Downthrust (again!) - Sailplane vs. F5B example.

[Andy W]

I've read the threads here..
I know how to set it, etc. I understand CG placement, angle of attack, incidence, etc.

What I'm struggling with is why my sailplanes, even those that can fly at moderate speeds, require significant downthrust, but an F5B model, that has similar power levels, similar prop sizes, has NO downthrust, and does not pitch up under power.

*all models have CG very carefully set, have very little incidence, trimmed for hands-off flight, etc.

Why?

The only thing I can think of is that the sailplanes have "high lift" airfoils, the F5B ships have "high speed" airfoils. The prop wash creates sufficient additional lift to require significant correction..

..a

[vintage1]

I think you have it there Andy.

Downthrust is mostly about inherent pitch stability - the fact that

- the wing lift increases dramaticaklly with speed
- the model is set to have a nose down moment at low speed, and a nose up at high speed.

It would seem that moving the CG aft and reducing wing incidence would solve the power on problem, but, in power off conditions you might end up with non optimal sink rates as a result. And a model that was hard to settle into a 'best glide'.

I don't know enough about sailplanes to give more than a hypothetical answer though.

I did a very little playing with contest free flight years ago, and we did find that rearward CG and not very much downthrust at all, coupled wih a spiral left climb, seemed to get us there for hands off climb and glide.

[Salto]

Andy,

If your model needs downthrust to counter a pitch up under power then it sounds like the CG is too far forward and there is too much longtitudinal dihedral. Moving the CG back and reducing longtitudinal dihedral accordingly should get you to a point where there is no pitch up with increasing speed and therefore no need for downthrust. To manage the high speed range, F5B models presumably must be set up like this (I have no direct experience of these).

Think about the classic dive test used for setting the CG placement. When you have a model that is neutrally stable, ie. one that doesn't diverge up or down from the dive, then by definition, you have created an incidence geometry that doesn't pitch up with increasing speed, and consequently won't need any downthrust.

If anything, your "high lift" airfoils, by which I think you mean high cambered, will have a stronger pitch down moment with increasing speed. But this simply requires a more rearward CG placement to get neutral stability over a workable speed range. A symmetrical airfoil (no camber) won't have any pitch down moment at any speed (ignoring nasty low Reynolds Number seperation bubble effects), and will have the neutral stability CG location very close to the neutral point of the airframe. The cambered airfoil will require the CG slightly more rearward.

About the only other reason for requiring downthrust would be a thrust line significantly below the vertical centres of drag and gravity of the airframe, but I assume your above comparison is between models all with relatively high thrust lines (classic sailplane layout). Remember that any dihedral will significantly raise the centre of drag of the airframe, so this needs to be considered in your comparison.

There may be second order effects due to propwash over the wing and stab, and due to seperation bubbles mentioned above, but my guess is that what you have observed is primarily due to the above CG placement issue.

Graham.

[Andy W]

I tune all my models with a dive test, the CG is not forward on my sailplanes that require significant downthrust.
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[Salto]

So you've tuned them for neutral stability? So by definition they don't zoom up after being pushed over into a dive? If so, then the increased speed under power is not causing the problem and we can rule out any longtitudinal dihedral related effect.

Is there any wing dihedral? How far is the thrust line below the centre of drag of the models? Perhaps this is the cause.

Graham.

[HELModels]

What about sizing Horizontal stabilizer? What effect will that have on the "phenomenon"?

go on... I get most of my answers myself. ;)

[Andy W]

Yes, the models do not zoom up, they do have a very small amount of correction, so I am very close to idea CG. The thermal sailplanes do tend to have more dihederal than the F5B shops.
The models are all sailplanes on one kind or another - thermal, F5B, etc. - the wing is mounted to the top of the fuselage, but they are all pretty much "in line", they don't have fuselages that hang down a signifiant amount below the wing.
..a