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Old Oct 22, 2012, 04:29 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Thanks Trevor,

I see what you mean about down elevator having the effect of lifting the plane, if the canard is disconnected from the elevator.

Another thought .. I don't believe I need to disconnect the canard from the elevator at any time. What the gyro does is to try to compensate for any changes in pitch other than those from the pilot. So I can use the elevator to set whatever pitch I need. The gyro should maintain that pitch until I send another instruction from the control stick.

Until I want to escape from GE, I leave the elevator alone. After switching on, the gyro will jiggle the canard whilst I'm moving the plane around and putting it on the ground. Then it will settle down and keep the pitch at that angle until further instructions. I might need to trim a bit.

I can anticipate some tricky setting up - the gyro has a little "gain" dial, which adjusts the amount of travel. Also the balance between the rear elevator and the canard will be interesting. I'll start with the CoG as if there were no canard but with the static margin at 10% and the canard's incidence at zero.

When the plane first starts to move, the tail lifts up first, so the gyro should raise the AoA of the canard until it's off the ground at both ends. If it doesn't raise it enough I can give it a bit of up elevator. I imagine I'll need to program the rudder to link it to the aileron inputs. Once it's flying level I should be able to leave the elevator alone till I want to leave GE.

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Old Oct 22, 2012, 06:42 AM
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As I understand the point mentioned by Nick is the aerodynamic center of the main wing airfoil moves forward when the model leaves the ground effect. I think it happens because of the pressure increase for the rearward portion of the main wing while in ground effect.

However I suspect there is a second issue at play. I am thinking about the difference in aero characteristics between the stabiliser and the main wing. It seems with Nick's design the main wing flys in ground effect, but the stabiliser does not. I think that is true because of the shorter span of the stabiliser and the fact it is high mounted. Because of the ground effect acting on the main wing, there is likely to be a difference in the lift curve slope for the main wing versus the rear stabilser. That's the "dee-see-ell,dee-alpha" that Don Stackhouse often writes about. What it means is that for stable flight in ground effect, a small positive increase in aircraft alpha causes the main wing to carry disproportionately more load than the stabiliser. This makes the aircraft neutral point further forward when in ground effect than it would otherwise be. The main wing lift curve slope is reduced when leaving ground effect and its effect on the aircraft neutral point should partially counteract the change in aerodynamic center of the main wing.

If what I wrote above is true, maybe its possible to adjust the parameters of the model so the two factors can cancel out more completely so the model is flyable without needing a large trim change when leaving the ground effect. I suggest any of three possible changes to improve the situation.
1) Move the stabiliser further rearward.
2) use a larger rear stabiliser.
3) Use a higher aspect ratio main wing.

Its unlikely the two factors will ever cancel out perfectly so maybe the gyro for pitch control is a good idea. I don't think a canard wing is going to improve the stability when leaving ground effect - I think its more likely to make the problem worse.
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Old Oct 22, 2012, 07:57 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,773 Posts
Hmmm..

I don't have any problem flying the existing WBT model, as long as I fly it normally. In other words, if I increase the power, it simply leaves GE and heads for the sky. And if I don't, the nose stays on the ground and it becomes more of a ski-doo than a WIG plane.

I was interested in the idea of setting it up so that I could cruise steadily in GE. Ideally, there should be a fairly wide tolerance of different speeds at the height where the pressure under the rearward portion of the main wing starts to drop.

Most planes that are set up to fly that way do not transition smoothly into the wild blue yonder. With my Flightship model, I did try several configurations of rear stabiliser and thrust line before giving up the whole plan. If cheap gyros had been available at that time I would have given it a try without the canard wing. The elevator on that model was well away from the props as well as the GE. But the plane has long ago been recycled.

With the WBT, the rear elevator is very effective, being in the propwash. I don't think the gyro will work well in propwash. It didn't when I was trying it out for roll stability with ailerons. I was hoping that the gyro, linked only to the forward elevator, might compensate for any unwanted movements in the neutral point. I have to say I don't really understand why the use of the canard wing with the gyro might make matters worse.

Any extra clarification would help

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Old Oct 22, 2012, 08:53 AM
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Quote:
Originally Posted by nickchud View Post
I was hoping that the gyro, linked only to the forward elevator, might compensate for any unwanted movements in the neutral point. I have to say I don't really understand why the use of the canard wing with the gyro might make matters worse.

Any extra clarification would help

I was trying to explain two different aspects that I see affecting the pitch stability for your model in ground effect. 1) is due to the trailing edge pressure under the main wing when in ground effect. 2) is the variation in the balance of lift between the main wing and stabiliser. I think the first one is more intuitive, but I'm going to try again to see if I can explain the second one more simply.

When in ground effect the main wing generates additional lift than it would when flying at altitude. My assumption is the stabiliser is not affected by ground effect, so it has the same lift characteristic in both situations. The higher lift from the main wing in ground effect brings the neutral point closer to the main wing and further away from the stabiliser. For a conventional WBT model the change in lift moves the neutral point further forward when in ground effect, whereas for a canard it moves the neutral point further back. In the case of the canard model coming out of ground effect, it moves the neutral point forward which will destabilise the model. I think that's undesirable and will contribute to the problem your model had already. Anyway, I've never built a ground effect model, so I could be completely wrong about all this!
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Old Oct 22, 2012, 09:58 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Thanks a million John

What I was hoping to do was to keep the AoA as steady as possible when in GE. I've been impressed with the gyro so far, using it on the aileron channel I've been able to do lots of hovering and high AoA flying. It works by making corrections little and often.

Quote:
In the case of the canard model coming out of ground effect, it moves the neutral point forward which will destabilise the model.
We can avoid moving the neutral point forward because the gyro will try to keep the plane level by using the canard to move the neutral point back as we come out of GE. This will require extra speed to generate the extra lift instead of a higher AoA. As long as it succeeds in this, it should be able to avoid the problem of point 2 above.

If I need to rotate to escape from GE then I have to use the elevator control. This will move the neutral point forward, but I hope not to destabilise the plane because the CoG will be still be in front, in the normal flying position.

I was thinking of using an all-moving flat-panel canard, default to zero incidence, with the CoG set for normal flight and ignoring the canard. In my theory the gyro would add lift at the nose in GE to prevent rotation of tail-up. Approaching the limit of GE it would have to move to zero and possibly negative.

The pre-gyro method of adjusting the CoG rearwards for WIG flying can be avoided if we can adjust the neutral point in real time instead.

Thanks again

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Old Oct 23, 2012, 01:09 AM
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Hi Nick, The gyro will help stabilise the pitch of the model, so it seems generally a good idea for this style of model. However it doesn't actually change the static stability and won't completely fix a model that's lacking in pitch stability. Rather the gyro just dampens the response so the changes happen more slowly and the pilot has more time to react in order to maintain control, or make trim changes etc. Hopefully that is all you need to make the model flyable.
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Old Oct 23, 2012, 02:41 AM View Post
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Old Oct 23, 2012, 03:27 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Thanks John

I'm sure you're right. Something extra I was hoping to achieve that I can't do now with that plane, or any other, is stable pitch in GE whilst still being able to fly normally.

If I'm following you correctly, the gyro won't actually maintain a level condition. It only damps deviations in pitch. So, what I need, prob'ly in conjunction with the gyro, is a switch to raise the AoA of the canard in GE and lower it after leaving GE and before the drastic up-pitch which results from the loss of pressure under the rear portion of the wing. The gyro should at least give me time to flip the switch.

Alicee07 That link is nothing but a commercial promotion. I hope our moderator removes it.

Nick
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Old Oct 23, 2012, 11:10 AM
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Lurk mode off:

Hi Nick

The one thing that struck me about that reverse delta is that the main wing while in GE has an AoA of maybe 15-20* [@ average cord location] to the horizontal while the rear stabilizer is ~0* to the horizontal. Thats what WIG requires but when transitioning to flight the main wing must go to ~3-5* which means the stabilizer becomes 10-15* up! When transitioning to flight you have an effective decalage of 15* up elevator. I would say that a fully movable stab needs to be fitted and switched between the two flight modes as required which would likely be a WAG.

Richard

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Old Oct 23, 2012, 02:35 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Hi Richard
Quote:
When transitioning to flight you have an effective decalage of 15* up elevator.
Aha! That figures! I guess the Flightship was never meant to fly both in and out of GE. The problem with the model seemed to be that it was not easy to stay in GE without simply scooting along the ground like a ski-doo. At the limit of GE that frightening amount of up elevator made for a dangerous time. Maybe a model Flightship would work best with much less incidence for the main wing. Anyway, my model got so battered that it ceased to be a worthwhile experiment. Also, if I had wanted 2-dimensional remote control I'd have got a model boat.

I came to this thread because there are plenty of people here with good ideas and a wealth of knowledge. But especially because I thought a canard and gyro arrangement might solve the problem of moving the neutral point on leaving GE.

Cheers

Nick
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Old Oct 25, 2012, 04:32 AM
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Nick
I missed all this good stuff.
A few pages back I posted a video of my WIG in a stall test and also another IGE. It doesn't have a tail.
I got it to work because I got aft migration of cp with height gain as opposed to the normal fwd cp migration.
Instead of your wide body try your Polaris and or polarstuntcat with a far fwd cg and see what happens.
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Old Oct 25, 2012, 02:48 PM
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Quote:
Originally Posted by The Tellurian View Post
Lurk mode off:

Hi Nick

The one thing that struck me about that reverse delta is that the main wing while in GE has an AoA of maybe 15-20* [@ average cord location] to the horizontal while the rear stabilizer is ~0* to the horizontal. Thats what WIG requires but when transitioning to flight the main wing must go to ~3-5* which means the stabilizer becomes 10-15* up! When transitioning to flight you have an effective decalage of 15* up elevator. I would say that a fully movable stab needs to be fitted and switched between the two flight modes as required which would likely be a WAG.

Richard

Lurk mode on:
The dreaded pitch up comes from the movement or migration of the aerodynamic force from almost 40% Mac to approx 25% Mac. The normal art uses oversized tails to keep the pitch up to a minimum.

The problem with a tail is that as the aircraft gains height the downwash increases pushing the tail down which causes an even greater forward shift of overall CP.

A reasonably close coupled canard may work better as the upwash should increase IGE causing overall CP to move forward losing height and aft gaining height.

Cheers
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Old Oct 25, 2012, 07:34 PM
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Quote:
Originally Posted by The Tellurian View Post
The one thing that struck me about that reverse delta is that the main wing while in GE has an AoA of maybe 15-20* [@ average cord location] to the horizontal while the rear stabilizer is ~0* to the horizontal. Thats what WIG requires but when transitioning to flight the main wing must go to ~3-5* which means the stabilizer becomes 10-15* up! When transitioning to flight you have an effective decalage of 15* up elevator. I would say that a fully movable stab needs to be fitted and switched between the two flight modes as required which would likely be a WAG.
I'm curious where you got the figure of 15-20 degress AoA for the wing in GE. For the same speed and lift, I believe the AoA should be lower when in GE due to smaller down-wash angle.
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Old Oct 25, 2012, 09:10 PM
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St Catharines Canada
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Quote:
Originally Posted by John235 View Post
I'm curious where you got the figure of 15-20 degress AoA for the wing in GE. For the same speed and lift, I believe the AoA should be lower when in GE due to smaller down-wash angle.
John235

I mentally drew a line at the approximate average cord from the LE to TE so that its parallel to the fuze. That line looked to be about 15-20* from the horizontal. No consideration to upwash or air flow, [ AoA may be a bad term to have used] just the geometry and it was just a visual guess.


Richard
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Old Oct 28, 2012, 04:48 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,773 Posts
Hello my friends!

Just back from a short walking holiday.

Good to see all this useful discussion. I guess the whole problem is, basically, the difficulty of keeping the nose up under the influence of GE without any violent reaction on transition out of GE.

A special issue with the WBT I'm thinking of is that motors are at the back and lift is at the front. So weight and counterbalance are well away from the centre. Maybe that's a good thing when you're trying to dampen these re-actions.

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