Delta head question

[iter]

With a Delta head, a blade's AoA goes down when it flaps up and up when it flaps down. How does a Delta-head rotor spin up? At "neutral", when the blades are level, should the blades be set at a negative AoA?

Ari.

[boberos]

I set blades at about 1.5 degrees neg for testing. Then shim as required from there.

Good luck,
Bob G

[georgs]

I set the flat bottom airfoil at 0 degrees with level blades. So there is even a very slight positiv AoA.
To my understanding the big advantage of a delta hub is the easy spinning up.
As long as rpm is low the centrifugal forces are low, resulting in a high coning angel when the air comes from below. The high coning angel results in well negativ AoA resulting in easy spinning up.
then high rpm -> high centifugal force -> low coning angle -> neutral or even positiv AoA -> good lift.

It worked well with me, but I did only handstarts.

Georg

[iter]

I guess with the rotor still, blades will hang down with a positive AoA even if you mount them with a negaive angle... Shouldn't that make for a more difficult spinup?

Ari.

[georgs]

yes, you are surely right.
My description was based on the observation of the single one I built ( I am definitly no expert!!).
As mentioned I made only handstarts. Giving the rotor a slight push I started walking/running in the wind with a extremly high AoA (45-60°) of the rotor disc. this resulted in flapping up of all blades -> negativ Blade AoA -> spinning up in seconds -> as soon as speed was gained I reduced the Disc AoA and started.
Maybe it´s much more difficult to get the rotor to speed while taxiing.

Another important point is surely the airfoil used. I finished a Rotor yesterday evening. Blades built from 3m pine leading edge + 4mm Depron resulting in a Foil of only 8% thickness. The rotor would not spin up unless substantial neg AoA of the blades in level position.
Maybe my mistake was also to give the airfoil I slight S chamberline to avoid torsion forces.

Georg

[soarrich]

The PT Gyro's Ninja rotor head, (it's a delta rotor head), has built in down stops so the can't sag down.

Most airfoils still make lift at negative AOA. The Aerobalsa high performance SG6042 blades make lift at anything greater than -4.92* AOA. You can set them -2* and still get good spin up and be making good lift.

http://www.flyingbalsa.com

http://www.aerobalsa.com

[iter]

Surely you mean geometric AoA, as in referencing the airfoil's (flat) bottom or the line between LE and TE? This has come up in this forum just recently, the definition of aerodynamicly neutral AoA for a given arifoil is the angle at which it produces no lift. "Negative" in this sense means "when the lift vector is pointing down". The aerodynamic definition is more useful because "x degrees negative geometric AoA" is meaningless without reference to a specific airfoil.

In a sense an airfol will always produce lift, it's just that beolw neutral the lift vector will be pointing down (except at neutral AoA when the vector is 0)

Ari.

[soarrich]

Surely you mean geometric AoA, as in .........the line between LE and TE?


Yes.

the definition of aerodynamicly neutral AoA for a given arifoil is the angle at which it produces no lift. "Negative" in this sense means "when the lift vector is pointing down". The aerodynamic definition is more useful because "x degrees negative geometric AoA" is meaningless without reference to a specific airfoil.

I guess we're playing "Who's the smartest guy in the room." You win when you show us how to measure your aerodynamicly neutral AoA with a tool some of us might have.

"x degrees negative geometric AoA" is meaningless without reference to a specific airfoil.

SG6042 seems pretty specific to me.

[mnowell129]

I guess we're playing "Who's the smartest guy in the room." You win when you show us how to measure your aerodynamicly neutral AoA with a tool some of us might have.
SG6042 seems pretty specific to me.

I don't want to play the game, so I'm not taking sides. I'm just offering information from my background in the wind tunnel and an answer to your question.
This statement made earlier :
"Most airfoils still make lift at negative AOA." Is completely true only if you insert the word "geometric" before AOA.
In full sized aerodynamics we measure geometric angle of attack from the chord line, the line between the trailing edge and the forward most leading edge point, and the relative airflow. We measure the true angle of attack as the difference between the airflow and the zero lift line (ZLL). The zero lift line is that line drawn from the TE forward through the airfoil such that if you hold that line parallel to the airflow, the airfoil makes no lift.
The angle difference between the geometric and true angles of attack is easily obtainable for most airfoils by looking at the airfoil polar diagrams ( the graphs of lift coefficient vs geometric angle of attack). The point where the lift curve crosses the x axis tells you exactly the zero lift line. For a clark y its something like -4 degrees, for the SD it is something like -4.92 (don't have the data in front of me). This simply means that if you mount this wing at -4.92 degrees geometric relative to the airflow it will make no lift.
The reason we don't generally work with the ZLL is that is physically inconvenient. The chord line has a nice built in reference point for setting angles, etc. As aerodynamicists we like to have reproducible results from experiment to experiment so using a precise rigging angle like the chord line creates reproducible settings. Usually the first thing you do with an airfoil in the tunnel is find the ZLL setting to confirm that the rigging was the same as last time. Then your airfoil is known to be aligned with the balance the same as before.
So knowing the specific airfoil/polar data does give you all the info you need.
The tool you need is the airfoil polar plot. However you can pretty much assume that it's -3 to -5 degrees for most cambered airfoils. 0 for non cambered sections (flat blades, or fully symmetricals).
It model gyro world it really doesn't help a lot to know the theoretical ZLL because the variations in a real wing's aspect ratio, finishing, craftsmanship, etc. create a large variance from the theoretical ZLL anyway, so having to adjust the pitch angle is a given, so you might as well build in adjustability from the start unless you are going to put in too much negative to start with. In this case you give up some performance but you always get the rotor to start reliably.


mick

[iter]

Mickey, thank you for this clarification. I now get back to my original question: in a delta head, asuming a chambered airfoil with (say) 4 degrees between chord line and ZLL, do I want the chord line at a negative AoA or the ZLL at a negative AoA in the neutral position (blade at 90 degrees to the mast)? Or is this so specific to wing aspect ratio, finishing, craftsmanship, etc., that one can't give meaningful advice here and it's all a matter of experimentation?

Ari.

[mnowell129]

Mickey, thank you for this clarification. I now get back to my original question: in a delta head, asuming a chambered airfoil with (say) 4 degrees between chord line and ZLL, do I want the chord line at a negative AoA or the ZLL at a negative AoA in the neutral position (blade at 90 degrees to the mast)? Or is this so specific to wing aspect ratio, finishing, craftsmanship, etc., that one can't give meaningful advice here and it's all a matter of experimentation?

Ari.
It's really impossible to give you a real number because of all the factors. Generally the smaller the gyro the more negative you need. Generally you need to be able to adjust the angle while trimming, so I'd suggest you build in a way to do it. In mine I put metal blade straps on so I can bend with needle nose pliers.
My 30" 14 ouncer with SD blades has about -4 geometric and does just fine.
When you go bigger you can go more positive, smaller more negative.
I'd start with a few degrees (geometric) negative, then be prepared to lower if it won't spin up reliably, or add pitch if it spins up too quickly and flies to fast.
Sorry there's not a better answer, there's just a good algorithm for getting it right. Too much positive it won't spin up, too much negative it spins up too fast and flies to fast. What I do is start flat (geometric 0), and keep reducing until it starts up reliably, this is the most reliable, highest performance setting.
mickey