Rudeboy

As I am not a crack in aerodynamics I have the following question: what would be an appropriate low drag, laminar airfoil for a fast delta?
As the delta plane has a large root chord, the airfoil should be suited for Reynolds numbers in the range of 4.000.000-5.000.000.
Also, what would be the aerodynamic effect of "stretching" an airfoil to reduce thickness?

Ollie

Low aspect ratio deltas have substantial three dimensional flow over the whole wing and laminar flow is quite unlikely. airfoils are disigned for two dimensional flow. As a result almost any thin, low camber airfoil will likely do. There is the turbulence of prop wash, tip vortex and middle effect to prevent laminar flow.

Rudeboy

Well then what would be the effect of leaving the delta plane design and going to a simple low aspect ratio, high taper wing with no sweep or even a swept forward wing?

Is it possible to reduce tip vortices on delta planes in a meaningful manner by using some sort of tip plate?

probligo

Rudeboy,

It really is a matter of "What are you trying to achieve?"

If you are only trying to reduce induced drag (reduce the size and power of the tip vortices) then there are many other means of attaining that objective.

Are there other reasons for wanting to adopt a delta planform? If there are then what are they - it will make the selection of other compromises easier.

For example if maneuvarability is not required then winglets become a serious option.

If you are wanting to do highspeed aerobatics, then other options might be used like high aspect ratio.

BMatthews

The simple answer is NO.

Delta wings produce a lot of their lift from the action of the vortices. As Ollie said the airflow on deltas at any sort of significant lift coefficient is largely vortex based with the airflow over the leading edges spilling inwards towards the fuselage. This is a primarily the reason that the delta winged planes can pull off those crazy super high angle of attack landing approaches with no flaps. And also why they are so hard to stall. So trying to add tip plates would not only be counter productive but a waste of effort since the main entry point for the air to flow into the upper surface vortices is the whole leading edge.

It's only at very low angles of attack that the delta wings work almost like regular wings with the air flowing more or less parallel to the direction of flight.

Or at least this is how I understand it.....

Dan Taylor

My Feedback: (2)

On a Long Eze
the use of fin lets on the tips is supposed to reduce vortices
and produce additional lift
according to the manual with my new kit from Fusco Aircraft Co.

I don't have a clue though

Selcuk

I think your suggestions is very important for delta wings. Could you add some schematic drawings?

Bruce Thompson

while on the subject of deltas,,,, what is the easiest why to figure out C/G?

Ollie

Extend the centerline, aft from the trailing edge, a distance equal to the tip chord, to a point A. Extend the tip chord line forward from the leading edge, a distance equal to the root chord, to a point B. Draw a straight line between points A and B. Draw another straight line between the midpoint of the root chord and the midpoint of the tip chord. Where the two drawn lines cross is the centroid of the wing planform. Draw a chord through the centroid. That chord is the Mean Aerodynamic Chord (MAC). Project the MAC to the center line. The CG will lie somewhere between about 15% of the MAC from its leading edge to 25% of the MAC from its leading edge, depending on how much static stability is desired, and how much the CG shifts from a full tank to an empty tank. The "dirtier" the aircraft, the more forward the CG needs to be. Props, uncowled engines, landing gear, exposed control linkages, etc. require a more forward CG within the above mentioned range.

rsieminski

If you put the CG at 25% of the MAC, make sure there's no-one around, because it'll be uncontrolable. Deltas usually have the CG at 11-17% of the MAC, and are more sensitive to CG change.

Selcuk

Thank you very much for your briefing. I am currently working on a delta wing plane with pusher prop (engine is at the back of the plane) and with forward horizontal stabilizer. Nobody knows anything about delta wings in our club in Ankara, Turkey. The wing has an triangle shape and the length of the root chord is 32.6 inches, tip chord is 1.6 inches and half of the wing span is 22 inches. Stable flight is my first aim because this is the first delta wing in our club. So which cg I must use? For a stable flight what can I do or add to the plane?
Any other confusion is function of the elevators. A friend told me that for the planes with forward horizontal stabilizers the function of the elevators should be reverse according to tail horizontal stabilizers. Is he right?

Ollie

The elevator in front picks the nose up by lifting the nose. The elevator in back lifts the nose by pushing the tail down. The elevator in front moves down to increase lift in the nose. The elevator in back moves up to push the tail down.

For pitch stability, the center of gravity must be ahead of the aerodynamic center of the whole aircraft. The bigger the forewing is compared to the aft wing and the greater the distance between them, the more forward the location of the aerodynamic center of the whole model. Calculating the location of the aerodynamic center of the whole aircraft is not a simple task. The simplest way to find the answer is to build a small, exact scale model glider of the model you are designing and adjust the CG of the small scale glider until it glides well and then measure the CG location. It will have the same relative location in the bigger model.

Selcuk

Is there any simple method for calculating the aerodynamic center of such a plane?

And the second question is 'Is there any known down or up and left and right thrust angles for the rear engines with pusher props?

Ollie

Selcuk, See:
http://aero.stanford.edu/Reports/MultOp/multop.html

I think you would be much better off building the small scale glider to get the CG rather than try to do the calculations.

Rudeboy

Down thrust on the engine stays the same: lower the shaft of the engine. Do you really need down thrust on that delta?

With right and left thrust it is the other way around, as the engine is spinning in the other direction when viewed from the front of the plane.

As for the AC and CG question, I don't know. What plane are you building? Maybe there's someone around that has built one too.

Selcuk

I am currently working on a delta winged plane similar to Valkyrie XB 70. I am not a scale master but I already built the plane which looks alike, but nobody knows where is CG and aerodynamic center. I planned to put cg 13 inches back from the leading edge of root chord. But now I am not sure, whether it is the correct number.
Sorry about my questions but my club members do not know delta wing and rear engines with pusher props. So I want to learn not to meet a surprise at flight area and meet a person saying 'you had to have two degrees of thrust angle or something else'.
Again thank you very much.