RE: Reflexed Airfoil and Flying Wings - Downwash?  
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All Forums >> RC Airplanes >> Aerodynamics >> RE: Reflexed Airfoil and Flying Wings - Downwash?
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 7:12:27 PM   
Crane



Posts: 33
Joined: 2/18/2003
From: Socorro, NM, USA
Status: offline 		Bruce,

"It might be nice if we could get back to the ramifications of reflex as well."

Naw---It's obviously more fun to rationalize favorite analogies by fondling the reference points.

Anybody else interested in reflex?

(in reply to BMatthews)
       Post #: 101

RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 7:18:15 PM   
KenLitko


 

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From: Crown Point, IN,
Status: offline
quote:

ORIGINAL: adam_one

Please look again at the drawing posted by Ben, locate where the Drag vector is, you can't have a Drag vector in a steady flight without having Thrust.
That's all.


Drag acts through the AC... thrust acts at the point of application... which is not at the AC.

Likewise, weight acts at the CG, which is likely not at the AC of the wing.

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(in reply to adam_one)
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 7:25:50 PM   
KenLitko


 

Posts: 243
Joined: 1/3/2003
From: Crown Point, IN,
Status: offline 		Reflex is built into a flying wing for longitudinal stability... it takes the place of the tail by balancing out the pitching moment of the wing (mostly... still needs to be trimmed for different flight conditions... just like a stabilizer).

:-)

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(in reply to Crane)
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 7:42:24 PM   
adam_one


 

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quote:

Drag acts through the AC... thrust acts at the point of application... which is not at the AC.

Likewise, weight acts at the CG, which is likely not at the AC of the wing.

Could you show us your own drawing representing all the forces involved in steady flight?

< Message edited by adam_one -- 3/23/2005 7:43:32 PM >

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(in reply to KenLitko)
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 7:57:36 PM   
Crane



Posts: 33
Joined: 2/18/2003
From: Socorro, NM, USA
Status: offline 		Ken,

The model shown below is one I use in presentations on Bird Flight. It glides nicely and is longitudinaly stable. The section is thin and flat - it has no reflex.

Since it does have a Cl it creates downwash at an angle proportional to that Cl. However it does not have a Coefficient of Moment other than zero. So, I assume that the Coefficient of Moment is not related to downwash. Is that correct?

(in reply to KenLitko)
       Post #: 105

RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:02:05 PM   
Crane



Posts: 33
Joined: 2/18/2003
From: Socorro, NM, USA
Status: offline 		Ooops, I lost my drawing. Hope I got it this time - yep there it is

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< Message edited by Crane -- 3/23/2005 8:06:42 PM >

(in reply to KenLitko)
       Post #: 106

RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:09:32 PM   
Ben Lanterman



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From: St. Charles, MO, USA
Status: offline 		Adam-one - you didn't read what I wrote. I said these were the aero forces on the wing. It didn't include thrust or weight because those are external force and moment producing forces and are not related to thrust.

Every time we do a wind tunnel test it is done without thrust and the weight of the model is removed from the final data. It is the way all standard force and moment wind tunnel tests are performed.

Bruce - "  You're both right. There is no such thing as a Downward Force but the weight and thrust add up to that downward vector and balance the lift and drag force. But technically I've never seen any Downward Force in any of the literature I've ever seen. But I can see where you got it from. "

No we aren't both right for the following reasons ...

1. The weight and thrust are not necessary to show lift and the cause of it. What Adam-one is doing is throwing in some force as it seems necessary for his argument.

2. When an engineer is doing a force and moment accounting in flight of a real airplane then weight and thrust must enter into the equations. The standard six-degree-of-freedom equations are well known and used in industry. For over 25 years I ran a six-degree-of-freedom program that was capable of doing various things, roll performance, longitudinal stability, external stores separation trajectories, etc. It is not all that hard, just a big set of equations. Basically for our discussions here with our simplified airplane in flight, we do have thrust and drag, Lift and weight and when they are equal and opposite and when the moments are zeroed you will find yourself at some flight condition based on the trim of the airplane. However that has never been in question.

Edited


< Message edited by BMatthews -- 3/23/2005 8:40:22 PM >


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:16:09 PM   
Ben Lanterman



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From: St. Charles, MO, USA
Status: offline 		Crane - are you sure you don't sneak in some trailing edge up somewhere?

With a nicely rectangular wing with no reflex the ac is at 25%. For stability the CG must be in front of that. That automatically gives a nose down moment. You must have reflex of some sort built in or the drag on the vertical fin is acting to give the same kind of nose up moment.

There isn't anyway of getting around the need for the nose up moment, whether it is from elevons, horizontal tail, or a big draggy vertical tail. I have a series of flat panel wings that show this very nicely. It need not be much but indeed is needed.

On the vertical tail moment input thing. You can nicely control the pitch of an airplane with a high split vertical tail. As you open it up in a trailing clamshell fashion it increases the local drag and the up moment as a result. However it is not the best way of making a pitching moment.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:24:50 PM   
Crane



Posts: 33
Joined: 2/18/2003
From: Socorro, NM, USA
Status: offline 		Ben,

That's interesting, but there is no reflex in the wing. If any camber sneeks in either + or - the thing won't glide at all.

The idea that the rudder or dihedral could impart a small vector is interesting. The dihedral would be a more likely culprit than the rudder since I have used very small rudders. That may be why these are fairly reliable performers in demos.

However, you can take a 2 by 12, 1/16 sheet of balsa and with patience balance it until you get straight glides. Try it.

Assuming that the dihedral or rudder is creating a small vector that would still leave the Cm independent of downwash.

(in reply to Ben Lanterman)
       Post #: 109

RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:29:33 PM   
KenLitko


 

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From: Crown Point, IN,
Status: offline 		More than likely what is happening is you are balancing a teeter totter.... the weight of the aircraft is sitting at a position near the AC that also balances the moment of the wing.

If you throw your glider harder or softer I bet it doesn't glide so well.... in these cases, these forces and moments don't balance.

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(in reply to Crane)
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:33:33 PM   
adam_one


 

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quote:

Every time we do a wind tunnel test it is done without thrust and the weight of the model is removed from the final data. It is the way all standard force and moment wind tunnel tests are performed.

In a wind tunnel you blow air towards the wing in order to simulate thrust, you get some parameters regarding the actual airfoil's effectiveness, but you don't get the whole story, because an airfoil is not a complete final wing.
Anyhow, you can't have vector forces pointing at only one direction without having corresponding opposite vectors, and no matter where the actual forces are applied their net sum is zero in steady flight.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:45:57 PM   
dick Hanson



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Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		I am not formally educated in all of these fine nuances but here is a split rudder I did -------
and it does allow trimming for pitch-or yaw
and or a drag device

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< Message edited by dick Hanson -- 3/23/2005 8:50:32 PM >

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(in reply to adam_one)
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:49:32 PM   
BMatthews



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quote:

....I've also noticed your acceptance for his provocative arguments towards those who don't agree with him, which is unfortunate.


If you check you'll see that I've edited or deleted posts from pretty much everyone that is getting emotional about all this on every side. Pretty much the only one I've left alone is Dick because he's good for comic relief! ! ! !

As of this post downwash and flight vector posts will not be acceptable and will be deleted. I'm sorry but we are just rehasing the same material as has passed this way before and all that's happening is feelings are being rubbed raw.
Discussion of reflexed airfoils will carry on for as long as we can all keep calm.



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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 8:51:46 PM   
Crane



Posts: 33
Joined: 2/18/2003
From: Socorro, NM, USA
Status: offline 		Ken n Ben,

Fresh data. I went out to the shop and found one of these wings on the bench. I has no rudder and very little dihedral. It will produce a long flat glide on most launches. The stability margin is small. A bad launch is a zero. However, the plank I showed a while back would not take a hard launch and my Strucks New Ruler would plough in on a bad launch. Why is that significant?





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