Aspect Ratio - Wing vs H.S.

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Toobuilder

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Running through the books again and found a little nugget that caught my eye. Higher AR surfaces stall at a lower AoA than low AR surfaces - therefore, you want your HS to be a lower AR than the wing. Makes sense. You don't want the HS to payoff in the flare like the early Cardinals did. So, is there a rule of thumb for the relationship between the wing and stab, or do both surfaces need to be analyzed independently?

I bring this up because the Lancair Evolution has a constant chord stab of fairly high AR. This is appealing to me from a construction standpoint because I can make one mold which will work for left/right/top/bottom skins.
 

Toobuilder

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Perhaps my question was burried in my post too deeply - Let me try again:

Is there a rule/formula that defines the maximum A/R of the stab, or does it simply need to be "less" than the wing?
 

SVSUSteve

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The advice I was given was so long as the AR of the horizontal stabilizer is within the "normal range" of 3-5 (for a non-sailplane design) you pick the actual AR based mostly on aesthetics.
 

Toobuilder

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Yes, I'm sure that the TLAR method would get you in the ballpark, but I expect that there is a converging curve that defines the stall of the two surfaces (wing and stab), and eventually those curves will cross and diverge. From a knowledge standpoint, I'm just wondering how sharply defined is the crossing point, and if there is any benefit to getting close to (but still to the "good" side) that cross?
 
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fly2kads

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What I have seen so far is that the horizontal tail is analyzed independently of the wing. The design of the stabilizer and elevator is heavily influenced by the wing, of course, since it is there to counteract the moments of the wing. I haven't seen anything yet (that I recall) that directly links the aspect ratios such that it would tell you, "if the wing AR is X, then the best tail AR would be Y."
 

autoreply

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There are some small advantages and some bigger drawbacks. This topic covers most of those in-depth:
https://www.homebuiltairplanes.com/...ology/10870-horizontal-tail-aspect-ratio.html


Your tail is flying in the downwash of the wing, so tail stall, even with a much higher AR as the wing shouldn't be a problem, unless you have a very low AR wing (delta, flying saucer). Except for ground effect naturally. Higher AR means lower interference drag on the tail.

One interesting trade-off in composites is that increasing the AR might shrink your panel size (D-nose) enough so that you can get away without ribs, or even without a sandwich cored skin (bare composite skin instead). While the spar weight grows, the skin weight decreases, for zero or negative penalty and much simpler construction.

The increase in dCl/dAlpha (tail effectiveness and stability) is fairly large when you move from AR=3 to AR=5 on the tail. From 5 to 7 for example, the gain is much smaller. Bottomline, except in very special cases (like the topic I linked to above), I don't see a good reason to have a very high-AR tail, say anything above 6. Even sailplanes rarely go much higher.
 
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bmcj

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I've got some additional food for thought....

1. You have to include the wing downwash in your HS angle of attack calculations.

2. You mentioned the tail paying out first in the flare. What is the effect in all of this when you consider that tail-lift is downward, so an increase in AOA of the wing (and aircraft) might be seen as a decrease in AOA for the tail?
 

Rick McWilliams

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A high aspect ratio horizontal stabilizers work well. The higher lift curve slope makes it more effective. Reduced chord reduces the elevator force. The increased span takes space in the hangar. It will be slightly heavier due to the bending loads.
 

Head in the clouds

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Aerodynamics is not my strong point so I'd like to know the answer to this as well. I thought that the stall AoA was lower on a smaller chord rather than a higher AR, at the same airspeed, identified by the lower Reynolds number. Or does the AR affect it independently as well?
 

topspeed100

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I think the horisontal tail volume was in Raymers conseptual approach at page 111.

The wing span and chord and tail arm lenght + elevator area are only variables..the higher AR HS you have the shorter AC you can make...in principal.
 

autoreply

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Aerodynamics is not my strong point so I'd like to know the answer to this as well. I thought that the stall AoA was lower on a smaller chord rather than a higher AR, at the same airspeed, identified by the lower Reynolds number. Or does the AR affect it independently as well?
Yes, the AR is the main dominator. Re effects can play a big role (that's why sailplanes have a limited tail AR), but on most GA aircraft, the effects are small to none.

We're basically talking about downwash on the tail:
Induce_drag_downwash.png


A given amount of lift is caused by the mass of air we accelerate times the velocity at which we accelerate it (downwash speed). A shorter span will "influence" less air and thus we need a bigger downwash for the same amount of lift. You might expect a stalling wing/tail, but that's not happening, as you can see in the picture above, the downwash will increase, but because of the increasing downwash, the effective angle of attack won't rise as fast as the angle of attack with respect to the free airstream.

The effective stall angle will be roughly the same as with a very high aspect ratio wing/tail, but the "freestream" angle of attack will be much larger.

That means that for a given amount of change in the tail angle of attack, the increase or decrease of the actual lift is (much) lower for a low AR tail. Since this defines stability (and tail effectiveness), higher AR usually means better.
 

Head in the clouds

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...............A shorter span will "influence" less air and thus we need a bigger downwash for the same amount of lift. You might expect a stalling wing/tail, but that's not happening,............That means that for a given amount of change in the tail angle of attack, the increase or decrease of the actual lift is (much) lower for a low AR tail. Since this defines stability (and tail effectiveness), higher AR usually means better.

OK I see your point for the example given, high AR for a tail, and that is the topic of course. Thank you for the explanation, I have my head around about a half of that one :ponder:

But if I may ask - slightly off topic - specifically about just a wing (no tail involved in this example) that has a chord of say 3ft/0.94m and has an aspect ratio of, say 5, i.e. 15ft span, if it has a stall AoA of X degrees at Y wing loading and Z speed... So, ignoring tip effects etc would the same wing have a different stall AoA if it was 30ft span instead of 15ft span? The Reynolds would be the same I think. I know Y and Z are constants but I didn't want any other considerations to be misconstrued for this example.

Thanks, Alan
 
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autoreply

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OK I see your point for the example given, high AR for a tail, and that is the topic of course. Thank you for the explanation, I have my head around about a half of that one :ponder:

But if I may ask - slightly off topic - specifically about just a wing (no tail involved in this example) that has a chord of say 3ft/0.94m and has an aspect ratio of, say 5, i.e. 15ft span, if it has a stall AoA of X degrees at Y wing loading and Z speed... So, ignoring tip effects etc would the same wing have a different stall AoA if it was 30ft span instead of 15ft span? The Reynolds would be the same I think. I know Y and Z are constants but I didn't want any other considerations to be misconstrued for this example.

Thanks, Alan
You can't ignore them. The "tip effects" are a direct result of the downwash. That's like discussing Christianity, but ignoring the Bible and Jezus ;)

But yes, lower aspect ratio's will result in a higher angle of attack, relative to the free air flow (or the horizon) in a stall. If you compare a sailplane (AR=50) with a GA aircraft with AR=6, the difference in angle of attack/angle relative to the horizon is something like 1.5 degree, assuming everything else is the same. Barely noticeable between different airframes. If you go to much shorter aspect ratio's though, you quickly start to notice the influence of the low AR wing.
Delta's for example will stall at very high aoa's. Google Verhees Delta for a good idea.
 
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timberwolf8199

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2. You mentioned the tail paying out first in the flare. What is the effect in all of this when you consider that tail-lift is downward, so an increase in AOA of the wing (and aircraft) might be seen as a decrease in AOA for the tail?

Tail lift is not always downward. It depends on the position of your cg in flight. Many tail draggers actually have lifting tails. Of the two configurations a lifting tail is the more efficient as your wing does not have to carry enough to haul the plane up plus an additional amount to counter the tail's negative lift (weight). Instead it carries only the weight of the aircraft, and not even all of it. A trigear can be designed the same, but is usually a bit more challenging to get the gear, wing, and cg in the appropriate locations relative to one another.
 

clanon

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Does Induced angle of Drag and induced AoA have always similar absolute value ?
Or i'm far from it...
 

Head in the clouds

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You can't ignore them. The "tip effects" are a direct result of the downwash. That's like discussing Christianity, but ignoring the Bible and Jezus ;)

But yes, lower aspect ratio's will result in a higher angle of attack, relative to the free air flow (or the horizon) in a stall. If you compare a sailplane (AR=50) with a GA aircraft with AR=6, the difference in angle of attack/angle relative to the horizon is something like 1.5 degree, assuming everything else is the same. Barely noticeable between different airframes. If you go to much shorter aspect ratio's though, you quickly start to notice the influence of the low AR wing.
Delta's for example will stall at very high aoa's. Google Verhees Delta for a good idea.

Thanks for your response again. So it's actually the tip effects that cause the AoA decrease on a high AP wing? Because less of the total span is affected by tip effects? That makes sense.

I don't think the delta analogy is really relevant to my question though, because it brings in another factor - the swept leading edge and the vortices associated with it. A Cassutt and a Cherokee would be better comparisons perhaps?

Anyway this is off-topic and there is much more that I need to learn about aerodynamics so I will create a subject-specific thread and would appreciate your input there.

Regards, Alan
 
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