Which Aerofoil??

[Gordon]

I'd appreciate some advice on choosing an efficient aerofoil section for a fairly quick and streamlined electric-powered model with a wing loading around 35oz/sq ft.

The section will need benign stall behaviour, but will generally not have to operate near the stall. In fact cruising speed will be around 50mph, top speed over 100mph, and I want the plane to be able to cruise efficiently on low power so the flight time will be prolonged, but the plane will still need a good turn of speed so that it doesn't look "slow" on a flyby and will have a good glide ratio - I'll be fitting flaps and posibly air brakes for landing.

On previous models of this type I've used NACA 64A012 and similar, but would like to take advantage of a modern model aerofoil section. The plane will not need to fly inverted very much, so a symmetrical section isn't necessarily a good choice.

Presumably a slope pylon race section, or hotliner or electric pylon racer section would be about right, though the wing loading I'm going to use will be much more than these planes, so the section will need low drag at the higher loading.

I haven't mentioned the type of plane until now as I didn't want to turn everyone off to begin with. It's an EDF jet. I don't simply blast my jets around on full throttle all the time as I like to present the things attractively, and perform aerobatics on low power settings as well as high. So that's why I'm interested in efficiency, and using an aerofoil with the ability to glide well and quick.

Scrolling through the aerofoil list on the Selig website I note that many full-size jets use sections with a couple or 3 percent camber, and with undercamber in the aft half of the aerofoil making the TE droop. However, most racing model wing sections seem to use a reflexed camber line.

So I can't decide which I should plump for - undercamber with TE droop, or reflex, or just stick with symmetrical.

Any ideas anyone?

Thanks

Gordon

[Ollie]

The wing span and wing area will determine the wing chord. The wing chord and airspeed will determine the reynolds number. The reynolds number will determine the airfoil data to use.
The maximum lift coefficient of the airfoil and wing loading will determine the stalling speed and with a stall margin, the take off and landing speed. TheMaximum airspeed and wing loading will determine the minimum design lift coefficient of the airfoil. With the range of lift coefficients and reynolds numbers known, we can search for airfoils with the lowest drag at high speed.

As Number 5 said in Short Circuit,"Need data!"

Wing span?
Wing area?
Flaps?
Desired stall speed?
Desired top speed?
Thrust available?

BTW, any thing you can do to reduce the very high wing loading will improve every aspect of performance. Are you planning on LiPo batteries and a brushless motor?

[Gordon]

Hi Ollie

Thanks for coming back.

The model is a scratch-built 1/7th scale Gloster Meteor, which is a reasonably sleek plane though of low aspect ratio (3.95). However, each engine nacelle is almost the same size as the fuselage on my late 44" span 20-cell MiG 15, so the Meteor will be like 2 MiGs plus a Meteor fuselage in terms of weight. The MiG weighed rtf 7-1/4lb with retracts & flaps and was a very good performer on its 64A-013 root 010 tip, except for a sharp stall.

One section I've been considering is RG-14, but really want a 11% thick aerofoil at the root so that the retracts will fit inside the wing - the reason the MiG wing root was so thick, BTW. The full size Meteor has a 12% root section and 9% at the tip, and I was thinking of 11% root, 9% tip.

Span 63.7"
Area 7.14 sq ft
Projected weight based on experience around 14.5lb max, not very negotiable considering the amount of stuff there'll be in the plane. My current 3 single-fan EDFs all weigh between 7-1/2 and 8lb and are between 32 and 45oz /sq ft, so I'm quite used to landing heavily loaded planes, plus when they're up there, the only thing really suffering due to weight is climb rate and amount of vertical.

flaps - small flaps inboard of engine nacelles, each is 7.5" span x 2/75 chord

Air brakes above and below the wings inboard of the nacelles. Qty 4, 5.75" x 1.25"

Stall speed not critical, but low enough so that the plane doesn't begin porpoising through excess speed on landing.

Top speed? It'll be powered with a pair of my existing power systems, ie 2xSchuebeler DS-51-DIA-3ph fans with 20 cells each and 2xHacker B50-16Ls or Kontronik FUN600/17. Say about 47A x 20 zapped cells, times 2, ie around 2kW all up (I'm already getting 1kW from these setups on 20 cells).

Thrust, about 9lb total.

I'll be using nicads & NiMH as I have loads of 20-cell packs already and I'm not able to spend the £1500+ it'd need (in UK) to get enough Lipos + chargers to keep this plane flying almost continuously all afternoon as I can now with my existing planes, and I want more than one 10-minute flight per hour - I get 5-7 minutes with my present EDFs, the higher figure being easily attainable on GP3300s with the Vector II.

Gordon

[Gordon]

Here's the little beauty in question :cool:

Gordon

[Sparky Paul]

I was amused by Ollie's request for "desired stall speed"...
:)
20 mph would be nice, but with that "wings buried in the engines" design, 70 is probably attainable. :)
Long flat approaches....

[Vipula AD]

Gordon, have you looked into some of the Eppler airfoils. In particular the E197. This section has gentle stall characteristics, its fast and works well with flaps. If you need a slightly thinner section you can use the E195 or the E205, but the stalls are a little more abrupt. With 25% chord 2/3rd span flaps deflected 40 degrees you will be able to get about 27-28mph landing speed with that wing loading.

I do have one question, Isn’t 35oz/sf a little too high for 50mph cruising efficiently? At that WL you will need a cruising Cl of about .35, a 60 to 65mph cruise would provide a better efficiency I think with a cruise Cl of .22. Just thinking loud, I’ll have to look in to some of the data.

I once had an OS .25FP powered sport plane that beet every thing in our club, except the pylon racers, in level flight. I tested both the E195 and the E197 airfoils on this model and I liked the 197 for its better cruise and stall handling. I could not tell a difference at the top end between the two, never had the means to measure the actual speed of the aircraft but I believe It would pass 100mph with the 7X7 or the 7.5X7 props.

Hope this helps,

Vipula.

[Vipula AD]

Now that I see what you are building, I would like to re-state that landing speed, It would be more like 35mph. scale flaps arn't going to be that much affective, on that planform with the engins taking up a lot of wing area.

I do like the airplane, thats one of my favorites of all time.

Vipula.

[BMatthews]

Like Ollie said we really need more data but off the top of my head I'd say you have the same requirements as the F3B group.

So things like the RG15 (1.5% camber) or HQ 2.0-xx series would be good places to start. Both of these types share that trailing edge cusp you speak of. They also are meant to be used with flap function to extend the low speed or decrease drag for the high speed side of things by reflexing the surfaces slightly. This would be best accomplished in a more or less automatic mode by mixing in a flaperon function with the elevators and perhaps with a mix to the throttle as well. Low throttle adding in some positive mix. The possibilities are only limited by your imagination and mixing commands on the transmitter.

But 35 oz/sq ft is heavy in anyone's books if the model is on the low side of the 100 inch span barrier. And it's pretty much the kiss of death for anything under 50 inch span and 500 sq inches IMHO. For decent gliding ability you really don't want 500 sq inches to have to hold up much more than 3 to 3.5 lbs. You should really examine schemes for lightening your structure first and foremost. The ability to cruise as you want is going to be largely dependent on that aspect.

PS: I submitted an old answer long in the composing and didn't see the later posts. I'll take back what I said about the smaller stuff. The Quabek series runs up to that % thickness and may offer you a decent option. Inverted will suffer if you don't wish to ruin the scale aspects by adding a full span flaperon mod. If you do make the center flaps separate in function for large deflections during landing only. Full span flaperons act funny if deflected past about 25 degrees if they run all the way to the tips.

I still say you need to get the weight down but you already know about that. Best of luck.

[Ollie]

With the high parasitic drag of the combined tail, fuselage, engine pods and interference drag plus the high induced drag of the low aspect ratio wing operating at a high lift coefficient associated with the high wing loading, the profile drag of the airfoil will be a small part of the drag budget. That makes the drag characteristics of the airfoil selection uncritical.There are plenty of airfoils whose performance will be indistinguishable in this application.

The S8052 is 11.89% thick and was designed for pylon racing. It might be a good choice.

I hate to rain on your parade but this plane looks to me to be overloaded and under powered.

[Haldor]

Originally posted by Ollie

I hate to rain on your parade but this plane looks to me to be overloaded and under powered.

I am fully assured that the projected powersystem will fly this bird very well. It wont land at 10mph though :rolleyes:

[Sparky Paul]

I've seen a much smaller electric Meteor fly locally, emualting the turbo-prop version, which flew reasonably well.
It does have a daunting top view... all those vortex generators, and so little wing! :)

[Sail 'n Soar]

On previous models of this type I've used NACA 64A012 and similar, but would like to take advantage of a modern model aerofoil section. The plane will not need to fly inverted very much, so a symmetrical section isn't necessarily a good choice.

With your low aspect ratio and wing loading, RE is not going to be an issue - most likely around 300,000 at stall. Don't worry too much about the reflex and super low drag more recent airfoils, you have too many other drag generators to worry about the small CD change. I'd focus on something like 2 to 3% camber and 10 to 13% thick foils. They will give you a lower stall speed but good inverted flight capability at your cruise speeds. With those parameters in mind, check out the Eppler 195, 197, 205, 207 or 209. I believe they were designed to perform more in your RE range. Check out the following site for candidate airfoil polars. Not that many Epplers, but numerous others you might consider.


http://www.nasg.com/afdb/search-airfoil-e.phtml

[Sail 'n Soar]

Originally posted by Sparky Paul
I was amused by Ollie's request for "desired stall speed"...
:)
20 mph would be nice, but with that "wings buried in the engines" design, 70 is probably attainable. :)
Long flat approaches....

How does 30 sound?

[Sparky Paul]

35 oz/ft^2 might be unhappy at a mere 30 mph. :)
The SAE planes at 50 oz plus loadings land pretty close to their takeoff speeds.

[Gordon]

Many thanks for your really helpful comments and airfoil suggestions. Whilst i accept your comments that airofoil drag will be small cf all the other drag contributions, I always feel better using an established section in my designs.

As Haldor said, with 135W/lb the plane won't be underpowered, as the Schuebeler is the most efficient EDF unit available, and 9lb thrust is going to give the plane a genuinely sprightly performance and it’ll look like a jet fighter, though it won’t be in the pylon racer class which isn’t an aim anyway. Plus the pitot intakes are probably more efficient than lateral ones.

Nor will it be overweight at 32oz loading, as 35+ oz wing loadings are standard for ducted fan planes, my highest being a 17lb 1/6th scale BAe Hawk at 54oz/sq ft with all of 5 sq ft wing area and an OS .91VFDF/Ramtec for power. So high landing speeds of the order of 30mph are fairly normal, and the Hawk was (amazingly) a pussycat with its slotted flaps.

The Meteor is a different ball game with its low AR wing which should give it a gentle stall, and though the nacelles are streamlined, they do look huge, so I guess there’ll be some interference between them and the fuselage. The Me 262, with its uncluttered wing upper surface undoubtedly was a more efficient layout, and I have wondered in the past if this might be the reason why the 262 needed only 234 sq ft of area vs the original Meteor Mk1's 374 sq ft. The 262 had quite large area-increasing slotted flaps as well which would help on landing.

However, loads of model 262s are flying around (not many with the flaps), and there are plenty of kits of various sizes for that ship, so I’m intent on doing the Meatbox whatever its failings.

I find it interesting that the Epplers show up strongly, as I thought they had probably been supeseded during the past few years. I used E205 on a 41oz/sq ft 52” span P-38 Lightning which always handled nicely, even on one engine, so I’ll definitely look at those.

The real Meteor F8 had an aerofoil called EC1240 from the root out to the dihedral break, tapering to EC 1040 at the tip. This was designed by the Royal Aircraft Establishment at Farnborough, England, but I’ve never seen co-ords. I presume 1240 = 12% thick, max at 40% chord, which seems reasonable for the period.

So, having perused my Compufoil, how does this sound:

E 207 inboard, 12% thick and 2.5% camber,

E 205 tip, 10.5% thick and 3% camber?

Or will I be mixing up the wrong aerofoils?

A pic of 3 of my 20-cell Schubi-powered jets is below. All are flap/retract equipped with from 7 to 10 servos, and weigh from 7.5-8lb. Though they all span only around 45-48 inches, their flaps are bigger than the 64” Meteor will have :eek:

FWIW:

The El Bandito on the left uses thinned (6%) MH-43
The Bob-E-Cat (cente uses Naca 4-figure foils, about 12% to 10% at tip
MiG has the 64A-013 to 010 tip
The Vector II in my avatar has EH 1.5/9 at various thicknesses.

Many thanks again :)

Gordon

[Sail 'n Soar]

Your airfoil choices sound good to me. I actually prefer the E205 - 209 to many other airfoils because their shapes are easier to model and don't have the extremely thin trailing edges of some others. I believe that much of the loss of favor with the Epplers was that they were designed for higher Re's (i.e., Re > 200,000) than some of the newer, more in-favor foils. But has I indicated preciously, with your 4:1 AR and high wing loading, the Re over your entire operating envelope will be greater than ~300,000, so these older airfoils should work just fine.

[Gordon]

Sail 'n Soar

The thicker te is a bonus, esp as I want to fit the scale split flaps.

Actually I am also making another MiG to replace my late one which I pilot-errored into the tarmac on the final turn into landing after running out of electrons and stretching the glide beyond what the batts and Mr Naca thought was reasonable. I had around 150 great flights with the plane which had a 42oz loading, but landings were always too fast until I fitted the generous-sized split flaps which slowed landings down dramatically. It was a squeeze getting the servos in the wing, and I only got around to doing it half a dozen flights before its demise :(

The next one will be 5% larger to improve accessibility. I was also wondering about whether to change the aerofoil for the rebuild, and think I'll try replacing the Naca with a E209 root and E205 tip. It'll be worth the experiment, as nothing ventured, nothing gained, and I already know how the naca 6-figure works so don't necessarily need to try it again.

The 13% - 10% root-to-tip depth taper is needed to allow the retract gear mechanism to fit inside at the max aerofoil depth of 12% at the wing's semi-span. The retract unit has to be tilted 17deg wrt the chord line, and angled 17deg wrt the transverse axis of the plane so that the wheels will fit inside the wing root at the max root aerofoil depth and lie flat there, yet be at the correct position wrt the CG when the plane is on the ground. The legs angle forward 17deg when retracted, and 17deg aft when extended. The Epplers should allow this geometry as easily as the Naca did.

Gordon

[Sparky Paul]

Found dit...
T'was an EDF Meteor, not the Trent version.
Memory does fade...
It flew well with the equipment available 11 years ago..
Seen at Mile Square, Nov. '93

[Gordon]

Thanks Sparky :)

That looks cool. It's the Mk 3 version with long span wings & rounded tips (oops - I just noticed the book cover ).

It definitely has a lot more wing area than the later versions, except for some of the night fighter and photo recon developments.

They still use a Meteor T7 ex-trainer at Martin Baker for ejection seat testing. They have two, which fly about 6 or seven missions a year each, so they have a lot of life left in them yet.

Gordon

[Gordon]

I began drawing my Meteor plan today. However, I measured my car first and found I needed to shrink the plane a little. The span drops from 63-3/4" to 62", but the length reduces from 76-1/2" to 68-1/2", as I changed from modelling the 44ft 7in long square-tail F8 version to the 41ft long elliptical tail F-4. The wings and nacelles are the same for both marks.

Gordon