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Trim-Drag: Small Aircraft Vs Large Aircraft  
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Posted (6 years 3 months 2 weeks 5 days ago) and read 5547 times:

It generally seems that even 1950's era supersonic fighter planes did not seem to require trim-tanks or clever aerodynamic ideas for the most part in order to avoid excessively high-trim drag. Larger airplanes on the other hands such as the Concorde, the B-58 and such generally do require trim-tanks.

The part I'm not getting is this... a longer airplane generally has more pitch control than a short plane in that it's control-surfaces can exert greater leverage -- yet it still needs the trim-tanks and the smaller plane does not which one would think it would because it's shorter fuselage would provide less leverage.


Anybody have any explanation for this issue?


BTW: The only things I can think of that would work against a large airplane is that

1.) It has a lower thrust to weight ratio requiring lower-drag levels than the smaller planes which got by with brute force.

2.) Bigger planes have bigger wings, which would theoretically experience a proportionately larger shift in the center of pressure. But the larger wing would have larger control surfaces, wouldn't it?


10 replies: All unread, jump to last
 
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 1, posted (6 years 3 months 2 weeks 5 days ago) and read 5542 times:



Quoting Blackbird (Thread starter):
yet it still needs the trim-tanks and the smaller plane does not which one would think it would because it's shorter fuselage would provide less leverage.

Trim tanks????? Show me a Boeing that has those???? None of the 320 series have trim tanks either!!!


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 2, posted (6 years 3 months 2 weeks 4 days 23 hours ago) and read 5537 times:

In the case of large deltas like Concorde and the B-58 the rearward CP movement when going supersonic is large (25% mean chord approximately). Some kind of CG movement is required to compensate, hence the use of fuel trimming. Concorde, being commercial, couldn't afford to be penalised by excessive trim drag.

The idea that longer aeroplanes have better pitch control is questionable. Longer aeroplanes tend to be larger, heavier, aeroplanes with much more inertia.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 3, posted (6 years 3 months 2 weeks 4 days 21 hours ago) and read 5527 times:



Quoting Blackbird (Thread starter):
It generally seems that even 1950's era supersonic fighter planes did not seem to require trim-tanks or clever aerodynamic ideas for the most part in order to avoid excessively high-trim drag. Larger airplanes on the other hands such as the Concorde, the B-58 and such generally do require trim-tanks.

Fighters, by design, are very close to being neutrally stable (or, in the case of modern FBW fighters, unstable). That means they need very small control moments to induce large attitude changes. The trim drag on a fighter should be very low because the trim force on a fighter is very low. Hence not so much penalty. Bombers and commercial airliners are a lot more stable (ignoring the B-2 for a moment, which has other reasons for using weird control laws), hence they need larger control and trim forces, hence more trim drag.

Fuel economy is also a secondary concern for a fighter; maneuverability, performance, and survivability are bigger concerns. For something that's going to be shot at, relying on trim tanks isn't such a great idea.

Tom.


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 4, posted (6 years 3 months 2 weeks 4 days 18 hours ago) and read 5510 times:

PhilSquares,

I was talking about large supersonic aircraft. However since McDonnell-Douglas was bought out by Boeing, the MD-11 technically does use trim-tanks.


Jetlagged,

Why would the center of pressure shift on a larger plane be out of proportion with a small plane?


Tdscanuck,

That seems to make sense -- more relaxed stability. But the B-58 could maneuver (almost) like a fighter and it still required trim-tanks (which would suggest similar stability)


Andrea Kent

[Edited 2008-04-12 20:14:57]

[Edited 2008-04-12 20:17:26]

User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (6 years 3 months 2 weeks 4 days 17 hours ago) and read 5501 times:



Quoting Blackbird (Reply 4):
But the B-58 could maneuver (almost) like a fighter and it still required trim-tanks (which would suggest similar stability)

The B-58 is a bit of an odd duck. I'm not familiar with its detailed design, but it's a delta wing, which exaggerates the CP shift when you go supersonic. This might be a reason for adding trim tanks. It's also a delta with no horizontal stabilizer, which would favour trim tanks.

Tom.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (6 years 3 months 2 weeks 4 days 14 hours ago) and read 5476 times:

Supersonic trim drag will only hurt you significantly if you are going to cruise along supersonic for extended periods of time. Most fighters will not.


I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 7, posted (6 years 3 months 2 weeks 4 days 5 hours ago) and read 5423 times:



Quoting Blackbird (Reply 4):
Why would the center of pressure shift on a larger plane be out of proportion with a small plane?

Well it wouldn't be if comparing like for like, but a slender delta like Concorde has the worst problem to deal with, compared to a normal delta or conventional aircraft. Concorde was designed to cruise supersonically, something that fighters didn't do until recently. The only fighter with a comparable configuration I can recall is the Saab Draken. The B-58 is not a slender delta, but has a very large chord relative to overall length.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 8, posted (6 years 3 months 2 weeks 3 days 3 hours ago) and read 5338 times:

Tdscanuck,

Quote:
The B-58 is a bit of an odd duck. I'm not familiar with its detailed design, but it's a delta wing, which exaggerates the CP shift when you go supersonic. This might be a reason for adding trim tanks. It's also a delta with no horizontal stabilizer, which would favour trim tanks.

I thought deltas were not particularly bad when it came to trim-drag. From what I remember

Straight-wings experience the worst shift in the center of pressure
Slightly-swept experience a smaller shift in the center of pressure than straight wings but less than highly swept wings
Delta wings experience a smaller shift in the center of pressure than slightly swept wings but less than highly-swept wings.
Highly-swept wings experience a pretty small shift in the center of pressure


Jetlagged,

Quote:
Well it wouldn't be if comparing like for like, but a slender delta like Concorde has the worst problem to deal with, compared to a normal delta or conventional aircraft. Concorde was designed to cruise supersonically, something that fighters didn't do until recently. The only fighter with a comparable configuration I can recall is the Saab Draken. The B-58 is not a slender delta, but has a very large chord relative to overall length.

But wouldn't the fact that the chord was equal to the length also provide the plane with very good pitch-control too? The B-58 had pretty big elevons!


Andrea Kent


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 9, posted (6 years 3 months 2 weeks 2 days 20 hours ago) and read 5304 times:



Quoting Blackbird (Reply 8):
I thought deltas were not particularly bad when it came to trim-drag. From what I remember

Straight-wings experience the worst shift in the center of pressure
Slightly-swept experience a smaller shift in the center of pressure than straight wings but less than highly swept wings
Delta wings experience a smaller shift in the center of pressure than slightly swept wings but less than highly-swept wings.
Highly-swept wings experience a pretty small shift in the center of pressure

In terms of % of chord, this sounds right. However, you need to look at the absolute distance of the shift. A delta has a *much* longer chord than a straight wing. The actual distance the CP moves is, I think, greater.

Tom.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 10, posted (6 years 3 months 2 weeks 1 day 18 hours ago) and read 5218 times:



Quoting Blackbird (Reply 10):
That's actually a good point.

Exactly the same point I was trying to make in my first reply. Unsuccessfully apparently.  Wink



The glass isn't half empty, or half full, it's twice as big as it needs to be.
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