WW2 aircraft tail load, up or down ?

[Timppa]

This is a technical question. I know that the horizontal stabilator load does not have to be down for the aircraft to be stable. (I hope nobody disagree on this). I believe that most aircraft had basically upward tail load for most flight conditions, but are there any numbers ?

 

[drgondog]

Timppa - conventional tail behind a cambered wing must have at least a slight down lift to overcome CMac at zero lift. CM changes (decreases) with AoA. When AoA changes the contribution of the elevator must offset the Delta CM due to the new airspeed (as f(AoA)).

The question is about stability at all speed ranges - the elevator trim required is both positive and negative depending on where you are in the CL range with aft cg being the most scrutinized condition at low speed.

Near CLmax , after the aircraft is trimmed for equilibrium for a specific velocity, the elevator trim is 'up' (negative angle gamma by conventions- which decreases the Lift on the tail) as the velocity decreases below equilibrium.

Conversely - for a speed above the trimmed velocity, the elevator deflection angle is down (deflection 'down' by convention is 'positive') thereby increasing the lift on the tail

Just about any aero book will devote a chapter on Stability and the derivatives of Tail Lift Coefficient as function of AoA and angle of elevator deflection.. the more complicated game is solving for Static margins and associated trim drag contributions..

 

[swampyankee]

This is a technical question. I know that the horizontal stabilator load does not have to be down for the aircraft to be stable. (I hope nobody disagree on this). I believe that most aircraft had basically upward tail load for most flight conditions, but are there any numbers ?

Insofar as I remember (I don't feel like digging out my books on aircraft stability control), there must be a net down load on the horizontal stabilizer of an aircraft with conventional configuration to have positive stick-free stability. Some aircraft may have operated with sufficiently aft c/g to have net upward lift on the horizontal tail, but the technical term for these when maneuvered was "death trap," as they were liable to pitch up and go into a flat spin.

 

[gumbyk]

With a down-force on the tailplane, if the aircraft slows (engine failure, throttling back, etc), the aircraft slows, the down-force reduces, and the aircraft nose pitches down, which increases airspeed again (albeit with a descent).

 

[Ivan1GFP]

This is a technical question. I know that the horizontal stabilator load does not have to be down for the aircraft to be stable. (I hope nobody disagree on this). I believe that most aircraft had basically upward tail load for most flight conditions, but are there any numbers ?

In general, you can count on the horizontal stabilizer providing down force for normal flight conditions. There are exceptions however: The P-51D Mustang with its 85 gallon (IIRC) fuselage tank even half filled had the stabilizers providing significant lift. This made the aircraft very unstable and also "unsafe" to fly. With significant lift provided by the stabilizers, if the aircraft was stalled, the stabilizer would stall first which would increase the angle of attack of the wing and likely cause a structural failure because of the increased load. That is why the fuselage tank had to be burned off before the drop tanks and also why even though the tank was 85 gallons, it was never filled to capacity.

The Aircraft's Aft Center of Gravity limit is established to prevent this stall and structural failure condition from happening.
Aircraft are usually tested (by the manufacturer) with the center of gravity as far aft as allowable to get the best performance numbers. It helps to not have the stabilizer providing downforce to counteract the lift by the wings. The problem is that as the aircraft's center of gravity is moved rearward, the longitudinal stability is reduced. The aircraft gets very wobbly on the vertical axis. It often flies what is called a "Phugoid".

So, the short answer is: Yes, the stabilizers can provide lift and it will give the best straight line performance, but it is generally avoided because the controllability and stability are not so good.

Hope this helps.
- Ivan.

 

[FLYBOYJ]

This made the aircraft very unstable and also "unsafe" to fly.

I don't think the aircraft was "unsafe to fly' in normal operating conditions, unsafe while performing any kind of upset maneuvers or areobatics.

 

[tyrodtom]

My understanding ( which is little ) is you want the tail at less of a angle of attack so it will stall last, the wing will stall first. The nose drops, restores lift over the wing.
If the tail is providing lift, and it stalls first, the tail drops and you've got no way to restore the aircraft to flight attitude. You fall out of the sky in a flat spin from which the aircraft cannot be recovered no matter how much altitude you have.
Aircraft with canards, are set up with lift on the canards, that way if the canard surface stalls, the nose drops, restoring flight.

 

[KiwiBiggles]

Given that the tailplane is a less efficient lifting body than the mainplane (symmetrical section, lower aspect ratio, lower Reynold's number), I would expect that ideal trimming would have no load on the tailplane at all, giving the lowest tailplane drag. This trim would also give the highest stability for a given size of tailplane.

 

[gumbyk]

Given that the tailplane is a less efficient lifting body than the mainplane (symmetrical section, lower aspect ratio, lower Reynold's number), I would expect that ideal trimming would have no load on the tailplane at all, giving the lowest tailplane drag. This trim would also give the highest stability for a given size of tailplane.

That's also the most dangerous. That is where there is zero stability longitudinally.

 

[Timppa]

See especially chapter 4 in the following document :

 

[Ivan1GFP]

That's also the most dangerous. That is where there is zero stability longitudinally.

What you are describing is the "Neutral Point". It isn't the most dangerous CoG location. It is possible to be stable even with the CoG behind that point. At some point further aft, there will be enough load on the aft "lifting" surface so that it will stall before the front. THAT is the dangerous point, but most folks avoid that because by that time, the aeroplane will tend to wander all over the place.

We keep thinking conventional aeroplane with a big wing in front and little bitty stabilizer in back. The general rules apply regardless of configuration. It could be a canard or something like Langley's Aerodrome and the same rules apply: The aft surface cannot stall first.

For stability, what needs to happen is that if the aircraft is perturbed a bit, the aerodynamic force will straighten it out and restore the original direction of flight. If that restoring force is high, the aircraft is VERY stable. If the force is low, the aircraft has LOW stability. If there isn't any restoring force, the aircraft is NEUTRALLY stabile. If the force tends to move the aircraft away from its original direction of motion, it is UNstable.

- Ivan.

 

[swampyankee]

Given that the tailplane is a less efficient lifting body than the mainplane (symmetrical section, lower aspect ratio, lower Reynold's number), I would expect that ideal trimming would have no load on the tailplane at all, giving the lowest tailplane drag. This trim would also give the highest stability for a given size of tailplane.

I recall that one of the airline manufacturers (in the US, they're all Boeing now; this was about 30 years ago) concluded that the optimum for minimizing operating costs was a slightly stable conventional configuration.

As an aside, one of the issues preventing airlines from using aircraft with the canard configuration is that they tend to have more demanding runway requirements and with the wings largely behind the c/g, it's tough to find someplace to put the fuel.

 

[nincomp]

As an aside, one of the issues preventing airlines from using aircraft with the canard configuration is that they tend to have more demanding runway requirements and with the wings largely behind the c/g, it's tough to find someplace to put the fuel.

The Beech Starship solved the runway and poor max lift problems with a variable-sweep canard. The canard would sweep foreword to allow flaps to be deployed on the main wing. The wings would move rearward under cruising conditions and to compensate for changes in balance as fuel burned off. ...And it was beautiful! I saw it fly once.

 

[Timppa]

Insofar as I remember (I don't feel like digging out my books on aircraft stability control), there must be a net down load on the horizontal stabilizer of an aircraft with conventional configuration to have positive stick-free stability. Some aircraft may have operated with sufficiently aft c/g to have net upward lift on the horizontal tail, but the technical term for these when maneuvered was "death trap," as they were liable to pitch up and go into a flat spin.

There was a discussion in PPrune:

Static stability does not require a downward force on the horizontal tail if you care to read past the second chapter of any self respecting aerodynamics testbook.

In the first chapter, static stability is introduced with a nice little schematic of a CG ahead of the center of lift, and a downward force on the horizontal to balance the aircraft.

If you believe things are that simplistic, you are mistaken.

Most airplanes are perfectly stable with a small positive lift on the horizontal tail when the CG is near the aft limit. Think airfoil camber and pitching moment (nose down) for example, to give you a lead...

There are aircraft out there - either canard/foreplane configurations, or the 'double wing' types - where both airfoils produce lift in some normal flight regime.

Since the mathematics and physics behind the idea of pitch stability cares not one jot whether I call them foreplanes, mainplanes, tailplanes, wings or chickens, these aircraft alone are sufficient to show that download on the aft-most airfoil is NOT required for stability.

All that is required for stability is that the cg be forward of the neutral point (and manoeuvre point) for the configuration. Since the neutral point is derived by considering all the airfoil surfaces, it follows that it may well be aft of the aerodynamic centre of the foremost airfoil, and also may well be aft of the cp of that airfoil. In such circumstances there might have to be upload on the after surface to trim in pitch.

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.

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.

It is possible to have positive lift on the tailplane.This condition is met if the CG is aft of the centre of lift of the wing (roundabout quarter chord point) but not too far behind.This is because the combined wing plus tailplane centre of lift must remain ahead of the CG in order to ensure positive static stability.
I can assure you of this fact from experience.Many many years ago,as a flight test observer I was given the task of flight testing a so called tail load restrictor in a Lancaster.It simply froze the power controlled elevator motion when a certain upward tail load was reached (designed for bombers recovering too smartly from evasive manoevres).I tested over a range of CG's and at the aftmost the inevitable happened--we pulled sufficient g to lock the elevator in steady flight, but couldn't push the stick forward, because that would transiently
increase the upload further.Fortunately I had the foresight/luck to install a cut-out switch (I can see it until this day)
Keith

J.C. Gibson (British Aerospace, Warton), wrote an reply to Air International of this misconception:

It is an unfortunate fact that much of the aerodynamic and stability theory that pilots teach themselves is wrong,and has been for up to 100 years. In the first place, most aircraft have their CG aft of the aerodynamic centre where the lift acts and therefore have a basically upwards tail load for balance. Any wing camber adds a download and the total may be up or down, but its direction does not influence stability. This depends on where the neutral point is, controlled by the size of the tail, which does not affect the tail load. So if you want a large tail upload, instability is unnecessary.

(See also post 10, from the above author)

 

[Ivan1GFP]

I recall that one of the airline manufacturers (in the US, they're all Boeing now; this was about 30 years ago) concluded that the optimum for minimizing operating costs was a slightly stable conventional configuration.

As an aside, one of the issues preventing airlines from using aircraft with the canard configuration is that they tend to have more demanding runway requirements and with the wings largely behind the c/g, it's tough to find someplace to put the fuel.

I was told that a 747 will sit on its tail if the wing tip tanks are filled first.

- Ivan.

 

[bobbysocks]

i am not sure if i believe that just from fuel. i do know that on certain ac you had to be careful boarding passengers ( distributing the weight ) when the cargo holds were full or they would tilt back. when i worked the ramp loading the smaller commuter planes..some ( saab a340?) even had a brace you had to attach to the tail prior to loading pax or bags.

 

[nincomp]

i am not sure if i believe that just from fuel.

How about fuel and a couple of mothers-in-law sitting in the back seats?

 

[bobbysocks]

THAT.. i can believe....

 

[gumbyk]

We loaded a helicopter into a 747 freighter, and they had a stand under the tail to prevent it from sitting down while loading. IIRC it was only really a problem if all the freight was at the rear, such as when loading the first few items.