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Cermark Phoenix - flying with lifting stab?


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I’ve been interested in seeing if my Phoenix is flying with the stab producing down force as Im told should be the case for most gliders. 

 

So, I indirectly measured it in flight. By installing an AOA sensing vane, calibrated to the flat bottom of the wing and adjusting for the fact that the actual wing incidence is about 2 degrees more that that, I determined that during stable flight, the wing is flying at around 4 to 5 degrees incidence. However measuring on the bench, when the stab is horizontal (using bubble level), the wing is at 3.75 degrees. So supposedly when in flight, if the wing is at 4 to 5 degrees positive, the stab would be at +.25 to +1.25, thus producing a nose down moment instead of a nose up moment as expected. The CG is at a location where the plane flys fine. 

 

What’s wrong here?  Nothing?

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No science to this comment, but just what I see and feel as the model flys.

The fluid air offers something to fly through, and the gravity below offers a pull down.

A flimsy stabilizer does not hold the nose down well.

In my imagination,  I see the correctly set and sized stab holding the wing firmly DOWN,

That the wing can hold the model up.

Actually pushing the wing down into the fluid air.

I know that I am potty.

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  Your not potty Denis, most aircraft have a stabilizer provides downward pressure to keep the aircraft in balance. Lifting tail's tend to be found on early aircraft like my Bleriot and free flight models.

The problem with a lifting tail while it does help keep the aircraft in the air it has a narrow speed range.

 Take a normal model wing on its own, just a wing and launch it from your hands it will either rear up over backwards or dive to the ground as it will be critical how the wing meets the air flowing over it, no tail no balance. Therefore Slope445 your set up sounds correct.   A reflex section wing will fly with no tail as will a steeply swept back type like a paper dart.

  Modern airliners have their stabilizer set at very near neutral at cruising power, this is to save drag and fuel and rely on the computer controls to keep the aircraft in balance when passing through disturbed air.

  I am no expert but that is how I understand it.

10.jpg

Edited by J D 8
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I think I may do more testing. Maybe I’ll attach a short string above the stab. Maybe an inch above it and away from the fuse. Then do some normal flight speed flybys and observe the angle of the string compared to the stab. Maybe video it. 

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On 02/02/2022 at 00:43, Slope445 said:

I’ve been interested in seeing if my Phoenix is flying with the stab producing down force as Im told should be the case for most gliders. 

 

So, I indirectly measured it in flight. By installing an AOA sensing vane, calibrated to the flat bottom of the wing and adjusting for the fact that the actual wing incidence is about 2 degrees more that that, I determined that during stable flight, the wing is flying at around 4 to 5 degrees incidence. However measuring on the bench, when the stab is horizontal (using bubble level), the wing is at 3.75 degrees. So supposedly when in flight, if the wing is at 4 to 5 degrees positive, the stab would be at +.25 to +1.25, thus producing a nose down moment instead of a nose up moment as expected. The CG is at a location where the plane flys fine. 

 

What’s wrong here?  Nothing?

The air flowing backwards from the wing does not follow the flight line exactly, but is deflected downwards by the (positive AoA of the) wing. This is the so-called "downwash". So, relative to the deflected air, the AoA of the tailplane (horizontal stab) is slightly negative; hence, the tailplane is generating slightly negative lift.

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I did wonder about the down wash effect. I wasn’t sure how far away from the wing it extends. The Phoenix is a T tail so the stab is maybe 2 ft behind the wing and several inches above it. Assuming it’s in the down wash then it makes sense it’s pushing down. 
 

 

Denis, do you have a link or website for one of these wind tunnel apps?

 

Edited by Slope445
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Well I found this free wind tunnel app (thanks Denis!). It’s got a lot of limitations but I was able to used their canned airfoil and draw in a flat stab that looks to have a slight positive incidence with respect to the airflow on the left of this screen shot of the results. Notice the blue (low pressure) over the wing and UNDER the stab. This means the stab is pushing down and illustrates the down wash effect as discussed by John S above!   This little test doesn’t perfectly reflect the Phoenix setup but it’s kinda cool and it’s telling me that down wash effect is probably what’s going on. 

 

 

 

1A99DF5C-22FC-41BD-8FFA-FF86AB6BB16A.png

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