aerodynamics Q: flaps in thin air when heavy

[Let'sgoflying!]

Yesterday we had another high, hot, heavy takeoff where v1 seems extreme (115kts) and the chart calls for 0 flaps.

Got to wondering why it says that. Could it be that at such speeds you derive a drag penalty that exceeds the lift benefit of using them?

 

[bbchien]

Yesterday we had another high, hot, heavy takeoff where v1 seems extreme (115kts) and the chart calls for 0 flaps.

Got to wondering why it says that. Could it be that at such speeds you derive a drag penalty that exceeds the lift benefit of using them?

In the Slotation it's because of the thermal limits of ITT. Even though you dial ITT max ~?575 and torque limits, it's producing less power. Former T54 guy.

 

[Ryan Ferguson]

Can't speak for the Slowtation, but the reason for this is different in the Hawker 700/800XP. Most hot and heavy takeoffs call for flaps 0 and higher V1s. Going to the tab data will give you longer runways, higher GWs, and higher available temps at flaps 0. The reason for this is called a "W.A.T." limitation (Weight, Altitude, and Temperature), a chart for which can be found in the AFM. The tab data will only give you distances and speeds for zero slope runways with zero wind, so if runway length is at all limiting at flaps 0 you go to the W.A.T. chart and compensate for those factors (this might shorten or lengthen your required runway.) Flaps 0 always gives better (higher) gross weights because your V1/VR is higher and therefore you're closer to Vfto or Venr which are both single engine climb speeds used during (Vfto) and after (Venr) the second segment climb. Basically you're going to the tab data and saying, "Here's what I want to do," and the tab data's saying, "You want to take off with flaps 15? Okay, but if you do, the temps will have to be lower and your maximum gross weight will be reduced." That might be fine with you on a shorter runway. When you have longer runways, you go back to the tab data and say "I can take off with flaps 0 now," and the tab data replies, "Great, you have the runway length needed for the longer takeoff roll required for the flaps 0 takeoff. Now you can take off at flaps 0 and have better second segment climb performance. I'll give you the extra weight you want to carry, and you can take off at the higher temps, and we're going to move the V1 up accordingly (as Vr and V2 have moved up) to reflect that." (Note that this essentially ignores brake energy limitation considerations, which don't seem pertinent to your question right now.) You end up having a conversation with a manual in that case, but maybe it helped shed a little light? So the simple answer is second-segment climb performance (in the Hawker.) I really couldn't speculate on all other jets, but I suspect many would be limited in the same basic way based on similar factors.

 

[poadeleted20]

It sounds like the big issue on both jets is climb after engine failure with the flaps set for takeoff (0 or whatever extension is "normal") -- at hot temps/high weights, it can't make the climb gradient with the flaps extended. Note that while flaps usually increase maximum lift coefficient, the drag coefficient goes up even more steeply, resulting in a reduced climb gradient with flaps even though you can get in the air at a lower speed (same reason you want zero flaps for max glide range). Thus, in a thrust-critical situation (e.g., OEI with high DA), the flaps degrade your post-liftoff climb angle -- only with less flap and more airspeed can you make the minimum required climb gradient.

 

[Ryan Ferguson]

It sounds like the big issue on both jets is climb after engine failure with the flaps set for takeoff (0 or whatever extension is "normal") -- at hot temps/high weights, it can't make the climb gradient with the flaps extended. Note that while flaps usually increase maximum lift coefficient, the drag coefficient goes up even more steeply, resulting in a reduced climb gradient with flaps even though you can get in the air at a lower speed (same reason you want zero flaps for max glide range). Thus, in a thrust-critical situation (e.g., OEI with high DA), the flaps degrade your post-liftoff climb angle -- only with less flap and more airspeed can you make the minimum required climb gradient.

Yup, that minimum single engine climb gradient (either 1.6% VFR or 3.3% IFR) is the determining factor, a minimum performance requirement. Making that minimum 1.6% (100 ft./nm) or 3.3% (200 ft./nm) is the goal during the "second segment climb" which begins at 35 ft. AGL. Second-segment climb performance is what makes most bizjet IFR departures out of places like ASE a no-go. (I believe the Lindz Four DP calls for a minimum 460 ft./nm up to 14,000, which is a gradient far above what most aircraft can do with one engine inop.)

With flaps extended, that climb performance is unnaceptably degraded (below those gradients) at certain combinations of weights and atmospheric conditions. That's when the tab data will simply exclude the temp/weight combination from the Flaps 15 table, and you'll have to hit the AFM to see if the runway slope (hopefully down) and wind (hopefully a headwind) will help you out if the runway's short. Otherwise... it's time to leave people, baggage, or fuel behind.

BTW, just for the record (I know Ron's already aware,) second segment climb always refers to single-engine climbouts. If you're climbing with both engines running, there is no second segment climb.

 

[Let'sgoflying!]

You guys will need to dumb it down a bit for me I think

Does this:

the flaps degrade your post-liftoff climb angle

mean something similar to what I was suggesting:

Could it be that at such speeds you derive a drag penalty that exceeds the lift benefit of using them?

I realize that the airplane performs better, and needs to be configured to satisfy the certified climb requirements, it was more of a 'air molecules passing over the wing question'. As in, why does it perform better?

Thanks!

 

[Ryan Ferguson]

I realize that the airplane performs better, and needs to be configured to satisfy the certified climb requirements, it was more of a 'air molecules passing over the wing question'. As in, why does it perform better?

Thanks!

Right. Sorry I missed the answer you were looking for. It's a matter of the extended flaps' effects on performance. Short field T/O in a C-172 is done with 10 deg. of flaps, while 'normal' T/Os are done with zero. Why the 10 deg. flaps? Better climb angle. Why do normal T/Os call for flaps 0? Better climb rate. On a hot day in Denver in a fully loaded C-172 - bad idea by the way - you won't (assuming the runway is long enough) use flaps 10 for takeoff because there is no performance benefit. You're looking for the most altitude you can get in the shortest amount of time, and that is by definition a best-rate climb. The increase in induced drag caused by the extended flaps decreases climb rate. This is not true "only" at high density altitudes (i.e. 'thin' air); it is true all the time. (Note before a nitpicker gets his or her teeth in it - there may be some aircraft that have better OEI climb performance with flaps in the T/O position - especially large swept-wing aircraft - I'm talking bizjets and below here.)

I'm sure you're already aware of the L/D curve, in which L/Dmax is shown as the most optimal airspeed between high induced drag/low parasite drag (low airspeed, high AOA) and and low induced drag/high parasite drag (high airspeed, low AOA). You know that slowing down or speeding up from that airspeed will increase drag. Therefore if you're looking for best single-engine climb performance, it stands to reason you'd want to be right AT V2, not above, not below, for the same reason. (Interesting side note - ever notice that best glide, Vg, and best angle of climb, Vy, are pretty darn close together in most piston airplanes?)

Well, those flaps in the breeze are increasing your drag, helping you out with your climbout angle but making it harder to attain (and maintain) V2. And the higher temps are causing a performance detriment in both lift and thrust (just like any piston single) so your ability to power through to the front side of the power curve is also hindered. If the charts tell you you're okay to take off with flaps 15, don't sweat it, you'll still meet that minimum 1.6% climb gradient mentioned earlier. Why not take off with flaps 0 all the time, then? Because you'll need longer runways and you'll have to accept higher V1/Vr and possibly run up against brake energy limitations, too.

 

[gismo]

Right. Sorry I missed the answer you were looking for. It's a matter of the extended flaps' effects on performance. Short field T/O in a C-172 is done with 10 deg. of flaps, while 'normal' T/Os are done with zero. Why the 10 deg. flaps? Better climb angle. Why do normal T/Os call for flaps 0? Better climb rate.

And in at least some airplanes (Bonanza is a good example) flaps decrease the takeoff roll length and climb angle after rotation. For a low obstruction that's close in, you will be higher at the obstruction if you use flaps, but for a more distant, taller obstruction zero flaps will provide the best margin.

 

[Let'sgoflying!]

OK, I'm going to mull that one over, thanks for your thoughts.
New topic:
in the limitations sections, you will get wt limits such as MTOW, Mrampwt, MZFW, etc...
but it never says, "WHY"

I imagine MLW is a limit of gear strenght or more likely, brake energy and landing performance charts?
MTOW strictly a performance limit?
MZFW any ideas?
M ramp wt ?? structural limits??

amazingly the full fuel payload on the CJ is only 500 (five hundred) lbs! my guess is a lot of these have been flown overgross.
Bruce it may be the Slowtation but one model holds/held the record for the fastest bizjet, the "X" model at m0.92!

 

[Ryan Ferguson]

I imagine MLW is a limit of gear strenght or more likely, brake energy and landing performance charts?

Usually. There is no one-size fits all answer. Often, it's because of the most limiting factor for that particular operation (if brake energy limits are met before structural limitations, the lesser of the two will become limiting.) In the Hawker we have restrictions on landing with fuel in the dorsal and ventral tanks (can't do it) and therefore any weight with fuel still in those tanks is above MLW. Other aircraft will have their own unique restrictions.

MTOW strictly a performance limit?

Not necessarily, but often so. To be certified under part 25, the aircraft has to meet many minimum performance requirements. The MGTW is often derived by the manufacturer as a weight which will meet those requirements and still provide reasonable performance/capability to the customer under normal conditions. Achieving the target MGTW is one of many engineering challenges.

MZFW any ideas?

Usually it's wing-bending moment. The more fuel you carry in the wings, the lower the wing-bending moment when the wings are producing lift. I.e., the extra weight there actually increases structural integrity. The limit forces a reasonable ratio between cargo weight (carried in the fuse) and fuel (carried in the wings and/or other possible locations) to provide this benefit.

M ramp wt ?? structural limits??

If it's not the same weight as your MGTW, it's probably slightly higher to allow for some fuel burn on taxi. You're not putting the gear assembly through much stress when you taxi, so sometimes the AFM will allow you to carry more weight on the ramp.

amazingly the full fuel payload on the CJ is only 500 (five hundred) lbs! my guess is a lot of these have been flown overgross.

Probably.

The Hawker 700's MGTW is 25,500, BOW is 14,300, full fuel is 9,440 lbs. That leaves 1,760 for pax and cargo. But BOW (Basic Operating Weight) already includes two crewmembers, so this isn't actually that bad. The margin, though, is probably pretty similar to the 500's.

9,440 lbs. gives us at least 5 hours of flight time with NBAA reserves, so the airplane RARELY takes off with full fuel. Quite easy to stack it up with 8 souls in the back, riding in comfort, with a few hours' fuel on-board.