Tech Support - Load Limitations


Load Limitations under SP and how to apply them.

Although wing mfg have in the past labeled their wings with gross weight limitations and still do, this only works correctly if the entire airframe/wing combination is known. i.e. A gross weight for a certain wing could actually be different depending upon maneuvering limitations of the total aircraft in question. In most cases in the past the wing in question was meant for a particular airframe so the labeling made sense. It was also primarily for rectangular wings on known PPCs and as you will see below, other factors than strength can limit gross weight. 

A wing that has no particular airframe and maneuvering limitations for that airframe specified for it simply cannot always be labeled for gross weight. Sure, some one can write it on there with a permanent marker, but does it make sense for every case? In a word NO. 

The new SP stds ASTM F2426 "Wing Interface Documentation" (Wing Pass Thru Standard) and ASTM F2244 "Design and Performance Requirements for Powered Parachute Aircraft" have addressed this issue so that it can work for any application. The wing mfg "passes thru" the ultimate load limitation for the wing. This is the load limit to which the wing can be subjected without damage, but it is NOT an actual in flight limitation. The in flight limitation would be the maneuvering limit taking into account how many Gs the wing would pull based on it's flying speed and turn rate. Gross weight limitations are then based on safety and gust factors which derate the ultimate load limit to the maneuvering load limit which is in turn converted to gross weight limit depending upon how many Gs are pulled in a turn. The maneuvering limit load factor.

An example or two might help understand this. Lets take a wing rated for an ultimate load limit of 5000 lbs "passed thru" by the wing mfg. This is a common ultimate load limit. The gust factor and safety factors required by SP are 1.5 and 1.5 respectively for a total of the two combined factors of 2.25 (1.5 X 1.5). The maneuvering load limit for our 5000 lbs ultimate load limited wing is 2222 lbs (5000/2.25). 

Depending upon the speed of flight and the turn rate of our aircraft with its' resultant G loading, the gross weight is then determined. This can get a bit complicated and there is a table or formula in ASTM F2244 that you can use to arrive at this, but I'll just pick a number out of a hat for this example to make it simple. Let's assume a 2 G turn. BTW SP requires a minimum of 1.5 if the actual G load is less than 1.5. A 2 G turn is a reasonable choice however since it is right at the FAR Part 91 bank limitation for "normal" flight maneuvering. Any bank greater than 60 degrees under FAR Part 91 is considered an aerobatic maneuver which most PPC mfgs probably would not like to condone.

For our 2G maneuver which its' 60 degree bank, our allowed gross weight for our 5000 lb ultimate load limit wing would be 1111 lbs. Let us further assume our wing flies at 35 mph at this weight (a typical 500 sf rect). This means our turn rate must be limited to 60 degrees per second or a 360 turn in 6 seconds. Actually that is PDQ. 

There is however a problem with this selection. Although this is fine from a strength standpoint, there is a hitch. At 1111 lbs the power off sink rate of our 500 rect is about 800 fpm. Such a sink rate is too high for a safe power off landing so we would have to limit this wing to about 890 lbs where the sink rate is a more reasonable 700 fpm even though it is structurally strong enough for more weight. 

Elliptical wings often have much lower sink rates at any given weight. In the case of an Thunderbolt E340 the sink rate at 1111 lbs is only 611 fpm. With it's mfg specified "pass thru" 5000 lb ultimate load limit, an E340 can carry 1100 lbs (we round it to the nearest even number). As long as you do not exceed a turn rate of about 55 degrees per second at the 43 mph speed of this wing at 1100 lbs. This turn rate at this speed is a 2 G turn. That's 360 degrees in about 6.55 seconds which is still PDQ. Except for special cases, we are suggesting an 1100 lb gross weight limit for the E340. 

Under SP the minimum turn rate cannot be lower than 12 degrees per second or 360 degrees in 30 seconds. This puts you below the 1.5 minimum maneuvering load factor so this wing would theoretically be limited to a load of 1481 lbs if you keep the turn rate at or below about 30 degrees per second or 360 degrees in 12 seconds. Still not that sluggish actually. The sink rate at 1480 lbs is a bit high though not over 700 fpm. A load of 1200 lbs however would not be out of line on any account for an E340 under the right overall airframe/wing design conditions. In order to keep flying speeds within the ranges of the usual PPC pilots skills however, it is recommended that an 1100 lb limit be set for the E340 for normal flight maneuvers.

For the case of the Thunderbolt E310, this wing has the same ultimate load limit as the E340 at 5000 lbs. However the smaller projected area of this wing makes it fly much faster at the same gross weights as an E340. At 900 lbs the E310 reaches a speed of about 46 mph. For most PPC pilots this is at the limit of normal skill levels. For this reason it is recommended that the E310 be limited to a gross weight of 900 lbs.

For special cases other limitations can be set as long as ASTM F2244 compliance is retained. If you are contemplating such a special case, we suggest you consult with us about your special needs.

Aerobatic maneuvers with canopies are not condoned by Canopies . We prohibit such use of our canopies even if technically feasible to meet ASTM requirements. The possibility of negative G maneuvers while performing aerobatics in a PPC is always a threat. PPCs do not have pitch control and averting a negative G aerobatic maneuver is not as easy as with a three axis control aircraft. Negative G maneuvers can quickly become a disaster with a PPC. The resultant canopy collapse and reinflation in flight makes damage to the canopy highly likely. That and the possibility of "falling thru" and becoming entangled in the canopy make aerobatics in PPCs something that ASAP Canopies does not condone with our products.

The accompanying table shows Maneuvering G loads as a function of airspeed and turn rate.

In addition a table showing airspeed vs gross weight is attached for the Thunderbolt E310 and E340.

With these two tables gross weight and maneuvering rates can be determined for about any combination desired using Thunderbolt canopies within the proscribed limits in the ASTM Sport Pilot standards for PPCs.