# Cheechako...can anyone explain how this works?

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#### Rktman

##### Eric
TRF Supporter
Can anyone explain how this works? I came across this old design (see below) that essentially uses a beveled edge at the rear of the wing to somehow channel air flow so that the glider's angle of attack is maintained in a way that keeps it stable and aloft.

It's understandable how a flap (elevator?) at a wing's aft edge works; for example you can feel the force exerted by air flow by sticking a hand out a car window. But a beveled edge? Does it somehow direct air upward to push the glider's tail downward? Is it creating an area of lower air pressure to pull the rear down in the same way that the top surface of an airfoiled wing works?

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Does it somehow direct air upward to push the gliders tail downward?

Doped tissue, now that's old school!

But how does the movement of air upward past the glider's aft end move it downward if it's not physically pushing against anything? I don't understand the aerodynamics (or physics?) of it. Can you explain?

But how does the movement of air upward past the glider's aft end move it downward if it's not physically pushing against anything? I don't understand the aerodynamics (or physics?) of it. Can you explain?
Probably creates a vacuum on the tail end

Is it creating an area of lower air pressure to pull the rear down in the same way that the top surface of an airfoiled wing works?
Guess our posts crossed.
That makes understandable sense. Must be harder to trim something like that though, vs something with a flap that you can adjust the angle of.

Guess our posts crossed.
That makes understandable sense. Must be harder to trim something like that though, vs something with a flap that you can adjust the angle of.
I can try to explain it as I see it in layman's terms.

If wind is moving across a flat surface and there is a divot or depression, the wind creates a low pressure area in that divot because it's drawing across the top of it. This is basically what it sounds like is happening in the bottom rear of the rocket if the back bottom trailing edge is beveled while the top is flat.

So the lower rear edge has a lower pressure area which pulls slightly downward there, which moves the nose slightly upwards.

Disclaimer - this is my interpretation/guess of the design and the intentions of it.

Doped tissue, now that's old school!
I love the smell of doped tissue in the morning, smells like.... old school!

Tony

Larry "Legend" Renger. His Sky Slash was the first front engine boost glider. Many more model rocket and airplane designs to his credit.

If you had a tapered elevator, and you raised it a few degrees, you woukd get the same shape.

Hmmmmm....I might have that issue of American Aircraft Modeler. I will have to go look.

Interesting design from one of the masters.

Rockets that look like airplanes rock. They look and fly good too! Love the smell of fuel proof dope in the morning!

Ever heard of the tail end of the stabilizer being warped slightly upwards and holding it place with white glue so you can heat it later to adjust it? It's been done forever with hand launch gliders which is basically what boost gliders are.
Take a look at the old CMR Manta or Stingray BG.

It adjusts the pitching moment around the CM/CG. You can trim a glider for either range or duration depending on where the CG is as Doug Malewicki showed in the his Model Rocketry series.

The Sky Slash BG( Estes Falcon)that Larry Render designed actually had a pop-up elevator on the stab. Estes considered it too difficult so the wing was tilted a couple degrees upward. It worked for boost but it had a tendency to do death divesduring transition from post to glide phase.

Larry hated the wing and stab tips to the Estes Falcon.
He also created the first pop-pod BG.
G.Harry Stone insisted that he thought of it first, but Larry published his design first, so he got the credit .

He also designed the Sky Dart with the internal pop pod to change the CG along with the Estes Bomarc and it's Sky Dancer RC glider and the Astro Blaster .

https://www.modelaircraft.org/sites/default/files/files/RengerLarryH.pdf

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Many rocket gliders use a sliding wing which moves forward when the ejection charge burns through a thread that's holding it back. Worked fine for me on the one I built.

But a beveled edge? Does it somehow direct air upward to push the glider's tail downward? Is it creating an area of lower air pressure to pull the rear down in the same way that the top surface of an airfoiled wing works?
Yep, and yep. Other ways of approaching the concept include the Coandă effect and the Kutta condition. In any case, the air follows the surface, and the mass times acceleration of the air directed upward (thick aqua arrow) equals the force pulling downward (red arrow) on the tail of the glider.

Note that this is local to the tail. It provides nose-up torque about the center of gravity (at the expense of some downward lift locally).

Overall, to generate lift, the wing is directing the air downward behind it and is therefore torquing the air writ large. The equal and opposite reaction to that is a nose-down torque on the wing. That's what the nose-up torque of the beveled trailing edge counteracts, so that the wing's angle stays constant. Without it, the glider would just dive, like a stable rocket.

But how does the movement of air upward past the glider's aft end move it downward if it's not physically pushing against anything? I don't understand the aerodynamics (or physics?) of it. Can you explain?
Aha, it's for the same reason that a rocket in space shooting high-speed material out the nozzle experiences a force pushing forward, even though it's in a vacuum. Or the reason that standing straight on ice skates and throwing a baseball forward pushes you backward.

(In the example of a wing generating lift, a funny thing about it is that the majority of the lift comes from suction on the top. The pressure on the bottom contributes, but it's a lesser effect. Both upper and lower surfaces help turn the air downward; it just so happens that the upper surface is "working harder.")

It's kind of silly to talk about how much comes from the top and how much from the bottom, since it's the difference that matters. From one point of view, it all has to come from the bottom, since air does not actually withstand tension.

Yep, and yep. Other ways of approaching the concept include the Coandă effect and the Kutta condition. In any case, the air follows the surface, and the mass times acceleration of the air directed upward (thick aqua arrow) equals the force pulling downward (red arrow) on the tail of the glider.
View attachment 658774

Note that this is local to the tail. It provides nose-up torque about the center of gravity (at the expense of some downward lift locally).

Overall, to generate lift, the wing is directing the air downward behind it and is therefore torquing the air writ large. The equal and opposite reaction to that is a nose-down torque on the wing. That's what the nose-up torque of the beveled trailing edge counteracts, so that the wing's angle stays constant. Without it, the glider would just dive, like a stable rocket.
Much thanks, this is the explanation of principle that I hoped for. It's something that'll stick, especially since it's in layman's terms.

Ever heard of the tail end of the stabilizer being warped slightly upwards and holding it place with white glue so you can heat it later to adjust it? It's been done forever with hand launch gliders which is basically what boost gliders are.
I've had too many bad experiences with rocket-boosted gliders looping into the ground with this technique. IMO, the threshold between looping on launch and optimum glide is just too narrow. Just a fraction of a degree too much and you have a nose-plant under thrust. Not just embarrassing, but dangerous to bystanders and the person launching. It works fine for HLGs that aren't going to build up anywhere near the speed when tossed compared to a glider being rocket launched.

Thanks for all the great feedback gents! Since you all have been so impressive at demystifying this (for me) aeronautic enigma, I have another even more incomprehensible "how can this work?" question. This design comes from an old Estes MRN issue, circa 1964.

The Robin delta BG has a main wing that has a positive incidence of 1.6° and wingtips that have a negative incidence of 10° (see plans below). Given that, how in the world can this NOT loop backward into the ground on launch?

#### Attachments

• Robin BG.pdf
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Thanks for all the great feedback gents! Since you all have been so impressive at demystifying this (for me) aeronautic enigma, I have another even more incomprehensible "how can this work?" question. This design comes from an old Estes MRN issue, circa 1964.

The Robin delta BG has a main wing that has a positive incidence of 1.6° and wingtips that have a negative incidence of 10° (see plans below). Given that, how in the world can this NOT loop backward into the ground on launch?
Tractor motor?

(I can't really make things out since plans look like they were written on a cave wall Lol)

I traced everything out. This should help makes things easier/clearer.

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The CG changes as the engine burns. On these very light gliders this can have a significant effect. There were probably many test flights by the designer to tune the behavior correctly.
There could also be some down thrust built in to reduce the looping tendency under power.

From one point of view, it all has to come from the bottom, since air does not actually withstand tension.
Uh... methinks that's confusing absolute pressure for relative pressure? The baseline pressure in the case of a wing isn't zero, it's 1 atmosphere, and local pressures vary up or down from there. Sure, nowhere in the flow is the pressure zero; the upper surface of the wing is pulling not a full vacuum but a partial vacuum.

 I mean, now I get what you're saying that the air underneath is pushing up, and the air on top is just pushing down less, and so one could say that the lift comes from the bottom. But that would also be the case in the extreme hypothetical example of the pressure on the bottom being 1 atmosphere and the pressure on top being zero (a perfect vacuum). In that scenario, the bottom of the wing would be doing nothing to the airflow, whereas the top would definitely be doing something to it!

If anything, all of the lift force coming from the lower surface and none from the upper surface would be the description of a stalled wing. Which still has net lift, just not nearly as much as with the upper surface also acting on the flow.

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