Thread: 15 degrees vs 30/40 again

dave,

you write well, but i am going to rebut what you have to say, see below your quote.

Originally Posted by tlarry17

I took accelerometer readings in still air at a takeoff weight of ~ 1000 lbs and temperature of ~ 65 degrees F. Data was taken at different, constant airspeeds. Accelerometer readings were converted to pitch angles. The numbers are the angle between a triangular plane made by the nose wheel and the main gear, and a horizontal runway. Positive angles mean the main gear is below the nose wheel.


ASI 0 deg. 15 deg. 30 deg. 40 deg.
Knots flap flap flap flap

55 2.3 -2.3 -5.7 -6.9
45 5.7 4.0 0.0 -1.7
40 10.4 5.7 1.7 1.1
35 12.1 8.6 3.4 2.3

Sorry about the "scrunched" tablular data!

At 30 or 40 degree flaps, there is less than +2 degree angle between landing gear and runway at 40 knots. The CTSW wheelbase is only 57cm, so if you touch down at 40 knots constant airspeed, the nose wheel is less than 2cm (0.8”) above the ground. That is a 3-point landing!

Of course, we don’t land at constant airspeed. After the initial flare, we try to hold constant altitude just above the ground. We maintain lift by increasing pitch as airspeed decreases, and the main gear settles gracefully onto the ground. The nose gear makes gentle contact as the aircraft succumbs to decreasing lift/gravity. Flare and touchdown are the most critical flight regime. If we flare at 50 knots, we have to hold both altitude and attitude for 5-10 seconds. Because we are so close to the ground, wind gusts require quick, accurate control inputs to avoid hard touchdowns. If I inadvertently touch down at 40 knots, 15 degrees flaps gives me an angular margin of safety of 5 degrees over 30 degrees flap. I like that.

At idle, (~2000 RPM at 50 knots), the CT sinks at 500-600 feet/minute. The rate is surprisingly independent of flap settings between -6 and 30 degrees. We should not, therefore, think of CT flaps as a way of increasing sink rate. A full-rudder slip works better. More positive flap settings decrease stall speed. This means short-final airspeed can be reduced. This, in turn, means steeper short-final glide angle. But this is not necessarily good.

Those of us who fly gliders become very aware of aileron “feel” over a wide range of airspeeds. As airspeed goes down, the ailerons become markedly less effective, and maximum roll rate goes down. At touchdown we commonly use full aileron deflection just to keep the wings level. The problem is more severe with flapped gliders as the inner, flapped wing panel is still flying while the outer wing panel is fully stalled with ineffective ailerons. This is one reason why some modern gliders use full-span flaps (flaperons).

The CT is not immune to this effect. With lower stall speed at high positive flap angles, roll-axis attitude becomes harder to control. At altitude, try simulating a landing with un-accelerated stalls at different flap settings. At 40 degrees flap and ASI < 35 knots, the CT is still flying, but it is hard to keep the wings level. With 15 degrees flap and ASI ~ 40 knots, roll-axis attitude is much more controllable.

The CT can land safely at any flap setting, including -6 degrees. Optimum flap setting for landing depends on conditions and pilot skill. I’ve found that landings at15 degrees flap are easier than landings at 30 or 40 degrees flap for two reasons:

1. There is less chance of a very unpleasant nose-wheel-first landing.
2. Aileron control at touchdown is better.

Note also that, while the CTSW POH doesn’t provide strong guidance on landing flap setting, the Flight Design USA website recommends 0-15 degrees flap for a typical touch ‘n go pattern.

Finally, scratching the CT tail skid occurrs at +12 degrees flap angle. I’ve found that at 0 degrees flap, touchdown happens at 40-45 knots. At 15 degrees, touchdown is at 35-40 knots. If pre-touchdown is done at constant altitude, the data above shows that the CT tailskid is unlikely to be scratched. I’ve done some VERY nose-up landings and have never scratched the CT tail skid.

Regards,

Dave Ellis
dellis@gmavt.net

you admit "We maintain lift by increasing pitch as airspeed decreases" the increasing of pitch means the deck angle is increasing and means your comparrison of deck angle and runway is only a snapshot in time and doesn't tell the whole picture. i have over 800 30/40 degree landings most of them at the nation's 2nd most dangerous field, dangerous due to gusty crosswinds and sheer complicated by high altitude conditions. i've kissed my nosewheel a couple of times but have demonstrated to my satisfaction that margin from nosewheel contact is not worth giving up the many advantages of landing with flaps.

your contention that the ctsw runs out of aileron authority doesn't agree with my experience. i have never used full deflection on a landing. you say it can be demonstrated at altitude at 35 knots, i question the validity of this given 35 kts is below stall speed. if i am at 35kts and need aileron authority i blew the landing and need full power to get myself back in a safe configuration.

you state that we should not think of CT flaps as a way of increasing sink rate but should slip instead, you claim sink rates are similar @ 50kts from neg six thru 30 degrees. once again your low speed makes your observations less then useful when it comes to landing a ctsw. 50kts @ negative flaps is less then Vs and should be avoided especially on a landing approach. same is true for zero, 15 and 30. think about the ctsw design intent to demonstrate how the reverse is true. the ctsw was designed to opperate in 300meter fields and still realize true airspeeds well above 120kts. if your claim that sink is the same in 4 flap setting at the same speed you are saying the l/d is the same. my experience is far different, it is actually easy to realize high sink rates in the ct and it is easy to control them if you approach at 1.3 Vs[o] and don't allow that energy to bleed off prior to a round out close to the runway.

when you state that you float 5-10 seconds i realize that you are describing a landing with too much speed. my take off roll is 5-6 seconds, your float = 750' approximately, this is not precission. the ctsw loses energy so quickly that the float phase of the landing can be elimnated resulting in a round out that seemlessly connects to the flare.

the 5-10 second float in volitile conditions makes you more vulnerable, and yet you are doing this to be less vulnerable. the 15 degrees vs 30 only means you are vulnerable longer and have more energy in an impact. you choose 15 to needlessly increase aileron authority at the cost of exposing your self to the need for that authority for an excessively long time.

i agree 15 is easier to learn and a more normal sight picture. i don't agree that the flaps are ineffective for increasing sink / descent rate and that slips are a better option. i don't agree that 15 should be your ultimate target for normal landings for the sake of extra safety margin.

the ctsw has great big flaps and flaperons to boot, if you are not willing to use them you opperate within a much smaller envelope

When I read this I found it interesting information. I am guessing the all the numbers were collected at altitude. The flight angles will not be the same for landing because of the decreased drag and increased lift caused by ground effect. I've had no problems landing a CTSW with 30-40 degrees of flaps while keeping the nose wheel off the ground. The same goes with the CTLS with 30-35 degrees. Tom

ground effect, throttle setting, and idle adjustments all change the angles as well

Originally Posted by Dave Ellis

... If I inadvertently touch down at 40 knots, 15 degrees flaps gives me an angular margin of safety of 5 degrees over 30 degrees flap. ...

Dave Ellis
dellis@gmavt.net

The quote above is the focal point worthy of 2 cardinal rules for landing a ctsw.

#1 achieve a landing attitude prior to slowing to stall speed or below stall speed
#2 sink to a few inches or less prior to slowing to stall speed or below stall speed

Energy management is extremely important in a CT or any very light aircraft. The CTSW runs out of energy easily and if you slow to 40kts without following the above rules you have just run out of energy and are in danger of hurting your airplane or worse. If you slow to 40kts but follow the rules above you are in for a good landing.

Originally Posted by Dave Ellis

... I’ve found that at 0 degrees flap, touchdown happens at 40-45 knots. At 15 degrees, touchdown is at 35-40 knots.

touchdowns do not simply happen at these speeds nor do the speeds at touch down determine the pitch attitude at touch down. the process of flaring increases attitude and decreases speed by using kinetic energy and the resulting speed/pitch ratio is not similar to what you demonstrate at constant airspeed.

i have contacted at 30kts by timing the flare well and getting the stick full aft just at contact after rounding out very low. i have also contacted at 70kts to deal with lee side rotors spilling over the runway.

touchdowns don't just happen our actions, coupled with the conditions determine them. pitch attitudes at touchdown are the result of our flaring techniques not a function of our speed.

I prefer flaps to slips. The only time I slip, except for practice, is if I blew the approach. I would not sign off on on anybodies CT transition until they demonstrated they could land with all available flap settings. The plane comes with flaps and each pilot should learn how to use them. After that, you may have a favorite you use most of the time but don't loose the ability to use them all.

The reason I prefer flaps instead of slips is that the approach is stabilized.

john,

like you i will slip if i blew the approach. the other reason i slip is to descend steeply at a higher airspeed, if the purpose of my descent isn't too land but maybe to get a close look at something in a place with limited options i might slip in at 95kts and have a bunch of energy in my pocket at the bottom of the slip.

Originally Posted by sandpiper

The reason I prefer flaps instead of slips is that the approach is stabilized.

Properly executed slips are stable. We slip gliders all the way from base to about 10ft above touchdown on a routine basis to practice inoperative spoilers. If it is done properly (i.e., not be a beginner student), it is stable.

Slipping is just another (highly useful!) tool in your energy management toolbox.

Doug

Doug - you are correct provided slip proficiency is maintained. But, I still prefer to use the flaps when available. If I was flying a no flap plane my proficiency would be much higher.

Doug - perhaps I answered too quickly. By stable maybe I wasn't thinking about it in the same terms you were. To me a "stable" approach is one in which airspeed, throttle and flaps are "stable" and no changes are made. This is the ideal when teaching new or low time pilots. A slip, although certainly not unsafe, does not fit this definition. Therefor the use of a slip is an "unstable" approach - not dangerous or unsteady, etc but significant control inputs have to be made at the bottom.