DomPotvin

Hi guys !

I need your experience here. Im doing a 1/6 version of the MQ-9B (also know as the Predator) and i need advice on airfoils.

The plane is going to be quite large (around 12 foot) and im sure it will be tail heavy since the engine will be at the back.

Ill probably need an adjustable counter weight, but even then wondering if it will be enough.

The nitro plane MQ-9 replica use a eppler 374, look good to me, but im really new to modeling so ill need your great advice.

The purpose of the plane is to be a drone, equiped with the ArduMega autopilot system. Im not there yet ! but i love the project, make me go back

to my woodworking skills.

So slow to medium speed, with great endurance for the fuel (might switch to a fuel cell in the future, yes it already exist some

system :http://www.horizonfuelcell.com), i know its not the best plane to have fun with, but its more a challenge for me and ... the computer will fly it not me hehe

Also if you can suggest airfoil for the tail, would be appreciated.

By the way ! im doing a lot of reseach but math arent my best skills !

Ill soon post picture of the frame of the plug.

Thanks !

Dominick

pimmnz

Any non-cambered (symmetric) airfoil will do. The tail will need to lift up and/or down through the various flight envelopes, so there is no advantage to using a cambered foil. So far as the balance is concerned, if it is a bit back, then the type of foil on the tail is not going to matter, it won't fly anyway. The single most important thing with any model airplane is the stability in pitch, although in this case, if you have a computer flying it with an aft balance, then when the computer fails, at least it won't go very far before impact. If it needs a computer to fly it, then human intervention won't be possible either, and again, impact will be reasonably close to loss of computer control. I suggest you do what is needed to balance the model so that the balance is in the stable range.
Evan, Wb #12.

DomPotvin

Hi Evan !

Thanks for the answer, about the computer fails, its really not important for me where the plane will crash, actually, the furter the better ! It means that the plane fly easily and that's is exactly what i want to achieve better fuel economy. Ill make sure that the balance will always be correct with the use of that counter weight, since i plan to have 4 tank ( 2 in the fuselage and 2 in the wings).

And about the airfoil, as seen in this picture, the lower camber should'nt get me a best support for the weight on the back ?

Sorry for my language, imsearching my word a lot ! its good for my english (first language is french).

charlie111

I agree for the most part.Your problem is not the airfoil itself but it's locatin.The cord line should be at the c.g. Either move the wing back so the plane balances on the cord line or 1/3 back from the front of the wing roughly or if you have plenty of power and it's not to tail heavy add some weight to the nose so it balances where it's at?

DomPotvin

Hi Charlie !

The lenght of the plane is 53.5''

The wings begin at 23'' and finish at 33'' . so its in the second third of the plane almost at the second third .

For now its place directly in the center of the frame.

Im thinking about the engine ... maybe i can install the firewall a bit more to the front and add a drive shaft to balance the plane !?!

The frame for the plug (going to be in fiberglass) is almost done, ill see how it balance once assemble. Ill tell you later today and post some pictures.

Thanks for you help , i really like the community !

BMatthews

To reinforce pimminz post. You can't make a tail heavy plane fly by giving the stabilizer an airfoil. Life, and aerodynamics, just does not work that way.

Check out the sticky thread at the top of the titles for Aerodynamics calculators and tools for the online CG calculators. Input the numbers and use "5" for the stability margin. What it tells you for the balance point is where it needs to go.

Try to position most of your equipment in the nose of the model so that it limits the amount of useless nose weight needed to achieve a stable CG location.

DomPotvin

Alright ! Seem ill have to work with CG calc !

Appreciate your help guys ! without you would never fly !

Here's a picture of the plug frame, just finished it ! Yay !


Thanks again !

tobuy3003

Some of the old timers has the CG far back. You can copy one of those.

RAPPTOR

My Feedback: (41)

YOUR CORRECT..YOU NEED A SEVER UNDERCAMBER AIRFOIL. Like the A-10 WARTHOG uses.

RAPPTOR

My Feedback: (41)

..YOU NEED A SEVER UNDERCAMBER AIRFOIL. Like the A-10 WARTHOG uses. Moves center of pressuer aft of N/P..C/G will remain the same..will cause neg pitch moment.. stab will need neg decalage. or "up". will fly slow

otrcman

The reason the old timers can fly with the CG so far back isn't due to the wing airfoil. The CG is far aft because of the size of the horizontal stabilizer. The CG is still safely ahead of the Neutral Point. The NP is just farther aft due to the big stab.

I know there's a lot of mystique surrounding "lifting tails", and some horizontals do indeed lift upward instead of downward. But it's due to the relative sizes of the wing and horizontal stab. CG still has to be ahead of the NP, even on a canard.

Dick

DomPotvin

Alright ! a clear answer , thanks Raptor ! ! !

I have worked all my data in a cg calculator and ill balance the plane that way.

Now for the wing, ill go with the Naca 6716 to naca 6713 at the wing tip, ill also add a tip fence to reduce the vortex. It is design to be a slow plane so no problem with the airfoil.

Raptor you telling me : Moves center of pressuer aft of N/P..C/G will remain the same..will cause neg pitch moment.. stab will need neg decalage. or "up". will fly slow

If i understand correctly in normal word this would mean : a sever undercamber airfoil will move my center of pressure toward the back of my neutral point, this should give the plane a push up from the back right ?Center of gravity will remain the same. That's exactly what i need (from my low engineering knowledge ! haha im a cabinet maker ! now thinking to do a certificate in aeronautic i love that ! furniture is soooo simple ! ).
Stab will need a neg decalage, so they will need to be oriented to make the nose to down by its natural position , is that right ?

Also another question, the angle of attack of the wing i think it should be around 6-8% compared to the fuselage and 0% compared to the ground on the landing gear, to give me some clearance for my prop and rudder ... you can clearly see what i mean in this picture.

Sounds good ?

(what aft mean toward the back ?? again my french give me some headache)

and also some picture of the carved fuselage plug

pimmnz

I will try to remove some of the fog...You say this will be a slow airplane...airfoil choice will not determine airframe speed, weight determines airspeed. If you want to fly slow, build very light. All model airfoils stall closer to 9 degrees than the usually quoted 'full size' 16 deg or so, this should concentrate your mind about the chord setting to the fuselage. 'Undercambered' (high cambered) sections have very high drag rises as speed rises, this limits the top speed they can reach, and yet they will stall about the same angle as any other foil, so this will reduce your 'speed envelope'. Choose a nice, gentle 'all round' foil so that you can fly in differing conditions. The ground attitude chosen will not affect the 'wing-body' angle, you will need to determine the lowest drag attitude of the fuselage, this will be your 'flying attitude'. Set the wing chord line about 1/2 a degree positive to this. Set the tailplane angle at this attitude. Combined with the weight of the model this setting will then determine 'cruise speed'. Balance the model at around 25~28% MAC for good 'hands off' stability. Set the mainwheels just behind the balance point and then the ground attitude will not really matter, the nose will rise without much airspeed and the model will fly off in the usual way. The extreme 'nose down' attitude of the original may be required to make sure that the thing 'sticks' to the ground after landing and before the speed has reduced below stall speed. Yes, aft is the back end, and no the ground attitude has little to do with prop clearance, but the long mainlegs do...Special 'anti-vortex' tips look pretty, but in model sizes do little than offer extra drag, vortex generation is a product of lift, if you try to eliminate the vortex, then you eliminate the lift...A nice, upward curved underside of the wing over the last few inches or so will assist shedding the vortex and lower the drag just a bit, per Mr. Hoerner.
Evan, WB #12.

BMatthews

I hate to say it but you've still got some way to go with this stuff. And frankly I'd say Raptor does as well. Just because an airfoil has a strong flap like rear curve does not mean that he center of pressure is moved back so strongly as to be behind the aircraft's overall neutral point. Especially since the neutral point location is set by the size of the stabilizer compared to the wing and how far back from the wing it is located.

The sort of "flaps down" like severe undercamber of an airfoil like the 6716 is going to produce a noticable nose down pitching action compared to a more normal looking airfoil. This means that rather than the tail needing to produce positive lift it would be the opposite and it would need to produce less lift to hold the wing from pitching. Or even, depending on where the CG is located, negative lift to control the wing's nose down pitching action due to the airfoil.

There is also the fact that the 6716 is a very extreme airfoil for model sizes. It's quite possible that it won't perform anywhere near the same for your size of model as it did on the A-10.

It would not be a bad idea to seat the wing with some positive angle. But your model will likely not be as highly loaded as the real thing. And especially if you're trying to keep it light so it'll fly slowly then you want to reduce the wing's angle to about half to two third of your 6 to 8 degrees so the fuselage doesn't end up pointing nose down in flight.

DomPotvin

Hi BMatthews !

First , dont be sorry ! I prefer that to crash my model haha ! And like i said i love learning aerodynamics ... ill need some math tho ! Algebra is sooo far behind me !

About the weight, it was "natural" for me to think that a light plane would react better, but im getting the point, lighter isnt better ! and i understand why. That explain why a 30 000 pounds plane can fly. Lift not weight ! and resistance to the air flow (in my own words) that's a good news since ill be able to fit stronger frame inside and more fuel.

Here's are my result for my cg calc and they seem alright and feasible.

MAC : 6.84"
MAC distance from root chord : 27.8"
Wing aerodinamic center aft of root LE (not sure to understand that one, i think its linked to the airfoil ... ? What they mean by LE ?) : 2.5"
Ideal center of gravity % of MAC : 42.9%
Neutral point % of MAC : 47.9%
Neutral point aft of wing root LE (NP) : 4.07 (again not sure about this one)

V- bar ration is : 0.626 wich is alright , no ?

I dont think ill have problem regarding the CG
For the NP ... how can i calculate that ? Wing lift, tail force etc etc ? make a model a trow it from the roof ?

BMatthews

First off your MAC of only 6.84 inches tells me that you're not going to have a lot of wing area to work with. Also the reynolds numbers for such a small and narrow chord wing are not going to be all that high. Especially if you expect the model to fly at all slowly.

There's some good arguments to be made for large jumbo scale models being of some minimum weight and wing loading to allow them to fly well in brisk conditons. But with your sort of wing and all the mass of the fuselage you're not going to end up too light by any means. And in fact at your model's size "Light is Right and Lighter is Righter". Realistically with the style of construction you're using it will be impossible to make your model TOO light. So work at keeping your weight as low as you can manage.

LE in the results is short for "Leading Edge" of the wing's airfoil.

The program has given you the location of the desired balance point AND the overall aircraft neutral point. It is just giving it to you using the MAC's leading edge as the reference point. THere's no need to find another Neutral point. To be stable the final balance point, or CG, needs to be ahead of the NP. WHich the program has done with the CG at 42.9% and the NP at 47.9%.

But those numbers are too far back for this design due to the short tail length and the fairly low horizontal equivalent of the tail area. I suspect you used the total area of the tails instead of calculating the effective horizontal area. To find the equivalent area use the calculations found at http://www.charlesriverrc.org/articl...tailsizing.htm . With that equivalent horizontal area run the calculations again.

And finally, with a wing chord of only 6.84 inches you're not going to be able to use an airfoil as extreme as the NACA 6716. Instead you'll want to use one of the higher lift airfoils used for 2 meter sailplanes or as found on some free flight models. Especially if your goal is to be able to fly fairly slowly.

One that I would suggest which is still thick enough to come out strong is the Selig S4233. Another good one that I know does well for slower flying at moderate wing loading is the S4061. Even the good old Clark Y has a great reputation for working well at this sort of size for the sort of flying it sounds like you are after. There's a few others but which you would be best to use will depend on how you're going to build the wing. In the meantime you can see what these two look like and see that they are a lot less extreme than the 6716. And less extreme is what you need.

charlie111

I know they had a similar problem with the phantom Jet.They angled the ELEVATOR TAIL FINS DOWN.I t Might Help with your Problem.I don't know about angle of incidese if there is one possibly nuetral.Though I think not?

BMatthews

Charlie, they did that to get some of the area down under the wing's blanking effect when operating at high angles of attack. Ideally they would have lowered the whole stabilizer but the jet exhausts were in the way. So the anhedral angle was a "patch". The prototype Phantom actualy had flat stabilizers but they changed them at some point through the prototype's development. The prototype also had flat wings instead of the dihedraled tips.

RAPPTOR

My Feedback: (41)

this should give the plane a push up from the back right ? Center of gravity will remain the same YES. Allmost ,all plane requier -up trim -when flaps are lowerd. Nose {down} over pitch.. to keep level flight with a, A-10 syle airfoil. Neg stab or decalge is needed.. this will cause the airframe to hav a lot of drag,, kinda flying in a bind ,,sorta.. airfoil like this FX63137 ZERO LIFT ANGLE -8.8 MOMENT @1/4 CHORD- -.238

RAPPTOR

My Feedback: (41)

Just do wing c/g.. Forget about the,,,, whole plane therory..,,, Telamaster has a, giant lifting stab. WING AIRFOIL IS A NEG PITCH NUMBER..Flys super.

RAPPTOR

My Feedback: (41)

GEORGE MILLER used this ,airfiol,{67-SERIES} on his A-10..Won all scale meets for years. Took off by it self.. Works great on models.. BY the way,i tried the airfoils you recomended for, a no tip stall wing..IT STALLED VERY SEVER AND NO RECOVERY.. I respect you comments and all the help,, BUT,real life testing does not allways follow. BOOK LEARNIN. IF so ,we would not need test pilots and wind tunnels.. Be safe and have fun,,

pimmnz

Actually RAPPTOR, all my models go nose up with flap application, from scale (Monocoupe) to aerobatic (Sicroly and Curare). The only model I have that displays little or no pitch up with flap application is my FW 190, but then it balances closer to 20% MAC than 25%. As BMatthews said, you'all have still got a ways to go with this stuff...
Evan, WB #12.

rmh

I read all the inputs on this thread
Not certain I understand what is going on-

RAPPTOR

My Feedback: (41)

At what speed?? How about a T-45?? Do some lookin up on the T-45. If you find that it pitches up on flap deployment, let me know please.. I have seen, and know for sure if more than 15 deg. flap is useded, the neg pitch will crash the plane. Elavator will not recover.. look on you tube and watch it happen.. I HAVE DONE A LOOP WHEN I PUT FLAPS DOWN- Whenfrom c.g.) must also be zero.
TOPTIC WAS TAIL HEAVY CORRECT?? THIS WILL FIX IT,, SIMPLE..
conventional airfoil with camber airfoil with reflexed mean line
Equilibrium State

This airfoil has a nose heavy moment. As stated above, the center of gravity is also the center of rotation of the wing. When it is shifted behind the c/4 point, the air force L* in front of the c.g. counteracts the nose heavy moment M* to achieve equilibrium. The distance between c.g. and c/4 point is depending on the amount of M*. A symmetrical airfoil has M*=0, which means we have to place the c.g. at the c/4 point. The reflexed camber line makes the moment coefficient positive, which means, that the moment around the c/4 point is working in the tail heavy direction. Therefore the center of gravity has to be located in front of the c/4 point to balance the moment M* by the lift force L*. The larger the moment (-coefficient) of the airfoil, the larger the distance between c/4 and the c.g. for equilibrium.
Disturbed State

When the angle of attack is increased (e.g. by a gust), the lift force L increases. Now L>L* and the tail heavy moment due to the lift is larger than the moment around c/4, which still is M=M*. Thus the wing will pitch up, increasing the angle of attack further. This behavior is instable and a tailplane is needed to stabilize the system. Here, we have the air force acting behind the c.g., which results in an additional nose heavy moment, when the lift increases. With L>L*, the wing will pitch down, reducing the angle of attack, until the equilibrium state is reached again. The system is stable.

Neutral Point and Stability
As we learned above, an unswept wing with a reflexed airfoil is able to stabilize itself. Its c.g. must be located in front of the c/4 point, which is also called neutral point (n.p.). The distance between the neutral point (quarter chord point for an unswept wing) and the center of gravity is defining the amount of stability - if the c.g. is close to the n.p., the straightening moment is small and the wing returns (too) slowly into its equilibrium condition. If the distance c.g. - n.p. is large, the c.g. is far ahead of the c/4 point and the wing returns quickly to the equilibrium angle. You will require larger flap deflections to control the model, though. If the distance is too large, the wing may become over-stabilized, overshooting its trimmed flight attitude and oscillating more and more until the plane crashes.

A measure for stability
you argue wih someone your just teaching them Thanks for help and info.

pimmnz

So far as my models tell me, at any speed above stalling speed, and the faster, the greater the reaction. When I was learning full size the Grumman I was using went a bit nose down with flap application, so you trimmed a bit nose up to maintain approach speed. Models appear, at least all those I fly with flaps, and it is a reasonable cross section, to go nose up. Why this should be I do not know, but the empirical evidence suggests that it it is so. What is a 'T-45'? I do have articals by Pete Russell, he of the famous 'STOL' model, and he reported the same thing when he nailed flaps on the STOL wing too, so it is not just my experience. Insofar as this discussion pertains to the OP, his proposed section is like having flaps down all the time, so it will be draggy and trying to put the model 'nose up' at anything over his 'trimmed speed'. This won't be much fun to fly. What Mr. Matthews and to an extent, myself, have been trying to say is that he has a short moment, high aspect ratio glider, with a fairly small chord, combined with a long fuselage extending forward. If this is to be in any way successful then it will need to be built both light and straight, with considerable care required with control surface size and movement along with a very safe balance. The section choices at this models size and chord, along with the wish to 'fly slow' dictates a wing section not unlike a 'Clark Y' or any derivative thereof. Even a nice symmetric foil will work better than the proposed section. And after all, this is only advice and you can accept it or not, and I for one don't mind one way or the other...
Evan, WB #12.