L/D, low wing loading

Hi

I am trying to design a plane that could fly over the sea and coastal areas. The plane will not be doing aerobatics, and will be flying in a straight line or circling most of the time. So, I must take high wind speed and gusts into account. I figure that the cruise speed should be around 90 km/h (60 mph).
Other requirements:
  • hand launchable (with flaps),
  • at least an hour of flight endurance,
  • able to land on a small field.

So the problem is getting high efficiency at 60 mph cruise speed with low or moderate wing loading necessary for hand launching.
I did a few test with different airfoils (cambered, symmetric, reflexed) at 44 g/sq dm (14 oz /sq ft) wing loading in XFLR5 and got pretty similar result - best L/D between 50 and 68 km/h (31-43 mph).

So, at the moment it seems that I would only be able to achieve 60 mph cruise speed at best L/D at very high wing loading.
Maybe I should lower the cruise speed a bit, but I'm afraid that the plane won't be able to headway against the wind if the cruise speed is too low.
Should I be trying to maximize the best glide ratio or trying to minimize drag?
Is there a way to satisfy all the requirements?

Regards
Martin
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Old Nov 08, 2012, 12:55 PM
B for Bruce
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The hand launchable part will automatically take care of the small field landing issue.

I'd suggest you look to the style of electric gliders called "hotliners" for inspiration. Clean airframes that easily fly 60mph or more on relatively minimal power yet can be hand launched and landed in small areas.

The flight duration is another issue though. Flying fast generates a fair degree of drag and that means you need to bring along enough energy in battery charge or fuel load to use for fighting that drag for that duration.

The best L/D for a given airframe is not necessarily the speed which it MUST fly at. It's simply the speed where the most ground is covered for the energy carried. But if you want to hit a given speed and still be able to fly as slowly as you need for hand launching and small field landings then you may need to give up a little on the L/D side of things and just fly faster than the best L/D indicates. And keep in mind that you don't need to fly at a 60mph cruise ALL the time. You can hold that in reserve for dashes against headwinds where needed and cruise at the more efficient best L/D speed the rest of the time. That alone will greatly ease the onboard energy requirements.

There's some other tricks you can do as well if you go with the right sort of airfoil. If you pick one that is intended for flaps used as camber adjusting surfaces you can typically pick up a little more high speed drag reduction by slightly reflexing the flaps and ailerons. The sort of airfoils intended for this use are often shown with test results for flap deflections of up to -5 degrees to +10 or so. By picking the right options between airfoil family and then adjusting the amount of negative flap angle you can move the point of minimum drag coeficient to the lift coefficient of the wing when in the high speed dash mode. This may not be the sort of thing that shows up in the XFLR graphs unless you've got coordinates for the airfoil with the flap displaced by the amounts you want.
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Old Nov 08, 2012, 02:03 PM
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Bruce has it right.

Take a look at Dr. Drela's glider glider airfoils. They were designed for high performance over wide speed ranges, with camber changing, and would work well in this application. The camber change is typically only 2 degrees of reflex for high speed, neutral for best L/D at a lower speed, and 3 degrees down for minimum sink or max endurance at an even lower speed. Bigger down deflections can be used for launching (+15), and even more for landing in small places (+45, variable to adjust glide path).

http://www.charlesriverrc.org/articl...t-airfoils.htm

You need to select the right airfoil for the Reynolds number at each wing station.

A bigger span wing and low weight will give you more endurance - minimum sink rate and minimum power are proportional to the span loading. Best L/D speed can be shifted to higher speed by increasing the wing loading though. A higher aspect ratio wing will usually give a better L/D, except at model scales the Reynolds Number effects limit the optimum aspect ratios. A bigger airplane will have better performance, carry a bigger load, and have a wider speed range. A 4m moulded electric glider would work great, but the good ones are $2000 plus equipment.

Prop and motor selection can make a huge difference to the endurance and speed.

Minimizing drag will move the best L/D speed higher. XFLR5 doesn't do fuselage and other parasitic drag at all well, so your real best L/D and best L/D speed will both be lower than it predicts. You do have to run the airfoils with the appropriate flap deflections in XFLR5.

A hot-liner glider with full span camber control is the type of airplane you need, except the space inside won't be that big for long endurance batteries. They are design for minimum drag and high speed. If it has flaps, it should slow down reasonably well. Some examples:

http://www.espritmodel.com/enigma-hotliner-arf.aspx

http://www.fvk.de/Englisch/Mini-Graphite.html

Kevin
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Old Nov 08, 2012, 02:42 PM
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Yeah, the suggestion to look at the hotliner designs in ONLY for a guide as to the sort of shape and style. I assume that it would be made bigger to match the needs for payload.

Mind you as the model becomes larger than you gain the ability to use a higher aspect ratio. In the end it's likely that your best style of design will end up being similar in wing shape and size as some of the faster flying clean style sailplanes but with a bigger fuselage to deal with the payload.

If the fuselage becomes too fat to easily hold for hand launching you can always look at some sort of fold in hand gripping doors or a retractable handle or something along this line. It can be pretty hard to get a reasonable support grip on any fuselage that is much bigger than about 3x6 inches in cross section even if it's a nice egg shape.
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Old Nov 08, 2012, 04:37 PM
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I've already looked at some hotliners. They look very clean.
I also thought about full span camber flaps. As I don't have any practical experience with planes flying so fast it's hard for me to decide what to consider.
I also thought about using the Supra wing design, because I know that Dr Drela has done a very good job designing it. Maybe I'll just scale it for my needs.

Choosing a good propeller + motor combo seems like a nightmare to me. I've taken a look at the various methods and calculators, but it's all too fuzzy for me. Then I stumbled upon a journal paper that concluded that using a large diameter high pitch propeller will give you the best endurance. They were using a 22x12 propeller with 9 degrees spinner twist.
So I have no glue on whether I should use a lo kV motor with a big prop or a higher kV motor with a smaller prop.

Bruce, you are right. The plane probably doesn't need to fly at 60 mph all the time. It just needs to have a reasonable ground speed. Maybe 20-30 mph.
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Old Nov 08, 2012, 07:20 PM
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A couple of thoughts on this:

Bruce has great advice, so the below is provided just to push some thinking to the other end of the design envelope to get the old brain working. As I'm sure he'd agree, there are many different ways to meet design requirements.

I don't know what you're planning to do, if you're carrying cargo, camera, sensors, etc. so it's hard to make a lot of detailed suggestions. If you have a minimum weight of cargo you're going for, it goes a long way for sizing the airframe.

For aerodynamic efficiency you might look at some F3B type sailplanes for design ideas. They have to be both efficient and relatively fast although you probably can lighten the structure quite a bit. Another design concept to look at (if you're going big) is cross country sailplanes. They are big, fast, and relatively efficient.

And as much as I like Drela airfoils (I've used AG series for the last couple of designs I've done, so I am definitely a fan) you also might take a look at MH-32 airfoil or similar Hepperell (probably misspelled) airfoils. If you're planning on flying FPV it has some really nice characteristics, like a nice easy stall rather that a break and drop like some high performance airfoils. As has been mentioned, flapping is probably the easiest method of getting high lift takeoffs and landings as well as low drag cruising.

Cruise speed. 60 is pretty high for most models, requiring a fair bit of power, and combine that with an hour flight duration, and you're talking about a fairly large and heavy battery setup. It might sound like blasphemy, but you might consider a small gas motor. As much as we like to ignore it, liquid fuel has a very high energy per weight content.

As far as hand launching, is that really the requirement, or is the requirement that you don't have a runway for taking off? A twenty foot bungee can loft a pretty big airplane fast enough to get you into the air and give you enough time to get on your way. (Granted, that is more difficult if you aren't using an electric motor.) I'd think this is your biggest constraint in making a model bigger, so out of hte box thinking here might make your design job much easier

As has already been mentioned, as big as feasible for launching and landing makes design a lot easier, especially at the speeds your looking for.

As far as propulsion, I tend toward using large low rpm props for endurance, but that's mostly from my experience with rubber powered stuff, so I'm certainly not a propulsion expert (I'm 96.7% a glider guy.) However, this is going to be one of the big areas to design carefully in order to get to your performance requirements.

Good luck! And I'd like to see pictures when you get this going!
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Old Nov 09, 2012, 01:23 AM
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Also, if a bungee is an acceptable solution, a retrieval net might be an acceptable solution too. I am thinking about launching and retrieving from a boat deck here, since the descriptions seems to be that of a SAR UAV. Another thing to consider to improve wind penetration is something used by full sized glider: taking on a water ballast. This can be then discarded to loiter over an area and to slow down the landing speed, though it will make the return leg slower if the loiter area is distant from the retrieval zone.
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Old Nov 09, 2012, 04:52 AM
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The plane will be carrying a camera and/or a spectrometer. The design AUW is 3 kg (6.6 lb). As I'm not sure how many batteries I need, that number might grow. I also wouldn't want to have more than 3 meters of span, for ease of transport and assembly.
I would like to avoid IC engines if possible. I'm not very keen on using a bungee or ballast either. I would use a bungee only as a last resort. But a 20 foot bungee doesn't sound too bad either.
Some launches will probably be from a boat or a hovercraft. Others from the forest or from a very small field.
I like the AG4x airfoil family because it's just that, a family.
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Old Nov 09, 2012, 07:44 AM
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I think I'm going to go with a flat Supra wing, no dihedral. -2 deg flap shifts the Cl/Cd polar to left and down like expected.
I've also changed the speed strategy a bit. I decided that the cruise speed into the wind should be reduced to ~70 km/h ( 45 mph). And the cruise speed downwind should be ~50 km/h (31 mph). This should make the plane more efficient.

I've also not yet decided whether to make make the wing 3 meters or 2 meters. Analysis in AVL shows that the 2 m wing should be more efficient in the entire flight regime, but a 75 g/sq dm (25 oz/sq in) wing loading would probably be too much for a hand launch.

On the other hand I've handlaunched a plane with similar wing loading, but it was really difficult. Had to be thrown into the wind or else it stayed on the ground. The weak motor and small prop were also culprits in that case.

So, a couple of questions have arisen.
What would be the never exceed wing loading for a successful hand launch?
What CL could I presume in the case of a hand launch? (CLmax is 1.2)
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Old Nov 11, 2012, 12:47 PM
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Here's the paper I found on powertrain efficiency -->http://www.engr.colostate.edu/~thb/P...o%20Markup.pdf
Over 3 hours of flight at near stall cruise speed. That's some endurance.
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Old Nov 11, 2012, 05:11 PM
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Keep in mind that the model was flying at the minimum sink airspeed. Your goal is to cover ground. Not that you may not want some minimum sink "loiter time" at some points in the flight. But just don't focus on the minimum sink duration and forget the other modes that you're after.

You want to range out to an area. If there was no wind that would be best done at the best L/D airspeed. But when you are fighting a headwind the optimum efficiency will be found at some point that is faster than the best L/D. Similarly if you have a tail wind your optimum efficiency will be more towards the Vms where you let the tail wind add the ground distance while the model minimizes the energy use.

Then, once on station, you can loiter in the area while flying at the Vms to minimize power used during the on station portion of the flight.
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Old Nov 12, 2012, 03:08 AM
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Quote:
Originally Posted by BMatthews View Post
You want to range out to an area. If there was no wind that would be best done at the best L/D airspeed. But when you are fighting a headwind the optimum efficiency will be found at some point that is faster than the best L/D. Similarly if you have a tail wind your optimum efficiency will be more towards the Vms where you let the tail wind add the ground distance while the model minimizes the energy use.

Then, once on station, you can loiter in the area while flying at the Vms to minimize power used during the on station portion of the flight.
Yeah, that's the strategy I'm going to use.

What intrigues me about the paper is that you need a grossly oversized motor for good efficiency. A 300 W motor would've been more than enough for that cruise speed.
I'm hoping that this setup would also be efficient at higher speeds.

I also found their youtube videos. Something is seriously wrong if they're having trouble launching it at full throttle. There should be enough thrust to rip the guy's hand off. Here's the video -->
Man Packable Long Endurance UAV (8 min 13 sec)
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Old Nov 12, 2012, 09:23 PM
B for Bruce
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Martig, there's a slew of "oopsies" in that video. First off they are flying during some rather hellacious wind conditions judging by the way the model is kicking around in the guy's grip as well as the sound pickup. But mostly based on the way the model is bouncing in his grip. Then he compounds the issues with a poor thrown that drives the model towards the ground. Then in the second launch we hear them saying that they do not have any left roll response. So either we're looking at badly warped or flexing wings or some other mechanical issue or they did not setup the radio correctly or they did not pick the transmitter program intended for that model. Or perhaps there was damage from the first "arrival". In any event it speaks volumes about the lack of their collective flying experience and lack pre-flight preparation and checks....

The model is also clearly suffering from the sort of higher wing loading that you "think" you need in order to fly fast. Yes, a heavier wing loading model will have it's best L/D occur at a higher flying speed. But that does not mean that it will be more efficient than a lighter wing loading model that is pushed to that same flying speed provided the airfoil is chosen such that the resulting low Cl is still enjoying a fairly low Cd value.

The "key" is to select an airfoil which has good low Cd values right down to a Cl value of 0 or darn close to it. That way when you push to higher speeds and resulting low values of Cl your Cd won't rise overly much.

You'd need to study the numbers to confirm all this but the way I see it is that you go with a somewhat but not overly generous wing area and span to ensure a low enough launch speed that you can throw the model and fly away. But at the same time you use a low camber airfoil that would normally be more at home on a slope soaring model to ensure low drag for the range of speed you'll use for the best L/D or slightly higher for "cruising" to the mission area. Yet by using a low camber airfoil you can then enjoy a low Cd value if it should be required to do a higher speed dash against the prevailing winds to return home or punch through to the mission area.

The bigger wing that encourages safe lower launch and approach speeds will certainly have SOME drag penalty at the dash speeds as well as tending to lower the best L/D speeds. But since it's important for your needs to be capable of hand launching and recovery in small areas that will likely require steep or spiral dive approaches you NEED the handling charactaristics that will come from the lower wing loading.

I would also STRONGLY suggest that 6 to 6.6 lbs as suggested in your first post is an absolute maximum take off weight. Even at that it's tough for most folks to really THROW a big bulky shape that weighs that much with grace, poise and, more importantly, good effectiveness. Again some sort of fold in hand grips or molded in recesses to allow the launcher to get a solid grip yet not have any risk of binding his hands at the release will be important. It's nearly impossible for most folks to throw something the size around of a football that weighs 6 lbs or more. Let alone a "football" with a slippery smooth surface. So provisions for a solid gripping sized and textured area will be important.

Myself? I'd work towards lowering the weight if at all possible. Even if it means slightly reducing the size of the model. Or better would be to work with a lighter structure. It's amazing just how light but still strong a model can be if the right choices are made in terms of the structural design. And don't forget that if you can lop off a half pound from the airframe that you then need to carry a few less ounces worth of battery power. So be sure to chant "Light is Right and Lighter is Righter" while draughting your design....
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Old Nov 13, 2012, 12:59 AM
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Bruce, I guess you are right about using a bigger wing.
Anyway, my last plane was a bit under 8 lbs and had a 3 meter wing, without flaps. Had no trouble hand launching it. And there was no need to really THROW it. Just took a few steps and moved the hand forward and up. The thrust that the motor provides during launch makes a hell of a difference.
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Old Nov 13, 2012, 10:18 PM
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8lbs and a 3 meter wing sounds to me like it's not that high a wing loading. Oh sure, it's higher than some 3 meter gliders but it's on par with lots of other perfectly hand launchable style models. Still, if you got it flying using the technique you describe safely then I'm thinking there was a goodly bit of headwind to launch it into that day. 8lb models even with generous wing area simply don't fly out of our hands with a casual push unless they are at least half way up to flying speed courtesy of the prevailing wind.

And now that I re-consider a 6.5lb model isn't really all that heavy at all for hand launching.

Watch the reliance on the power for getting you out of trouble though. You ALWAYS want ANY model to be at or darn close to the minimum flying speed as it leaves the hands of the launcher. If the launcher gets complacent or some other set of circumstances comes into play a bad launch that is well below the stall speed will someday result in one of those cartwheel crashes we saw on that last video. It doesn't take much either. A model which is 100% trouble free to launch into a 5 mph headwind might be on the ragged edge in dead calm.

I say this due to seeing far too many innocent model sailplanes launched into a cartwheel by bringing the pressure up on a winch or using a high start and simply letting the planes go with no throw at all. Granted this is an extreme case but it shows that there's a degree of room for a sub stall speed launch but not that much. And as the AUW grows the ability to accelerate up to speed drops fast as well. At full gross launch weight a hefty throw is all the more important.

Anyhow, I think I've about exhausted about all I can offer. Be sure to post pictures as the project develops.
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