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Raptor Composite Aircraft

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donjohnston

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IMHO they are a kludge for the wrong elevator moment. Ok on a prototype, but for production, you should probably develop the right elevator. Bits that are fragile and fall off have no place on an aeroplane. Especially if they are likely to go through the prop... I believe the quickie was the last Rutan design to use them. Could Marc or our other canard experts comment on later Rutan elevator feel?
I've never heard of a properly attached sparrow strainer coming off.

And they are not required. There are a number of Velocity's flying without them. They are there only to relieve the amount of force required to control the elevator IF the trim spring breaks. And even if the trim spring breaks and you have no sparrow strainer, the pitch can still be controlled. It just may require more force.
 

BBerson

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Could sparrow strainers be the popular solution just because they look purposeful and different?
I don't know that much about canards, mostly learning from this thread.
My guess is that a luxury canard like Raptor or Velocity could use a jack screw trim on the main forward surface like Cubs and Boeing airliners. But canards might have issues I am not aware of.
 

Wild Bill

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His idea for the production version shows flap style slotted track rollers mounted internally.
Not sure how that would work out.
 

BBerson

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His idea for the production version shows flap style slotted track rollers mounted internally.
Not sure how that would work out.
The Fowler flap type track elevator was installed. But removed after much discussion. Click search at top right, select "this thread only" and type in fowler flap as key words.
 

Tiger Tim

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I don't know that much about canards, mostly learning from this thread.
My guess is that a luxury canard like Raptor or Velocity could use a jack screw trim on the main forward surface like Cubs and Boeing airliners. But canards might have issues I am not aware of.
Same here, and that’s why I’m asking questions about this stuff. A cursory read about sparrow strainers on Quickies sounds to me like they’re there not exactly for trim as we’d normally think about it but to reduce the pitching moment of the canard’s elevator(flap? whatever it’s called here), or to even it out across all air speeds.

Of course, I still don’t understand why pitch trim springs are in vogue on canards instead of tabs so that could well be the key to the whole thing. Are canard elevators often too short chord for traditional trim tabs to be effective?
 

donjohnston

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Same here, and that’s why I’m asking questions about this stuff. A cursory read about sparrow strainers on Quickies sounds to me like they’re there not exactly for trim as we’d normally think about it but to reduce the pitching moment of the canard’s elevator(flap? whatever it’s called here), or to even it out across all air speeds.

Of course, I still don’t understand why pitch trim springs are in vogue on canards instead of tabs so that could well be the key to the whole thing. Are canard elevators often too short chord for traditional trim tabs to be effective?
The challenge with trim tabs on the elevator with a canard is A) the canard is not hollow (at least on the Velocity). So mounting the servo inside the canard would require creating a pocket with the necessary reinforcing. And whenever you do that, there's always the chance that it's going to show up on the surface after a couple years.

The second challenge is that the canard is small compared to most horizontal stabilizers. Mine is a little under 10". The usable space on the elevator is about 5". So trying to fit the servo, linkage and trim tab would be tight.

Finally, the trim spring works. It applies enough force to allow the plane to be flown hands off while still letting you move it to full up or down deflection. If you worry about the trim spring breaking, put on a sparrow strainer. Or don't. It will still fly.

I guess another way of looking at it is: If it ain't broke, don't fix it.
 

pictsidhe

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A Sparrow strainer could be replaced by adding some reflex to the elevator. A full span baked in tab. No change to production to the plain elevator. It would require a couple of test elevators made. You should be able to get close with some maths, or, properly done simulations. Reading some old threads, Roncz probably did this for the Long EZ.
 

donjohnston

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A Sparrow strainer could be replaced by adding some reflex to the elevator. A full span baked in tab. No change to production to the plain elevator. It would require a couple of test elevators made. You should be able to get close with some maths, or, properly done simulations. Reading some old threads, Roncz probably did this for the Long EZ.
Knock yourself out. I'd rather be flying than fixing something that ain't broke.
 

TFF

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If you change the incidence of the canard as trim, wouldn’t you change the lift balance and stall with the main wing? Each trim would have the aircraft handle different. I think there would be complaints that a certain CG would be faster than another too.
 

Tiger Tim

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If you change the incidence of the canard as trim, wouldn’t you change the lift balance and stall with the main wing? Each trim would have the aircraft handle different. I think there would be complaints that a certain CG would be faster than another too.
Isn’t that how it works for every other airplane with stab trim?
 

donjohnston

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Changing the incidence of the canard could be bad. One of the advantages of canards is that the incidence of the canard is set to stall before the main wing. If the main wing stalls before the canard, you're gonna have a bad time.
 

BBerson

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The canard would still stall first with more incidence. It might be an issue with deep stall with incidence and full elevator. Perhaps the elevator could be limited with full up incidence.
 

pictsidhe

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The canard would still stall first with more incidence. It might be an issue with deep stall with incidence and full elevator. Perhaps the elevator could be limited with full up incidence.
Increasing the canard incidence would lead to it stalling at a lower aircaft angle of attack. That means less lift on the main wing, resulting in an even higher stall speed. Something that the Raptor really doesn't need. Reduce it too far, you could stall the main wing before the canard. Neither option is a good idea. The canard incidence relative to the main wing should be set for a low and safe stall speed. Trim effects need be fixed another way.
 

wsimpso1

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Seems to me there has to be about a dozen more elegant looking solutions than that, and the mention that they’ve been lost in Flight a couple times is unsettling.
I always thought that it was an elegant way to balance off most of the control surface moment on a highly loaded lifting surface. Reflexed trailing edges have higher negative lift component for the control moment achieved than when the down force is further aft. I for one would love to see these "more elegant looking solutions"...
 

Tiger Tim

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I for one would love to see these "more elegant looking solutions"...
Bearing in mind that I’m well aware that I’m waaaaaay to the left on the Dunning-Kruger chart I’d lean towards a reflex in the trailing edge. Either designed-in or added incrementally in the initial test period by attaching triangle stock to the top of the TE, then glassing it in once satisfied. You could argue that a reflex is on a shorter arm therefore will have to be a larger surface than a sparrow strainer and come with a corresponding increase in trim drag. On the other hand, there’s got to be a good bit of interference drag on a sparrow strainer from the attach bracket to airfoils, and also whatever the airflow is doing in the gap.

I have no doubt whatsoever that they have their place, I was just wondering if their place was indeed everywhere. How come we don’t see them on modern conventional planes? Is it just that the Venn diagram of people who believe in canards and those who believe in sparrow strainers has a near 100% overlap?
 

berridos

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I am really having a hard time believing (uneducated guess) that the turboencabulator doesnt produce tons of interference drag between the canard and the main wing and other turbulent drag creatures i ignore the name of, compared to a reflexed airfoil. Would be lovely to see some wind tunnel test having the main wing behind.
 

wsimpso1

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Bearing in mind that I’m well aware that I’m waaaaaay to the left on the Dunning-Kruger chart I’d lean towards a reflex in the trailing edge. Either designed-in or added incrementally in the initial test period by attaching triangle stock to the top of the TE, then glassing it in once satisfied. You could argue that a reflex is on a shorter arm therefore will have to be a larger surface than a sparrow strainer and come with a corresponding increase in trim drag. On the other hand, there’s got to be a good bit of interference drag on a sparrow strainer from the attach bracket to airfoils, and also whatever the airflow is doing in the gap.

I have no doubt whatsoever that they have their place, I was just wondering if their place was indeed everywhere. How come we don’t see them on modern conventional planes? Is it just that the Venn diagram of people who believe in canards and those who believe in sparrow strainers has a near 100% overlap?
All of this is amenable to both logical analysis, and analysis by application of math models. Let's avoid the math for now...

Let's try to keep several things in mind - The elevator on a heavily loaded canard is a slotted flap. It has substantial control moments, and you want those moments, that IS exactly how you are getting more lift out of the canard to balance the rest of the pitching moment in the airplane...

Now to the usual route of making the elevator controllable with relatively low muscle inputs and travel at the stick.

When you reflex the trailing edge to balance that big moment, you are also unloading the flap. Looking at the foil, you have high pressure below, low pressure above the canard. This pattern comes in part from the flap bending the air flow down as it goes aft, and the flap's adjustability gives you pitch control... No what are you doing when you reflex the trailing edge? You are attempting to change that high pressure along the bottom to low pressure, while you are also attempting to change the low pressure on the top to high pressure. The air can never change pressures abruptly like that except at sonic shock waves. Our velocities here are way to low for that effect and it moves around a lot with density and airspeed. So the change in pressure difference you want at the tab tends to spread out over the elevator... So, when you reflex the trailing edge enough to balance the flap, you also are unloading the rest of the flap a fair amount. Now this can work, but now your slotted reflexed flap must get bigger to do the same amount of lifting to balance all the other moments of the airplane.

Next option might be to put a trim tab on the trailing edge of the slotted flap elevator. You do not need to unload quite so much and it might work with less than the full span of the elevator, but it pressure changes still spread out on the elevator. Drives your elevator size up, but not so much as reflexing the trailing edge.

Well, we could move the balancing element off the training edge a ways. That will require even less force on the air because it has a longer arm to get the moment needed. But won't the pressures from an elevator and aft element bleed towards each other? Well, they might, except that folks have figured out that vertical elements between the elevator and the external tab tend to serve as fences that greatly reduce this bleed-over of pressures. The also serve as end plates for the external tab and make it easy to fabricate, adjust and generally allow fooling with it until it works.

Being as the reflexed trailing edge, then tab, then external tab schemes are aerodynamic, they can be adjusted to provide substantial balance across the speed range, allowing the remaining adjustments to be achieved with smaller balance springs or other in-flight adjustable methods.

In a lot of ways, the "sparrow strainer" is rather elegant.

Other ways that I can think of (I do have patents and have served on Patent Committees for two major companies) might be to:

Shift the hinge line to largely balance the elevator. Unfortunately, this usually is at odds with getting the high CL increment needed for the canard to balance the airplane over its entire intended speed range while also allowing a smooth progression in stick movement with airspeed change. The canard is more heavily loaded than the wing, and to obtain pitch control and a balanced airplane at the low speed end of the envelope, substantial Cl boost is needed. These issues frequently conflict, but some progress can be obtained with hinge line location;

Trim with in-flight adjustable centering springs. Spring balance is not liked by some for the artificial feel that it sometimes imparts. A control surface that is not aerodynamically balanced but is then trimmed with springs results in speed stability, which is good, but also raise the pitch gradient the pilot feels in flight, which can be bad if large. In the Rutan and derivative canard ships, the canard and its elevator are small chord and so this method can be effective. Increase the size of the ship and spring trim alone may result in excessive control forces - Burt Rutan and company returned to the sparrow strainer for Voyager and Proteus.

Bill
 
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