Tailplane loading 1

[xfr8rflyr]

I am a pilot, not an engineer. I realize this is a forum for engineers, but I will ask for your thoughts anyway, for what better place to gain understanding than to come to the hallowed ground to ask the question. Jeez, why do I feel like I'm crashing a party so I can ask the pretty girl to dance?

Well, here goes-

There is a great deal of mis-information in the pilot world surrounding the C208 Caravan. The latest is highly controversial. Some folks are trying to say that the Caravan needs boundary layer energizing Vortex Generators on the top of the horizontal stabilizer because the tail is uploaded continuously under some circimstances.

It has been my understanding that any 14CFR23 or 25 aircraft must demonstrate positive dynamic pitch stability and that this stability is achieved by the tail producing a down load at all times in unaccelerated flight. The down load varies according to CG and configuration but is always present to some degree.

I don't believe the junk science being used to pursuade others that the Caravan is so poorly designed as to require an up load on the tail to help carry the airplane around when loaded near but within the aft CG limit. I am disgusted with the whole "beat up the van" mentality that some pilots maintain. They contend that the aircraft is stable under these conditions. I'd call this BS, it makes absolutely no sense to me, but I have no credentials to make my point stick, I'm just a pilot.

Just for my own edification, is there any truth to their contention that some Part 23 certified conventional airplanes (not canard equipped Beech Starships or three surface Piaggio's) are designed to fly with an continuously uploaded tail? If there is, I certainly don't understand how, but I'd like to.

If you want the bouncers to toss me out, go ahead, but I had to ask.

Thanks for your consideration.

 

[edbgtr]

Hi xfr8rflyr

I should leave this discussion to the Stability and Control experts, but let me add my twopence worth anyway.

I have dealt with a utility aircraft in my country, which exhibited similar characteristics to those you are describing. At certain speeds (economy cruise and aft CG) the horizontal stab can have an upload on it, as the pitching moment on the wing is low. At this setting, pilots did complain about a very long period phugoid, which was probably more a "hunting" action of the aircraft in pitch as it was probably on the "border" between upload and download, i.e. the HS load could have been zero.

The setting angle of the HS relative to the wing alpha-zero line determines whether it will recover from a dive on it's own or not. That is quite different from the longitudinal stability of the A/C which should return to neutral after a disturbance. Aft CG loading will affect the degree of long-stab of the A/C, but if designed correctly, should have a 15% margin on AMC (MAC).

As the Caravan is a real "Ute" A/C as I understand it, with a large CG range, then similar characteristics to the one I worked on, may be evident.

For the rest, I'll let an expert explain the ins-and-outs of long-stab and HS loading to you.

Regards,

Ed

 

[CESSNA1]

XFR8RFLYER: The force(up or down) on the horizontal stabilizer (exclusing turbulence and maneuvers)depends on the location of the center of gravity relative to the center of lift of the wing. If the CG is behind the center of lift the force on the horizontal stabilizer will be up. If the CG is in front of the center of lift the force on the horizontal stabilizer will be down. All airplanes have a weight and CG envelope, and as long as the airplane is witin that envelope it will be stable.

I do not understand the theory behind the statment about vortex generators. The purpose of vortex generators is to add energy to the air and keep it attached to the surface. It is the separation of air that causes a wing to stall. Vortex generators keep the airflow attached to the wing and as a result they lower stall speed somewhat.

Regards
Dave

 

[Sparweb]

The "misinformation" situation is likely. Pilot's I've talked to are satisfied with the Caravan, and these guys are flying survey runs where accuracy is crucial (no phugoids please) and they're dancing on the aft CG line, too. Their planes are, in fact, fitted with instrument booms that project 20 feet out the back of the plane's tail.

People think that vortex generators affect the flying attitude of the aircraft. I see no reason for this to be so. I don't think they change the slope of the lift curve, or its intercept of the x-axis, meaning the angle of attack associated with a given coefficient of lift doesn't change. The energizing effect on the boundary layer helps keep it attached to the wing surface at high angles of attack, permitting higher Gross Weights. That's the advantage, perhaps more controllability at low speed, too, but that's more speculation.

Associated with increased GW kits are the flight manual supplements that shrink/expand or extend the CG limits (the pointy peak of the weight & balance chart). The pilots you've talked to may have gone astray in these areas.



Steven Fahey, CET

 

[planedr]

The only thing I have heard of using vortex generators for is to simply energize the boundry layer to keep the flow attached. It should; expand the flight envelope by, improving the control surface response, as well as reducing drag in some conditions.

 

[aviat]

Tail lift may be either positive of negative as described in section 6 of the article in the site below. Most training books only show the contributions of the wing and tail to stability, and they show that the CG should be ahead of the lift. But that is only part of the equation. The wing and tail do make major contributions to static stability, but the effects of the power plant and fuselage can sometimes be quite important. If thrust line does not go though the CG, the effect of thrust may be stabilizing, if above or destabilizing, if below. If CG is not close to the chord line – a higher CG location (low wing) or a lower CG (high wing); it has an effect on stability also. Different configurations in the same plane can change CG, for instance in the Caravan adding floats changes the envelope.

http://www.av8n.com/how/htm/aoastab.html

This article shows that it is the total aircraft that is important. As long as the total aircraft CM vs. CL graph has a negative slope the aircraft is stable

http://142.26.194.131/aerodynamics1/Stability/Page9.html

The following is more detailed and is the same info as in Aerodynamics for Naval Aviators.

http://history.nasa.gov/SP-367/chapt9.htm#f133

I can’t say for sure that the 208 have positive lift, but other Cessna airplanes do. I wouldn’t say that a positive lift is a bad design; an airplane can have a positive lift on tail and still be stable. Airliners usually have negative lift on tail, and crews like to load airplane as close to rear CG as possible, because it makes for a lighter airplane. They do that at the sacrifice of stability because a forward CG is more stable – within limits.

The following is slightly off topic, but the Caravan’s aft CG, as well as altitude limits, also changes when flying into known icing condition. According to this NTSB report issued last year, a lot of pilots weren’t aware of this.

http://www.ntsb.gov/recs/letters/2004/A04_64_67.pdf

There has been a lot of work on tail plane icing lately by NASA, in particular, tail plane stalling. Some people believe that some of the Caravan accidents may be related to that.

http://www.avweb.com/news/columns/182696-1.html

http://www.grc.nasa.gov/WW/RT1998/5000/5840ratvasky.html

VG are installed on many different parts of an airplane including wings, flaps, inside engine air inlet ducts, vertical stabilizer, and top and bottom of horizontal stabilizer to name a few. Hence, there is no single use for them. The following is from an
Flying Magazine describing how VG are used to meet crashworthiness at higher gross weight.

http://www.flyingmag.com/article.asp?section_id=13&article_id=302

 

[planedr]

Maybe I'm missiing something in the article? The discussion in the piper article is in adding vortex generators (VG's) to control (or energize) the airflow to increase stalling speed. That's what was said above.

Doesn't matter where they (VG's) are installed on the aircraft they achieve the same function. That function may be used to whatever end the designer needs to achieve be it, control surface response, flow control, lift increase, or drag reduction.

 

[plasgears]

Reminds me of a trip I took many years ago on a rented C-172. I had four seats filled with some 30 lb baggage in the form of attache cases. Takeoff was uneventful, but when we got to our destination two hrs later, I had a surprise on short final. I ran out of down elevator, and I was hitting the stops trying to gain airspeed over the lights. I added power, and that made the difference.

The plane came from a marginal FBO outside Chicago. A week later this same plane was involved in a fatal, 3 killed. The FAA padlocked the facility. I personally believe the plane was out of rig. I had 600 hours in Cessnas at that time.

 

[Onemorechance]

XFR8RFLYER,

Although a bit late, I want to just reply in a few words on (your last) the following question although I think CESSNA1 has done a brief but good job.

Just for my own edification, is there any truth to their contention that some Part 23 certified conventional airplanes (not canard equipped Beech Starships or three surface Piaggio's) are designed to fly with an continuously uploaded tail? If there is, I certainly don't understand how, but I'd like to.

First the notion of natural stability which is the same for all kinds of aircraft, whether canards or conventional or even three surfaced or whatever.
Any wing or wing-like object (or even rigid combination of wing like objects) has an aerodynamic center. That's a position relative to the wing (or object) in which the moment coefficient is almost constant. This means: Say this object is flying at a constant speed and performs a change in angle of attack (say due to a vertical gust), there is a point with respect to which the pitching moment (due to air loads) DOES NOT CHANGE. That’s the point where in fact the change of air loads (or the CHANGE in resultant lift force) is acting in. If the cg of the object is located in front of that point, then the object is statically stable. Because an increase in lift force (due to an increase in angle of attack) will result in a (pitch down) moment about the cg that tends to decrease the angle of attack again. That’s also the first condition for natural stability for an aircraft. Another condition to real natural stability is dynamic stability, which will depend upon the proportions of air loads versus inertial loads of the object, but that ‘s another topic.

Now a word about the possibility of positive lift on the tail (or aft wing or whatever). Just imagine a conventional kind of airplane with a wing and a tail. Make the tail shrink in your mind to nothing, then the airplane aerodynamic center is located in the wing aerodynamic center. With the wing being a normal unswept cambered wing you would not be able to fly because it has a pitch down moment about the aerodynamic center. To counter this moment you would have to put the cg behind the aerodynamic center, but then it would be unstable. If you put the cg before the aerodynamic center it would only increase the pith down moment about the cg and tumble and fall. Now think your tail at some distance behind the wing. This tail also has an aerodynamic center that results in a new aerodynamic center of the whole aircraft somewhere BEHIND the wing aerodynamic center. Depending on the tail-arm length (how far it is behind the wing) and how big (in surface area and span) the tail is compared to the wing, this aircraft aerodynamic center will be more or less distance BEHIND the wing. It can perfectly be so far behind the wing that your cg will still be before the aerodynamic center (thus resulting in a stable aircraft) and for moment equilibrium about the cg, result in both a positive lift on the tail and on the wing. An extreme case of this thinking when increasing the tail surface compared to the wing surface is the canard aircraft that always has positive lift on both aft wing and canard wing.
So concluding, in theory there is nothing special about having a lift on the tail, but it will depend on what aircraft you consider, how the proportion of tail surface to wing surface and the tail-arm length…

Regards,

OneMoreChance

 

[Onemorechance]

XFR8RFLYER,

I would just like to continue to answer on two more of your statements:

1. It has been my understanding that any 14CFR23 or 25 aircraft must demonstrate positive dynamic pitch stability and that this stability is achieved by the tail producing a down load at all times in unaccelerated flight. The down load varies according to CG and configuration but is always present to some degree.

The positive pitch stability is independent of the direction of the load on the tail. In fact, in order to have positive pitch stability the DIFFERENTIAL load on the tail will always be positive. That means that the change in load on the tail due to a change in angle of attack has to be UPWARD instead of downward. But it is true that most conventional aircraft will fly with a downward tail load most of the time.



2. Some folks are trying to say that the Caravan needs boundary layer energizing Vortex Generators on the top of the horizontal stabilizer because the tail is uploaded continuously under some circumstances.

I can imagine that this aircraft is one of these that have their aerodynamic center rather far behind the wing and that you can comfortably fly with a positive load on the tail.
In that case, you could come into a potentially dangerous situation at low airspeed and high aircraft mass. The lower your speed, the higher the angle of attack needs to be to produce the lift on the wing. When also your tail lift is positive, it needs to grow too in order to keep equilibrium. But now it might happen that you reach the tail maximum lift before reaching the maximum lift of the wing which would cause your tail to stall before the wing! To prevent this to happen, you could boost the maximum lift capacity of the tail by placing some vortex generators to energize the airflow over the tail which would make it capable of a higher maximum lift!

Regards,

OneMoreChance