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-   -   Which aircraft have pos/neg elevator lift? (https://www.pprune.org/tech-log/367457-aircraft-have-pos-neg-elevator-lift.html)

FE Hoppy 29th Mar 2009 15:06

I would submit that a canard does not have to always produce lift. The same argument that a tail "could" produce lift applies to a canard producing down force.

Use a stick pusher to prevent any nastiness.

The point is that theory is one thing. Practical observation shows that so far in the entire history of aviation no commercial airliner has or has had a lift producing tail.

Many of the free flight endurance models I designed and built as a kid had multiple lifting surfaces. They were all trimmed to fly at only one speed. The most successful ones only had 1 lifting surface.

captjns 29th Mar 2009 15:41

Thanks for the spelling correction Beer Hunter... I guess I was absent from school that day. Probably taking flying lessons:E.

ChristiaanJ 29th Mar 2009 16:09


Originally Posted by Mad (Flt) Scientist
The key is that stability is concerned with the CHANGE in forces and moments in response to a disturbance - the absolute values matter for trim and control, not for stability. Which is why the branch of aerospace engineering concerned with these matters is called "stability and control" - both need to be considered, and they are not the same thing.

Thanks, MFL !
Exactly what I would have said if I had been able to get my tongue (errr... keyboard) around it in a clear and concise manner.
I graduated with "stability and control" as my main subject, but it IS forty years ago.

CJ

bjornhall 29th Mar 2009 16:34


I would submit that a canard does not have to always produce lift. The same argument that a tail "could" produce lift applies to a canard producing down force.
I don't see that, at least not by the same argument. For a positive stability contribution from decalage, the rear surface must have a lower angle of attack than the front surface, as explained in Denker´s book referenced earlier. (Not suggesting his is the only good description, but it is one readily available online).

The wing always has to have positive angle of attack in steady flight, obviously. This means that in a conventional aircraft, the tailplane can therefore have a negative or zero angle of attack, or a slight positive angle of attack (but never more positive than the wing). But in a canard aircraft, the canard must always have a more positive angle of attack than the wing does; otherwise the surface at the front would have a lower angle of attack than the one at the rear, and the decalage would give a negative contribution to stability.

To emphasize: A tailplane can provide positive decalage if lift is negative, zero or slightly positive, but a canard can only provide positive decalage by producing positive lift.

The only way an aircraft could be stable with a canard having zero lift, is if there are other positive contributions to stability, resulting in a net positive stability despite the negative decalage contribution. That might be the case if you take an already stable delta wing aircraft and add a canard for maneuverability. I wonder if there are such aircraft? Some Kfir, perhaps?

So yes, you could in principle have a stable aircraft with a canard producing no lift. But we can tell from the very high angle of incidence displayed by most canards that they are designed to always provide positive lift.

ChristiaanJ 29th Mar 2009 17:17


Originally Posted by bjornhall
That might be the case if you take an already stable delta wing aircraft and add a canard for maneuverability. I wonder if there are such aircraft? Some Kfir, perhaps?

Why something as extreme as a Kfir?
What about the Long-EZ?
I would submit it already manages to "look" like a dynamically stable wing in pitch, so that the contribution from the canards may be mostly better trim and manoeuvrablity rather than primarily added decalage.
If so, even an EZ might well end up with very small loads on the foreplane.

CJ

PS never had the occasion to study an EZ closely enough, hence my "look" between quotation marks :)

NO LAND 3 29th Mar 2009 18:55

Hmmm, I was under the impression that decolage, if you want to use it in this context, was primarily to ensure that the appropriate flying surface stalled first rather than for stability. Also, the only evidence the 172 article produces is a fairly dubious experiment where the author loads one at its rear c of g limit and finds it is still stable. Well it would be because Cessna wisely anticipated someone would try this and ensured the allowable c of g range would be far enough forward to remain stable.
I'd love to be proven wrong - are any of you guys aircraft designers with wind tunnel or computer modelling experience?

bjornhall 29th Mar 2009 19:18


What about the Long-EZ?
I would submit it already manages to "look" like a dynamically stable wing in pitch, so that the contribution from the canards may be mostly better trim and manoeuvrablity rather than primarily added decalage
I'd be surprised ... But who knows, might learn something! :)


Also, the only evidence the 172 article produces is a fairly dubious experiment where the author loads one at its rear c of g limit and finds it is still stable.
Far as I remember, what he did was to tie strings to the outer end of the stabiliser, and observe the direction of the vortices generated. From that one could readily observe the direction of the lift being generated. Have to admit I'm only 75% convinced though, would love to see it myself... :8

Negative on the aircraft designer question.... Anyone else?

ChristiaanJ 29th Mar 2009 20:45

NO LAND 3,
Re "decalage" as we are using it here at the moment, it's not an issue of what stalls first. That's determined by the separate profiles, etc.
We're talking about the "normal" behaviour around the steady state and small deviations around that steady state.

I agree about the 172 article.

bjornhall,
I wouldn't be surpised at all.... It would be typically Rutan.
I'll dive into it if I find a moment.

CJ

Machdiamond 29th Mar 2009 22:09

I have some limited wind tunnel exposure and I have yet to come across an aircraft that does not exhibit a small amount of positive lift on the horizontal tail with the CG near the aft certified range, at normal cruise speeds.
So when I read that none of them do, I know for a fact that the statement is incorrect. Actually I wouldn't be surprised that most if not all modern general aviation aircraft behave that way. I don't know about airliners.

TyroPicard 30th Mar 2009 10:16

What's the French for duck, Rodney?
It's canard.
You can say that again, bruv...

keith smith 31st Mar 2009 01:02

Mach diamond
 
Please see my entry no33.You can achieve positive lift on complete tailplaneand still have positive static stability of the complete aircraft, but it won't be very positive in most cases.
Keith

Intruder 31st Mar 2009 03:54


I have yet to come across an aircraft that does not exhibit a small amount of positive lift on the horizontal tail with the CG near the aft certified range, at normal cruise speeds.
Which airplanes were those, specifically? Got any relevant data?

Loose rivets 3rd Jan 2020 23:56

Running on memories these days but while going back down that pprune link I suddenly saw this mental image of the impossible.

I went back to DC3's for my first command and reintroduced myself to cyclic boots. Let a layer build up. Okay, it was forming fast, how much ice? I'd guess I'd got 20mm or so. I turned the big rotating valve behind the FO. I think it was there, but a 50 year old memory. It was then the impossible happened. The ice came off okay but then a 10 foot length of it rotated damn nigh exactly 90 degrees and stopped. We flew for several minutes with this chunk proving just how structurally strong ice can be.

No. Yet again, no darn camera when you want one.


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