G Limits in aerobatic aircraft

[generalspecific]

Ok am probably going to show up my woeful knowledge of physics and aerodynamics, but here goes.

If an aircraft is certified for a max +/- G loading then presumably;

1. This G load is permissable up to the VNE. Put another way does the stress on the airframe increase with speed or does the total G load "capture all", as it were.

2. Even I know that "compound" manouvers put additional strain on the airframe (I suppose a lomke..whatsit being the most extreme example i can think of) and back when i flew only spamcans one was always warned to be very careful of multiple inputs (like ailerons and elevator in concert when recovering from a spiral dive). So is it the case that again the G meter catches all and the spamcan warning only applies because it doesn't have one.

Have a feeling I am about to get a 101 on aerodynamics and loading but the hard hat is on and I welcome all thoughts!

[photofly]

There are different limits. g load limits are related to (eg) the weight the wings and load-bearing structure can definitely support, in a downward direction. Vne is related to the force of the air trying to rip the wheels off, tear the engine cowling off, the wing struts off, etc.

[generalspecific]

Ok I see. So to compound my show of lack of knowledge...

The g pulled is different for the same stick deflection depending on speed, but the G limit is irrelevant to the VNE limit because they are different limits. I.e you can pull max G at VNE because you are A) under the "things start getting blown off" limits; and B) under the "things start snapping" limits.

Put another way the G meter measures the total of all the G loads and any variation due to airspeed is already captured in the G load reading?

Ish ?? :\

[Pugilistic Animus]

Va is the maximum speed at which maximum load factor can be imparted w/o structural damage or failure. If care is taken aerobatic maneuvers can be done above Va but the limit load could be readily exceeded, if not careful, as the available load factor due to the high lift coefficients achievable at higher speed and the G -meter must be respected....

You have less and less structural margin for maneuvering loads as Vne is approached. Va is the speed at which if an airplane is stalled no more than a limit load will result; since maximum loading on the wings occurs at the stall.
Va does not protect against rapid control reversals and no aerobatic maneuver requires such control inputs anyway ...

even if you wanted to say reverse a snap roll it would require momentary centering of the control surfaces before going the other way i.e complete unloading first otherwise a resonance condition maybe set up which can destroy the structure very rapidly:\

of course in the above case you'd be forced to wait to get up a new head of steam anyway...which is good!

[generalspecific]

Very interesting and I think I follow. So hypothetically you can pull to the G limit upto Va, but beyond that it is "somewhat" less. If that is a correct assumption, how do you know hoe less is "less"?

[captainsmiffy]

Let's hope that we don't end up reading about you in the past tense after you experiment with your new-found info about Va! Be careful!!!

[generalspecific]

nope mr cautious me... hence the questions!

[Morrisman1]

From what I read you are asking if it is OK to max out both limits simultaneously: the G limit and the airspeed limit VNE. In the r2160 flight manual I cannot recall any limitation on this apart from their individual limits. The r2160 is the only aerobatic type I've flown.

[Air Tourer]

Re. Photofly, and bits falling off at vne, the Victa Airtourer (115 anyway)
vne limit was the engine reaching red line rpm with the throttle full back.
That I have seen, but how one could get to the +9G limit I don't know.'
A homicidal instructor took me into "grey-out", and we were probably only half-way there?

[dartman2]

Consider this example:

Vs1G = 50kts
Vne = 200kts
G limit = +6

VsAcc = Vs1G X √G

so for 6G...

VsAcc = 50 X 2.45
=122.5kts ie the aircraft will stall well short of the Vne of 200kts

[john_tullamarine]

I think that a quick review of FAR 23.333 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=f069f26099487db1ef745bb2179f8ca1&rgn=div8&view=text&node=14:1.0.1.3.10.3.70.8&idno=14) might be an idea.

[Checkboard]

http://www.rainierflightservice.com/blog/wp-content/uploads/2011/01/vg-diagram.jpg

You can see the vertical axis is the load factor ("G limit"), and the horizontal axis speed. The normal axis stall speed at various loads is the curve marked "accelerated stall", and it runs up until meets the quoted G limit for that particular airframe.

The basic theory is: Pull back on the stick below this meeting point (ie. below the manoeuvring speed), and the wing will stall, thus unloading it, before it over-stresses. Pull back on the stick above this speed, and you will overload the wing before it can stall ...

If an aircraft is certified for a max +/- G loading then presumably;

1. This G load is permissable up to the VNE. Put another way does the stress on the airframe increase with speed or does the total G load "capture all", as it were.

You can see that as the speed increases towards VNE, you enter the caution range. Gust loads here will have a consequently higher effect on the airframe, which may result in an overload. The Ultimate G limits reduce (you can see the reduction on the negative side, the positive side reduction is off the scale), I would guess due to flutter problems outside of the ability for the damping control to be effective.

2. Even I know that "compound" manouvers put additional strain on the airframe (I suppose a lomke..whatsit being the most extreme example i can think of) and back when i flew only spamcans one was always warned to be very careful of multiple inputs (like ailerons and elevator in concert when recovering from a spiral dive). So is it the case that again the G meter catches all and the spamcan warning only applies because it doesn't have one.

No - compound manoeuvres reduce the G-limit for the airframe, the G meter only measure G along the normal axis.

[PittsS2A]

Va is the speed at which the pilot can deflect the controls to the maximum inputs and not exceed the G limits of the aircraft. The aircrafts wings will stall before the G limit is exceeded.

At all speeds above Va the pilot must limit the amount of control input so as not to exceed the design G limit, the aircraft is quite capable of pulling up to the design limit G force up to Vne.

It should also be noted that whilst pulling the maximum G at speeds above Va it is quite possible to overstress the wings if a roll is executed, as the G force at the tip of the rising wing will be greater due to the increased angle of attack caused by the aileron inputs. This is known as rolling G, something that all good aerobatic instructors should know about and re-inforce to their students. An example of where pilots have come unstuck due to rolling G is during recovery from spiral dives, in this instance it is paramount to ensure that the wings are rolled level first and then recover from the dive, rather than attempting to roll and pull at the same time.

Generally the rolling G limit for most aerobatic aircraft would be 2/3 of the max G limit ie. 4G rolling limit for an aircraft certified to 6G.

[Basil]

dartman2,
=122.5kts ie the aircraft will stall well short of the Vne of 200kts

Just in case anyone misunderstands (present company excepted), if you fly this aircraft at 200kn then the theoretical stall would be around 16g (is that right? seems a lot).

Once went slightly over the g limit and, upon reporting the infringement, the kindly old Chiefy (prob about 50) said "Ah, yes, we know you young gentlemen will get a bit enthusiastic so your limit is a little bit below the aircraft limit!" :)

[photofly]

Va is the speed at which the pilot can deflect the controls to the maximum inputs and not exceed the G limits of the aircraft.I don't think that's exactly right. I think it's the max speed at which the sudden and full deflection of any *single* control is sure not to cause structural damage.

I believe in pratical circumstances, the most limiting control input is full up-elevator, which is why Va is usually set at the square-root of the g-limit times VS0, and why Va decreases as the weight goes down.

I do recall that if at Va you input full up elevator and full aileron you could well damage the airframe.

[henra]

Regarding Va it has to be noted that there are two different aspects which define Va:

The first as has been already pointed out is the stall.
In this case the risk of exceeding the structural integrity via dynamic maneuvering is limited to some extent, as the wing will stall at that angle no matter how dynamic the entry is. As has been pointed out you will probably still be able to destroy your aircraft using clever combinations of loads. But it will take significant effort.

The second aspect potentially defining Va can be control Authority. (However, this usually doesn't apply for an aerobatic plane).
In this case for instance the Elevator or Rudder authority does not allow to exceed a certain amount of Pitch authority and thus AoA at a given speed, i.e. g load.
This limitation has the distinct disadvantage that it is by no means a 'hard' limit and can easily be circumnavigated by very dynamic handling, i.e. reversals.
This is also how the famous AA587 shed its tail below Va.
In a control authority restricted Va aircraft it will be much much easier to shed some imporrtant bits'npieces than in a stall limited Va craft.

So it might be a good idea to check what factor determines Va in your aircraft (and for which axis) to know what particularly to avoid.

[photofly]

There are different ways to define Va, but that doesn't mean they're all correct.

See Maneuvering Speed Limitation Statement (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFinalRule.nsf/0/395c3f064247018a862577b200495b5a!OpenDocument) for the FAA's guidance on the definition of Va, and how one pilot's misunderstanding of it contributed to a fatal accident.

Per the FAA: VA is a structural design airspeed used in determining the strength requirements for the airplane and its control surfaces. The structural design requirements do not cover multiple control inputs in one axis or control inputs in more than one axis at a time at any speed, even below VA.

Can anyone point out a definition of Va from an accredited body that even mentions the word "stall"? I've never seen one.

[Brian Abraham]

Can anyone point out a definition of Va from an accredited body that even mentions the word "stall"? I've never seen one.

§ 23.335 Design airspeeds.

c) Design maneuvering speed VA. For VA,the following applies:

(1) VA may not be less than VS√ n where—

(i) VS is a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force coefficients, CNA ; and

(ii) n is the limit maneuvering load factor used in design

(2) The value of VA need not exceed the value of VC used in design.

(d) Design speed for maximum gust intensity, VB. For VB, the following apply:

(1) VB may not be less than the speed determined by the intersection of the line representing the maximum positive lift, CNMAX, and the line representing the rough air gust velocity on the gust V-n diagram, or VS1√ ng, whichever is less, where:

(i) ngthe positive airplane gust load factor due to gust, at speed VC (in accordance with §23.341), and at the particular weight under consideration; and

(ii) VS1 is the stalling speed with the flaps retracted at the particular weight under consideration.

(2) VB need not be greater than VC.

[photofly]

That's FAR part 23 - airworthiness standards.

I think you're confusing the definition of Va with methods of determining its value.

[Brian Abraham]

There are different ways to define Va, but that doesn't mean they're all correctNo there isn't.

The definition of Va is "VA means design maneuvering speed". That is the FAA definition. As you say no mention of "stall" exists in the definition. The definition in itself tells you absolutely zip. For an understanding of what Va is, and trying to acomplish, you need to refer to 23.335, which is centred basically around the stall and structual strength.

PittsS2A has put it very well in his post.

Further guidance is provided by (note the stall reference);

§ 23.1507 Operating maneuvering speed.

The maximum operating maneuvering speed, VO, must be established as an operating limitation. VO is a selected speed that is not greater than VS√n established in §23.335(c).

I do recall that if at Va you input full up elevator and full aileron you could well damage the airframe.

Rolling 'g' limits, at least on those I've flown, were two thirds of the normal limit. I assume that derives from (b) below;

§ 23.349 Rolling conditions.

The wing and wing bracing must be designed for the following loading conditions:

(a) Unsymmetrical wing loads appropriate to the category. Unless the following values result in unrealistic loads, the rolling accelerations may be obtained by modifying the symmetrical flight conditions in §23.333(d) as follows:

(1) For the acrobatic category, in conditions A and F, assume that 100 percent of the semispan wing airload acts on one side of the plane of symmetry and 60 percent of this load acts on the other side.

(2) For normal, utility, and commuter categories, in Condition A, assume that 100 percent of the semispan wing airload acts on one side of the airplane and 75 percent of this load acts on the other side.

(b) The loads resulting from the aileron deflections and speeds specified in §23.455, in combination with an airplane load factor of at least two thirds of the positive maneuvering load factor used for design. Unless the following values result in unrealistic loads, the effect of aileron displacement on wing torsion may be accounted for by adding the following increment to the basic airfoil moment coefficient over the aileron portion of the span in the critical condition determined in §23.333(d):

Δ c m=−0.01δ

where—

Δ c mis the moment coefficient increment; and

δ is the down aileron deflection in degrees in the critical condition.

A good paper on Va is here http://http://www.flightlab.net/Flightlab.net/Download_Course_Notes_files/8_Maneuvering.pdf

As the referred paper says "here’s a conservative, inclusive, legalistic mouthful: Maneuvering speed, VA, is the maximum speed, at a given weight and configuration, at which any one (and only one) flight control surface can be abruptly and fully deflected—not to include rapid control surface reversals—without causing aircraft damage."

[photofly]

PittsS2A has put it very well in his post.Except in so far as saying "controls to the maximum inputs" - it's only one control.

[Brian Abraham]

Except in so far as saying "controls to the maximum inputs" - it's only one control. Correct, but Pitts had that covered by his reference to rolling g.

Back to your "Can anyone point out a definition of Va from an accredited body that even mentions the word "stall"." The authority, had they wished, could have defined Va as "the maximum permissable stall speed when maneuvering". ;)

[Pugilistic Animus]

just to add -assymetrical changes in spanwise normal incremental loading and a decrease in limit load on the due with aileron deflection is one reason why compound mauneuvers are not covered by Va

but physically Va is a function of stall speed and design limit load...it is the highest speed at which an accelerated stall will imapart a limit load as shown by the flight envelope get closer to Vne you have to be careful with as lift coefficients are available for ultimate loading well before a stall...but increased dynamic pressures offer some protection...one reason why stabilators are not seen in aerobatic aircraft is the low stick force/g which can lead to overstress
Va= Vs(nlimit)^.5 double stall speed you have 4 g triple it 9g due to high limit loads on some of these planes up to 20g Va could be set quite high especially if stall speed is high ...:)

[Pugilistic Animus]

...also rudder deflection coupled with inertial input in other axes may be limiting because side loads generally are less than vertical loads so the factor is lesser but the same explanation with regard to change in normal incremental loading with high lift surface deflection including flaps...when compound maneuvers such as a snap roll are done the entry speed will be less than Va but the total load allowed is up to a limit load but unless you're professional well conditioned and well paid with many modern aerobatic plane with limit loads well above FAR23 requirements few of us really have to worry...you can't sell the plane either when you're done:\

[john_tullamarine]

While the limit stall case normally is equivalent to Va, sometimes other control limits may be more restrictive. For instance, the early 125s had Va dependent on rudder input and consequent yawing dynamics - no idea whether that applied to the later versions or not.

[clivewatson]

I don't recall any mention in the Pitts S2B flight manual about roll inputs at Va, although it is a valid point. I do however know (or did) that no factory manufactured Pitts had ever broken up in flight, which is a solid testament to Curtis's design.

Re the 125, I believe that no longer applies, and the 800 aileron rolls very nicely, provided the toilet is emptied first in case one gets a bit slow over the top. The 900 variant seems to lack the roll rate, and thus a bit more energy is required, probably due to the winglets. In the sim, of course!

[zzuf]

Well, AC 23-19A has a different take on this.
48. What is the design maneuvering speed VA?
a. The design maneuvering speed is a value chosen by the applicant. It may not
be less than Vs√ n and need not be greater than Vc, but it could be greater if the applicant
chose the higher value. The loads resulting from full control surface deflections at VA are
used to design the empennage and ailerons in part 23, §§ 23.423, 23.441, and 23.455.
b. VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane structural limits, nor
should it be interpreted as a gust penetration speed. Only if VA = Vs √n will the airplane
stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where
VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would
exceed the limit load factor.
c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507.
VO is established not greater than VS√n, and it is a speed where the airplane will stall in a
nose-up pitching maneuver before exceeding the airplane structural limits.