Flat plate Delta wings & differential

[Nigelp]

Hi,

Have built an 'Estrados' (see 'Foamies' forum) Flat plate Delta wing planform, controlled with 'elevons' & rudder (3mm - 1/4" upward reflex on elevons).

Finally got the CG sorted out and set but now comes trimming. The rolls are awful - very slow and extremely barrel shaped ! Am now considering large amounts of aileron differential.

I know the theory and practice of aileron differential when applied to a soarer (high aspect ratio) but have no experience of how it affects low aspect ratio.

Anyone got any experience ?

Nigel

[BMatthews]

Nigel, that sounds like a lot of reflex to my thinking. And just how much aileron throw do you have?

Generally differential won't work as effectively on lower aspect ratios. And if you think about it using differential on an elevon controlled model is like automaticaaly inputting lots of up elevator whenever you request roll only. I think this would make it worse, not better.

I'm also thinking that you may need to re-check your mixes and look for any throw volume differences from up to down. A difference in throw in the two directions would cause this in that it would act like differential and induce some up elevator when only roll is wanted. And any up elevator in a roll will force it into a barrel roll style.

[Nigelp]

Having read through your reply, you are absolutely right ! Aileron differential in a roll would be equivalent to inputting more up elevator which will make the roll even more barrel like ! If anything - 'reverse differential' would be more effective on low A.R.....

Regarding the reflex, it is 3mm over 95mm (width of elevon at that point) which doesn't sound too bad ?

One problem could be the central vertical stabilizer / rudder which is large and the rudder is too effective (results in a roll rather than a yaw)...I have just reduced the height by 25mm to see the effect.

Aileron throw is quite small at low rates - haven't dared go to high rates yet !?

Thanks for your reply, will have to have a re-think on this - will let you know how I get on.

Nigel

[BMatthews]

....Regarding the reflex, it is 3mm over 95mm (width of elevon at that point) which doesn't sound too bad ?......

You're right, that's so little it's almost flat... :D

There may be some interaction with the fin and rudder area that is causing the model to not perform axial rolls. especially if you're adding in rudder when rolling.

[Nigelp]

BM,

Paul (Reflex) on the 'foamies' - Estrados thread has used 'reversed differential' (more down than up) on his ailerons with a smaller version of the Estados. He feels it effectively reduces the 'up' reflex in the roll thus making them less barrel shaped....seems to make sense to me and I have heard of reverse differential on Delta's before - just wondered what you thought ! (of course, ultimately, the only way is to 'suck it and see' which is what I shall do when this gale abates !!!!!)

Take care,

Nigel

[Texas Buzzard]

Good stuff. Aileron differential surely is needed on a high-winged monoplane. Use more up than down. Adverse yaw will result if down and up throws are the same. But the way most guys yank 3-Ds around adverse yaw might not be noticed.

This thread started out discussing a "flat plate" airfoil. May I throw in a few words here?

From all I can find from aerodynamics's and practical ( empirical data ) model fliers it seems that there is a window for the flat-plate airfoil. When I say flat-plate airfoil I mean just that....a flat pate such as we use in most of our foamies. I use 6mm Depron mostly and the chord is mostly about 6". The leading edge and T.E. is just a square cut.

The GOOD thing is that the wind tunnel shows the flat-plate operated at speeds less than 22 MPH shows a lift/drag ratio as "normal" airfoils. It uses a Grayson motor - high K/v ratio. The flat-plate will have an abrupt stall but that is covered up because the 3-D ship is flying on it's prop anyway.

The new "modeler" is attracted to Foamies for their simple and quick construction. Easy building and easy to repair attract lots of guys. So if they don't know a thing about airfoils they can still fly and fly well. The plane certainly does not know it has a flat plate for a wing- so it flies ...IF... the speeds are kept low and there is excess thrust from the prop. But at higher speeds the "normal" airfoils win out. The pic is of one of my "flat-plate ships. A "T-Jet 007" The red-nosed Jet is from a kit I got from <http://www.rcpowers.com/> This kit maker will work with you. Go see his site, excellent videos for flight and construction. The Mig-29 just cost $29.

[BMatthews]

TB, flat plate on a delta is more forgiving than on a regular (or reasonable swept) wing. On a severe delta there's a strong vortex flow up and over the leading edge that really delays the stall and softens it to the point that it's almost not an issue. You'll see this with the super high angles of attack of the Concorde and old school Delta Dart and Dagger fighters during landings where they used this high drag semi stalled stability to slow the planes down instead of flaps. You'll see much of the same thing from models with lower aspect ratios and larger LEX's than on your models. A LEX more like what you see on F18's and F14's will help to force a strong overwing flow at high angles of attack and greatly delay and likely soften the effects of stalling.

And speaking of your models. Those are cuter than a bug's ear. Nicely done!

[Texas Buzzard]

BMatt, You are dead center in your discusssssssion of a highly swept Delta being able to achieve a high angle of attack in order to slow for landing. How the air flows to get a soft stall isn't understood by me....but it works. Then at slower speeds the plane is happy to have some lift derived from the upward pointed prop thrust. I keep thinking about how a housefly flies, constantly changing direction....maybe the planes are "flying on the prop" most of the time so the airfoil doesn't matter too much. The Ritz airfoil used on gliders and FF work very well and it is essentially a curved flat plate. They look a lot like the curved plate airfoils the Wright Brothers used on their first airplanes. But hey, the Wright Brothers flew at slow speeds too. :rolleyes:

I may be wrong but when I carefully watch a Flat-Plate Airfoiled elec. jet make a flyby, It seems that the plane is flying with a bit of positive angle of attack. We get by with that and ........Hey, the plane doesn't mind at all. :) :cool:

That red-nosed jet I pictured is a Mig-29 - a kit from RCPowers.com. The motor is a Grayson mounted aft on the elevator for Vectored Thrust. Wt. is about 16 oz. The VT will produce a very tight loop & quick change of direction in the verticle axis. :p

[macboffin]

For wings with more than 27 degrees sweep angle,(measured at quarter chord point) lift is more dependant on angle of attack than airfoil section, so a flat plate section is nearly as good as a symmetrical section. They can go to high angles of attack and high lift co-efficients provided that there is enough power to counteract the high drag also experienced. Cambered sections on Deltas are not a good idea for models unless you have a pitch gyro set-up which varies the amount of up-elevater,( making the section effectively a reflex section) dependant on airspeed/angle of attack.
Such sections are used on full-size Deltas, (Mirage for example) which have heavy underwing payloads which when dropped greatly reduce the amount of AofA necessary, (and enable a much faster speed for the "clean" aircraft.)

[flypaper 2]

The full size Avro Arrow had a flat upper surface and a just under 1% camber on the bottom surface so essentially no reflex was needed. Makes for a very low drag wing.

Gord.

[flyingwings]

The full size Avro Arrow had a flat upper surface and a just under 1% camber on the bottom surface so essentially no reflex was needed. Makes for a very low drag wing.

Gord.

The attached image shows inboard and outboard cross sections of the wing.

Both would appear to be reflexed.

Paul

[flypaper 2]

They both have leading edge droop for low speed flight. the top of the wing is flat. Do you see a raised trailing edge?? Any reflex is from the camber in the lower wing surface.

Gord.

[flyingwings]

They both have leading edge droop for low speed flight. the top of the wing is flat. Do you see a raised trailing edge?? Any reflex is from the camber in the lower wing surface.

Gord.

Leading edge droop is irrelevant to whether the wing is reflexed or not. The reflex is based on where the camber line crosses the chord line.

Take the drawing I sent you and draw the chord line. Chord line starts at the leading edge and is a straight line to the trailing edge.

Now draw your camber line. Camber line is the mid point between top and bottom surfaces at each station along the chord line. Break the chord line into 20 equal distances. Work out where the centre is between top and bottow wing surfaces. That will be your camber line.

Where the camber line crosses the chord line is the start of reflex.

Reflex means where the lift vector changes from positive to negative or down.

A reflexed airfoil provides a down load on the aft portion of the wing to balance the moment caused by the CG being forward of the Mean Aerodynamic Centre. In other words the forward down load of the weight of the airplane times the distance to the Mac must equal the reflex down load load centre times the distance from Mac. In short Sum of moments around Mac must equal zero.

In other words a delta does require some reflex as it doesn't have a horizontal stabilizer.

From the sketch you made it should be apparent that the Arrow wing is massively reflexed.

Actually the airfoil design for the Arrow was based primarily on Transonic flight and slow speed characteristics were a minor consideration.

During Transonic flight the MAC shifts forward and they needed that extra lift from the droop to prevent pitch down.

Paul