How To Determine GLIDE SPEED For A Model . . .
NOTE : The example is for a Glider with a w Wing Area of 60 sq. in.. .
Dave F.
NOTE : The example is for a Glider with a w Wing Area of 60 sq. in.. .
Dave F.
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How To Determine GLIDE SPEED For A Model . . .
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Are you assuming some wing airfoil, or is this assumed to be wing agnostic?
Just a general flat-bottom HLG airfoil . . . Nothing fancy !
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Hmm . . . With the higher drag and zero lift ( flat sheet ) from the wing "airfoil" ( a positive angle of attack would be mandatory ), the "glide" would consist of "mushing" at a lower airspeed. It would probably be very prone to stalling and have difficulty recovering, afterward.
This is the type of glider I was referring to . . .
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That's not the behavior I see in all of my gliders in fact they're much more docile, and much more forgiving in stall in my experience with the flat plate..
Do you use a positive AOA on the wing, or only Decalage on the Stabilizer ? Are you talking about an RC glider ?
Also, remember that the is a Free Flight glider, not RC controlled . . . A Pop-Pod Boost-Glider with a 12"-18" wingspan, flying on BP motors from 1/2 A - C/D impulse.
A glider like this would be used in a Maximum Duration competition event, very light wing loading with a slow rate of descent.
There is a huge difference in performance between the two models pictured.
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I'm talking about an RC glider no decalage or anything but I thought this was a general posting regarding how to determine glide speed based on wing area not something very specific
Too many variables with RC . . . The pilot can re-trim the glider and has constant control over the AOA, etc. Decalage is not required, due to movable control surfaces.
Now, taking the glider I showed and building one with a precision airfoil, compared to an identical one with flat "plank", un-airfoiled wings would clearly show the difference ( it would "glide like a brick" ).
The formula must take into account some given angle of incidence. You could increase the angle of incidence, which would in turn increase induced drag and make the glider fly slower. Or vice versa. And other things are going to have a huge effect on speed as well.
We all know it's way more complicated than that when it comes to a FF glider when were trying to get it to launch, transition and glide properly.
I'm just getting back into FF gliders. Building a tip launch model currently.
Interesting discussion!
We all know it's way more complicated than that when it comes to a FF glider when were trying to get it to launch, transition and glide properly.
I'm just getting back into FF gliders. Building a tip launch model currently.
Interesting discussion!
"Back in the day" Dr. Gerald Gregorek, and others , did some in-depth testing . Here are a few items.
The equation assumes a zero incidence glider, at zero AOA . . . The theoretical "equilibrium glide", mentioned by Dr, Gregorek.
The equation assumes a zero incidence glider, at zero AOA . . . The theoretical "equilibrium glide", mentioned by Dr, Gregorek.
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This thread may be helpful to you . . . https://www.rocketryforum.com/threads/glider-design-trimming-library.155758
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Is that a B58 or a century series interceptor (e.g., 101, 102, 106)?
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They look quite similar without engines
Did you see the link I posted before ?
https://www.rocketryforum.com/threads/glider-design-trimming-library.155758
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Cool!
I Like this post!
My compliment to Ez2c that is an inexhaustible source of information , data and photos.
About the discussion on angle of attack and wing profile, the formula that Dave posted in the first message comes from the below one.
It is the lift formula
Lift = Cl x p X V^2 x S
Cl=Lift coefficient
P = air density
V = air speed
S= wing surface
When an airplane flyes at constant vertical speed you have
Aircraft weight = Lift force = Cl x p xV^2 x S
if we solve respect V you gets
V = SQRT ( Aircraft weight / Cl x p x S )
The one reported by Dave is
V = SQRT( Aircraft weight x 160 / S)
The 160 is a factor that consider Cl , P and also some unit conversion.
Now it is important to know that Cl is a function of the angle of attack.
For flat wings Cl = 0 if angle is 0.
Here a chart Cl / alpha about (see the curve , flat plate)
Flat plate has best Cl at around 7, 8 deg.
So when Burke flyies for sure he will keeps angle that is not 0.
Now we have another actor , Cd. Drag coefficient. This is well known by the rocket guys.
In this case the formula is very similar.
Drag force = Cd x p x V^2 x A
with A = fron area section
We need to consider that both Cl and Cd have different values in relation of the angle of attack.
Here below an example
To get the free gliding speed we will have to balance drag and weight component along the descending direction as well the lift and weight.
We will have different speeds and descending angles depending by the angle of attack we will set on our wings.
So the 160 is a constant determined with some experiments. But this assumes that Cl is not 0.
So if you have the wing profile that is a flat plate you can set a Cl at 0,5 and do the calculations with a resonable approximation.
You will fly with an angle of attack that is around 7 deg.
When Burke replies to Dave's sentence about flat plate
"It would probably be very prone to stalling and have difficulty recovering, afterward. "
"That's not the behavior I see in all of my gliders in fact they're much more docile, and much more forgiving in stall in my experience with the flat plate.. "
this in reality depends by the shape of Cl curve. Cl grows with the angle of attack, but when it exceeds a certain value... the wing stalls!
What is different is how the stall comes . If you see the curves of the 1st diagram the two naca profiles , these have two different ends.
One is sharper the other more rounded. In one case there will a sudden loss of lift, in the other more progressive.
Flat plate have quite rounded end. This is why the behaviour is more "forgiving".
But let see next !
My compliment to Ez2c that is an inexhaustible source of information , data and photos.
About the discussion on angle of attack and wing profile, the formula that Dave posted in the first message comes from the below one.
It is the lift formula
Lift = Cl x p X V^2 x S
Cl=Lift coefficient
P = air density
V = air speed
S= wing surface
When an airplane flyes at constant vertical speed you have
Aircraft weight = Lift force = Cl x p xV^2 x S
if we solve respect V you gets
V = SQRT ( Aircraft weight / Cl x p x S )
The one reported by Dave is
V = SQRT( Aircraft weight x 160 / S)
The 160 is a factor that consider Cl , P and also some unit conversion.
Now it is important to know that Cl is a function of the angle of attack.
For flat wings Cl = 0 if angle is 0.
Here a chart Cl / alpha about (see the curve , flat plate)
Flat plate has best Cl at around 7, 8 deg.
So when Burke flyies for sure he will keeps angle that is not 0.
Now we have another actor , Cd. Drag coefficient. This is well known by the rocket guys.
In this case the formula is very similar.
Drag force = Cd x p x V^2 x A
with A = fron area section
We need to consider that both Cl and Cd have different values in relation of the angle of attack.
Here below an example
To get the free gliding speed we will have to balance drag and weight component along the descending direction as well the lift and weight.
We will have different speeds and descending angles depending by the angle of attack we will set on our wings.
So the 160 is a constant determined with some experiments. But this assumes that Cl is not 0.
So if you have the wing profile that is a flat plate you can set a Cl at 0,5 and do the calculations with a resonable approximation.
You will fly with an angle of attack that is around 7 deg.
When Burke replies to Dave's sentence about flat plate
"It would probably be very prone to stalling and have difficulty recovering, afterward. "
"That's not the behavior I see in all of my gliders in fact they're much more docile, and much more forgiving in stall in my experience with the flat plate.. "
this in reality depends by the shape of Cl curve. Cl grows with the angle of attack, but when it exceeds a certain value... the wing stalls!
What is different is how the stall comes . If you see the curves of the 1st diagram the two naca profiles , these have two different ends.
One is sharper the other more rounded. In one case there will a sudden loss of lift, in the other more progressive.
Flat plate have quite rounded end. This is why the behaviour is more "forgiving".
But let see next !
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Dave, are you aware of literature specific to increasing paint/surface “roughness” to reduce drag on LPR/MPR/HPR’s? Similar application as dimples on a golf ball improving flight characteristics. If you are, could you please steer me in the right direction? Thanks!
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Are you maybe referring to turbulators?
If so, here's some good info from one of the RC forums: https://www.rcgroups.com/forums/showthread.php?1445248-turbulators
You can make adding them real simple with zig zag or dimple turbulator tape available from places like this: https://wingsandwheels.com/turbulator-dimple-tape.html
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Thanks so much for your help Dave. This is GREAT information. I gave it a cursory look through and it does address parts of the issues I was asking about, but only for low/very low speed applications. Highly relevant for rocket powered gliders in the recovery phase, but less so for full-on high velocity vertical applications (I.e., most of high power model rocketry).
What I have learned is that “air doesn’t scale” and that drag reduction data for small diameter missile-type craft is hard to find. I’ll keep looking, but if you can think of high velocity boundary layer laminar flow enhancers ... please let me know. Thanks again for your help. It is greatly appreciated!
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That's a great clear description, thank you for posting that.
Frank
Frank
I Like this post!
My compliment to Ez2c that is an inexhaustible source of information , data and photos.
About the discussion on angle of attack and wing profile, the formula that Dave posted in the first message comes from the below one.
It is the lift formula
Lift = Cl x p X V^2 x S
Cl=Lift coefficient
P = air density
V = air speed
S= wing surface
When an airplane flyes at constant vertical speed you have
Aircraft weight = Lift force = Cl x p xV^2 x S
if we solve respect V you gets
V = SQRT ( Aircraft weight / Cl x p x S )
The one reported by Dave is
V = SQRT( Aircraft weight x 160 / S)
The 160 is a factor that consider Cl , P and also some unit conversion.
Now it is important to know that Cl is a function of the angle of attack.
For flat wings Cl = 0 if angle is 0.
Here a chart Cl / alpha about (see the curve , flat plate)
View attachment 420944
Flat plate has best Cl at around 7, 8 deg.
So when Burke flyies for sure he will keeps angle that is not 0.
Now we have another actor , Cd. Drag coefficient. This is well known by the rocket guys.
In this case the formula is very similar.
Drag force = Cd x p x V^2 x A
with A = fron area section
We need to consider that both Cl and Cd have different values in relation of the angle of attack.
Here below an example
View attachment 420948
To get the free gliding speed we will have to balance drag and weight component along the descending direction as well the lift and weight.
We will have different speeds and descending angles depending by the angle of attack we will set on our wings.
So the 160 is a constant determined with some experiments. But this assumes that Cl is not 0.
So if you have the wing profile that is a flat plate you can set a Cl at 0,5 and do the calculations with a resonable approximation.
You will fly with an angle of attack that is around 7 deg.
When Burke replies to Dave's sentence about flat plate
"It would probably be very prone to stalling and have difficulty recovering, afterward. "
"That's not the behavior I see in all of my gliders in fact they're much more docile, and much more forgiving in stall in my experience with the flat plate.. "
this in reality depends by the shape of Cl curve. Cl grows with the angle of attack, but when it exceeds a certain value... the wing stalls!
What is different is how the stall comes . If you see the curves of the 1st diagram the two naca profiles , these have two different ends.
One is sharper the other more rounded. In one case there will a sudden loss of lift, in the other more progressive.
Flat plate have quite rounded end. This is why the behaviour is more "forgiving".
But let see next !
You might want to read Hoerner's "Fluid Dynamic Drag". It is not sport rocket specific, but it is generally known as the "Drag Bible".
Flow separation is bad for drag, and usually not good for lift. There are some band-aid fixes that can sometimes be effective, such as turbulators and vortex generators.
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Thanks Eric!
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