Looking for an airfoil with high lift and gently stall

[nick_cool]

Hi, I am looking for an airfoil with high lift and gently stall, I want it to re-made the wing of a Great Planes Gee Bee.

I do not need high speed or the best performance, I only want to make easier the flight, specially the landing.

I have seen the Eppler 197, but I would like a thicker one. I saw too the Eppler 68 but I have no reference about it.

Does anybody have a suggestion?

Good landings,
Adrian
:cool:

[nick_cool]

here are the polars for E 68 and E197

[JetPlaneFlyer]

You wont go far wrong with a good old Clark-Y.. Add generous washout to give a nice safe stall and you should be in business. The Clark -Y also has the advantage of being easy to built due to it's (mainly) flat bottom. Regardless of what airfoil you use the Gee Bee is unlikely to be a 'floater' on landing as it will most likely carry a pretty high wing loading.

Best avoid very thick airfoils because they dont work well in anything smaller than giant scale models.

Steve

[eflightray]

If it's the ARF Gee Bee from Great Planes -

http://www.greatplanes.com/airplanes/gpma1326main-2.jpg

The spec on it says - Wing Loading: 34.9-38 oz/ft² (106-116 g/dm²)

Gentle stalls and low speed come from low wing loading, 34 to 38 oz/ft² to me on just 68" span, IMHO is like a brick with stubby wings.

That said, it is a very good looking model and probably flies extremely well, but it is not designed as a slow flying trainer, it is a scale sports pylon racer and should be flown like one.

I personally don't think any change to the wing section will make much difference. Now if you want to double the wing area, biplane ?, (and increase the stab accordingly)..........

[JetPlaneFlyer]

I personally don't think any change to the wing section will make much difference. Now if you want to double the wing area, biplane ?, (and increase the stab accordingly)..........
Agree 100%... in my previous reply i assumed you 'had' to make another wing due to the original being damaged.. It would not be worth making another wing simply to try to reduce stall speed. Any advantage of a different airfoil would be marginal at best because there is probably nothing 'wrong' with the original airfoil anyway.. it's weight not airfoil that's the 'problem'.

Steve

[nick_cool]

Yes, the plane is the one in the picture, and yes, the wing was broken and I have to make a new one.

It will never fly slowly, yes, and the factory model tip stall and is hard to land; but using a high lift profile plus some tricks, it can be domesticated.

I found HQ 3,5-14 and I think it has all I need. Does anybody tried it?

Thanks
Adrian

PS Anyway, the good news is that I will tell you what happened in a few mounths

[Joel K. Scholz]

Cant go wrong with an eppler 197. High lift, low drag, gentle stall. Flies as good upside down as right side up.

[Butch777]

You may think about adding a inch or two to increase the wing area.

[JetPlaneFlyer]

Here are the airfoils you have been talking about plotted in Profilli at the Re number typical of your model's landing speed. I've added my recommendetion (Clark-Y) to the graph also.

In my opinion the Clark-Y is best.. joint highest Cl max and gentle stall. It's also easiest to build and closest to scale in profile and has reasonably low Cm.. no contest as far as I'm concerned. The HQ is next best having similar performance to the 'Y' but non of the practical advantages.

The other practical issue is picking an airfoil that has an upper profile very close to the original used on the model.. Otherwise achieving a fit up of the wing to the fiberglass fuselage will be all but impossible without some major rework of the fuselage....

Steve

[vintage1]

If its tip stalling on finals, put in washout or reflex outboard ailerons up.

That should at least turn a snap wingdrop into a mush..

[HugePanic]

what about an kfm-"airfoil"????

[nick_cool]

According profilli all of them are similar, I have to make my mind.

Regarding to add generous washout I agree. Are 2°~ 3° enough???

And yes I will add some extra area, may be 15%, as well as remove some weight.

A

[Butch777]

Weight removal does help.

[vintage1]

2-3 degrees makes a huge difference to any plane. I added that to a sail plane that ALWAYS spun in on last turn. (I fly in small spaces). Completely fixed it.

[nick_cool]

I already did the autopsy, and I think I can remove 300g~400g from the original 5kg. :cool:

I will go with 2,5° of wash out, I like this better than use two different airfoils. ;)

The original model has 8° of total dihedral, or 4° each wing from the horizontal. I think this is too much, and may be 4~6° is enough.

Still thinking about the airfoils… the noble E197 vs. the lifter HQ 3,5-14… :rolleyes:

[JetPlaneFlyer]

The real Gee Bee had 4 Deg per side dihedral (near as i can measure from my 3-View).. I cant see any reason to reduce it to less than scale?.. Certainly it wont help the slow speed handling.

Why are you not considering the Clark-Y?.. In the simulation if performs clearly better than the (less cambered) E197 and at least as good as the HQ.. plus it's easier to build than either, closer to scale in looks and well proven in use.

If you want a REALLY high lift airfoil then you need to go for something with lots of camber. Something like the GOE 528 with 6% camber would in pure lift terms outperform anything we have talked about previously with a Cl max about 15% higher than the next best and close to 40% higher than the E197.. Dont expect sparkling top speed or any inverted performance though ;) (new graph attached)

Have you resolved how you would address the fit up to the fuselage.. This could be a real show-stopper especially with the higher cambered and thicker airfoils.

Removing some weight and adding 2.5 Deg washout are both good suggestions.. i'm amazed that the original wing did nor have some washout built in.

Steve

[nick_cool]

Thanks for the info about real dihedral, I will keep it then. :D

I know that I have to fit up the wing to the fuselage, but I am willing to do it, because to change the airfoil is a must and the fuse is anyway broken.

Clark Y is a choice too, but as I am going to make the wing in foam and balsa, then the construction is not simplified by the flat bottom.

Here you can see that the wing has not wash-out, and that the airfoil it is very close to a symmetrical one.

[vintage1]

Anothet good trick is to use a fattish root section and taper to a shallower section at the tip. With a 'flat bottomed' section this gives natural washout without the wing looking 'twisted'

[MCarlton]

Would it not make sense to use a Clark Y rigged at 0 degrees incidence? That would give decent flight performance in normal trim, but add a few clicks of up trim in for landing, and you have a nice floaty section. A sort of cut price variable geometry?

[nick_cool]

The original wing is at 6°... may be 0° is too low

[BMatthews]

The Clark Y in this case MAY, I'm not saying it will, produce too much pitching moment for the size of the horizontal tail.

There's a tailor made airfoil for heavy scale models. It's the Selig S8036 and S8037. They are thicker but they are designed to work in exactly this case for this type of model. By all accounts they offer up excellent pitch stability and a delayed and gentle stall. Selig designed the airfoil for the Top Flite scale model series and all the reports of flying with that airfoil say that the new wing really transformed the models from scarey to just worrisome..... :D

So I'd have to say that in this case I'd put my money on the 8037 which has a touch more camber to the shape so it should be that touch better for slower flying. But if you'll be trying some inverted you may want to go with the lesser camber of the 8036.

These two airfoils come package in Profili2 which you can get from www.profili2.com . Good luck with the new wings.

[Pauld109]

If I may Hijack Adrian's thread for a minute I'd like somebody to explain how you would choose the optimum alpha for the GOE 528 from the attachement contained in Post 16. Thanks in advance.
Later
Paul

[JetPlaneFlyer]

If I may Hijack Adrian's thread for a minute I'd like somebody to explain how you would choose the optimum alpha for the GOE 528 from the attachement contained in Post 16. Thanks in advance.
Later
Paul

Depends on what you mean by 'optimum'

Best l/d (lift/drag - i.e. best glide ratio) is the point where the value of lift at a certain alfa divided by drag at the same alfa is greatest.. Profili produces a seperate l/d graph makes identifying this point easy.

Minimum sink rate is usually just before the stall point.

Steve

[Pauld109]

Depends on what you mean by 'optimum'

Best l/d (lift/drag - i.e. best glide ratio) is the point where the value of lift at a certain alfa divided by drag at the same alfa is greatest.. Profili produces a seperate l/d graph makes identifying this point easy.

Minimum sink rate is usually just before the stall point.

Steve

OK thanks for that. So at an Re of 110000 according to profili optimum alfa would be 7. Is this correct? If not perhaps you could paste the correct solution.
Thanks for your help
Paul

[JetPlaneFlyer]

Just run a new graph... Yes 7 deg is the best l/d but the curve is almost flat between 5 and 9 deg so anywhere in that range.

[Pauld109]

Just run a new graph... Yes 7 deg is the best l/d but the curve is almost flat between 5 and 9 deg so anywhere in that range.

So would there be a design reason for using another angle other than at 7 degrees, given that the curve is virtually flat, to obtain an advantage elsewhere in the flight envelope.
Many thanks for your help Steve, I've learnt a thing or two here.
Paul

[hweeler]

If it's the ARF Gee Bee from Great Planes -

http://www.greatplanes.com/airplanes/gpma1326main-2.jpg

The spec on it says - Wing Loading: 34.9-38 oz/ft² (106-116 g/dm²)

Gentle stalls and low speed come from low wing loading, 34 to 38 oz/ft² to me on just 68" span, IMHO is like a brick with stubby wings.

That said, it is a very good looking model and probably flies extremely well, but it is not designed as a slow flying trainer, it is a scale sports pylon racer and should be flown like one.

I personally don't think any change to the wing section will make much difference. Now if you want to double the wing area, biplane ?, (and increase the stab accordingly)..........

Heres a pic of Delmar Benjamen flying the real Garand Brothers Gee-Bee replica ship...The wing was redesigned updating the pilot killer design from back in the day to where DB is able to put the ship through all aerobatic tricks without near the danger. Saw this plane with DB at the helm at the Reno Air races in 1993. I had a video somewhere but havent been able to find! Sorry...EJ

[windywestflyer]

I ran into the same problem as you I used the GOE 593. It's slightly faster than the clark Y because it is thinner and has slightly higher cl before stall. I attached it below as well as it's calculated polar data for Re=75000. Sorry it's not pretty

[windywestflyer]

sorry can't get the polars. xfoil is wierd

[JetPlaneFlyer]

I ran into the same problem as you I used the GOE 593. It's slightly faster than the clark Y because it is thinner and has slightly higher cl before stall.

Hmmm.. According to the airfoil data in the Profili database the Clark Y and the GOE 593 are very similar in thickness, the GOE is just a shade thicker (11.9% for GOE 595 vs 11.71 5 for Clark Y).. They are near as 'a rub of sandpaper' the same thickness.

I ran a Profili plot and the two airfoils perform very similar but the Clark Y probably has the slight edge in this simulation being lower drag over most of it's range and very slightly higher Clmax. The two look and perform very much alike so in practice would be hard to seperate i'm sure.

[ThermalSeeker]

This is all good stuff but also consider moving the CG further forward. That will help to tame her, possibly a considerable amount depending on where it balances at now. A CG too far aft guarantees an overly sensitive elevator and a mean stall.

Mike

[windywestflyer]

I don't think we're talking about the same airfoil. When I plotted it the GOE was thinner than the clark y. Maybe the source of the ploting data is differant than the data in profilli.

Maybe i should get profilli and not have to deal with xfoil command line.

[windywestflyer]

We weren't talking about the same airfoil. I was thinking GOE 389 but wrote differant.
sorry.

[JetPlaneFlyer]

Yeah, the 389 does very well in terms of lift and drag. It does have some downsides compared to the Clak-Y though; it's slightly undercambered so would not be as easy to build. It also has a sharper stall and worse inverted performance.

[windywestflyer]

Ain't it strange that the best airfoils were built when airplanes were still in their infancy. The Clark Y is pretty old. From the 20's. 90 years later we still can't beat some of the old tried and true.