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CAS vs IAS

Thats well and good. But, if you’re having to look at that airspeed instrument while landing at that “short mountain strip”, you are very apt to fail.

It doesn’t take very long to fly by feeling what the plane is telling you. Go to altitude, slow the airplane in landing configuration until it is hanging on that fine edge prior to the stall. Maintain altitude with a bit of power. You want to have the speed such that ANY increase in pitch or bank or reduction of power will precipitate a stall buffet.

Now, fly the airplane around, maintaining altitude, on that fine edge, at altitude, maneuvering (which will require slight added power to prevent a stall), but all the while hanging right on that fine edge.

While doing this in a Cub, more than likely the airspeed instrument will be pegged on zero. But the airplane will be talking to you all the while. And, feeling what the airplane is telling you will permit you to SAFELY land at that short mountain strip.

Trust me, it won’t take long playing the true slow flight game at altitude before you’ll develop a good feel for the plane.

But, Frankly, an airspeed indicator in a Cub is over rated as to its utility. So forget about CAS vs IAS and go fly the airplane and feel it’s paces.

I agree with you Mike, Another way is over a large relatively flat area with no obstructions like trees or power lines. A mile each way is a great size to start with....the trick is to stay within 5 to maybe 10 feet of the ground higher to start then lower as you gain some experience and confidence. At 5 feet or less if you stall one wing or the other wing or both wings or the tail all that will happen is you will touch down, your not high enough to get a wing tip into the dirt or the nose to fall enough to get the prop....fly around with power behind the curve,,,,it will give you a much quicker “feel” for the speed and feel and sight picture....as you gain more time and experience you can make shallow turns and practice holding it off with power with the nose high,full flaps just don’t climb out of ground effect or high enough to get a wing tip. Even if everything went wrong if your close to the ground the deflection angle is so low that damage would be minimal....getting comfortable close to the ground is the first step towards landing short unimproved places. If you still are looking at the airspeed to tell you it’s close to a stall you haven’t practiced enough to go where the airplane is capable of taking you on even 8:50 let alone 31s.
ps:
your airspeed will be reading zero or awfully close to it but you really don’t have time or altitude to look just like in the real world of bush flying
 
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(The text below is not from an experienced Cub pilot, but rather a few things experienced over a few decades of aviating.)

Consider combining slow flight and steep turns.

I flew gliders once up on a time. If I recall, a full 360 turn in 12 seconds was my goal when coring a thermal. It took +/- 60 degrees of bank, very slow speed right on the edge of a stall. If the yaw string was out of center just a little bit, a wing might dip (spin entry?). But an instantaneous recovery was done by agile recognition, rudder application and the slightest release of back pressure. The turn never stopped...we just kept on going....turn after turn until the lift gave out. We felt the stall in the turn ...that was ok. That was the expected standard. The experienced glider pilot community is confident, and proficiency/pilotage skill levels extremely high in this realm. I just thought of this as I read about slow flight practice in this thread.

Some 12 second turns might not be achievable in a cub....A glider will fly slower. (The Slower speed will improve heading change /second.) Can you do 14 second turn? Pick a cloud on the horizon as a heading reference, start the turn and count like a metronome. Don't look at the airspeed, or the clock. Focus outside, see the bank angle, feel the stall.

Doing steep turn slow flight on the edge (at altitude) and getting your proficiency with a wing drop (spin entry?) and recovery to a level where its no big deal seems to be missing in our discussion here and I suspect in aviation outside of acro and soaring. You might need an instructor (do I have to say good instructor these days?) to introduce them to spins and get rid of fear doubt and uncertainty of the wing drop/spin entry.

Make sure your wt/balance places it in the utility category and Mods to your stock airframe may also restrict this. Do it with an instructor until you are safe and confident. I'm sure I'll get flamed for an unsafe suggestion "Never Spin without and Instructor." Let him know your goal of doing spins solo. Your really safe when your knowledge, skills, and proficiency and confidence levels are high and can do it without any concern in the world. Never show off to passengers or other pilots. Build your own skills and confidence with quiet humility.

We'll never know how many low altitude stall spins have been avoided by pilots who recognition and recovery from the wing drop was second nature. We don't keep those statistics. Know where the highest risk area is when flying slow during pattern work. (Overshoot turn to final due to tail wind on base let)
 
So I went up yesterday and did an hour or two flying around at MCA. I'm finding that without power and full flaps my cub just starts mushing around 38mph IAS and goes into a 800fpm descent. There is no real stall. Without flaps the same thing happens at about 41mph.

But if I add power to about 2000rpm I can maintain altitude and get the speed way down into low 30s IAS (off the scale on my AI so not exact.) I'm guessing this is because the power gives more airflow over the elevator and so more authority.

Which got me thinking about adding a burst of power during the flare after a slow approach to make sure elevator maintains authority (and prevent dropping in with a thud). I had always thought a bit of power was about slowing the rate of descent, but I can see that maintaining elevator authority is important too.

How do you guys do it for short field landings? No power, a short burst of power, or a steady rpm of say 1300-1500 into the flare? I'm thinking of the technique for short strips, not long comfortable ones. And I'm aware that I'll have to watch the nose pulling up and to the right if I add power.

jsbc, you’ve go some good counseling’s on how to get proficient at slow flight as well as approaches. Tons of good videos with Pov’s To glean from on the tube, as has been posted. You are at the point where the fun is gonna get a lot funner 8).
kinda like water skiing or any other sport, The enjoyment really starts when you get comfortable with your mount.

https://vimeo.com/352308670
 
There's a world of visual and feel difference between 40 and mid-30's. Watch Flyrite's vid and note the AS and GPS. My light flapped PA-11 landed at 40 flaps up and 36 down GPS. Almost like walking fast vs slow.

Gary
 
Not at all sure about that advice about always having an instructor on board when learning slow flight and spins. Most instructors have little experience in either realm. Choose a really good aerobatics instructor, and choose not to join the club that counts total number of turns before recovery. Emphasize the slow flight - the regime where pulling back more makes the nose fall. For the spin, get it fully developed, and recover immediately. That can be a half turn, with a clean entry.
 
There's one thing that bothers me. Let's say my IAS at stall is 38mph, and my CAS at stall is 44 mph. Given that the big error in IAS is very close to the stall, which number - IAS or CAS - should I be using for 1.3Vs, 1.2Vs and other approach speeds?

I have been using 1.3Vs IAS on base and 1.2 Vs IAS for final. That gives me approx 50mph IAS for base and 45mph IAS on final.

But if the IAS/CAS error at 50mph is much less than at stall, my 50 mph IAS may actually be close to 50mph CAS which is only 1.15Vs.

Here's what Imeson says: "If the pilot goes out and stalls the airplane, noting the indicated airspeed and multiplies this value by 1.3 (for approach) there is an error introduced into the equation that will result in the approach being made at too slow an approach speed."

He then goes on to suggest, though he is not totally clear on the subject, that the IAS should be converted to CAS and then multiplied by 1.3. In my case that would give more like 55mph on base and 50mph on final which is significantly different.

First of all, the plane I'm flying is a Citabria, instead of a Cub, so my airspeeds are probably considerably higher than yours...

That said, I think you left out something that Imeson probably said in that explanation about converting IAS to CAS and then multiplying by 1.3. At that point, you've got a target Calibrated Airspeed for your approach, but to be useful, you'll need to convert your "1.3 Vso" back to IAS for it to be of ANY use to you. Most of us don't have "Calibrated Airspeed" instruments – we only have our standard ASI, which reads "Indicated Airspeed" (IAS) only.

Let's create a hypothetical example, based on the POH for a Citabria 7ECA. Here's the relevant portion of the Airspeed Correction chart from that POH:

I.A.S. (MPH)C.A.S. (MPH)
5058
6066
7075
8083


Let's say I go flying, and determine that the stall occurs at 41 mph IAS. Let's assume I'm flying at max gross weight. Calculating the base leg airspeed (1.3 Vso) using only that IAS, I would determine a target speed of 54 mph IAS.

But Imeson says that speed is not accurate, and he's right. Very much so, in the case of the Citabria! Since how airplanes fly (and stall) is based upon their "real" airspeed (which is Calibrated Airspeed), we need to determine our target airspeeds in CAS. The chart above says we need to add ~10 mph to our 41 MPH IAS observed stall speed to obtain the actual CAS stall speed. Making the conversion, my real stall speed is 51 MPH CAS. (Sanity check: The Citabria POH shows stall speed at gross weight, power off , 0º of bank is 51 MPH CAS. So my "observed" stall speed is right on target. So far, so good.)

Now let's calculate our 1.3 Vso target speed for the base leg using the two different methods:


First, using Indicated Airspeed only:
41 mph IAS stall speed x 1.3 = 53 mph IAS target speed for base leg

Now using Imeson's
Calibrated Airspeed method:
51 mph CAS stall speed x 1.3 = 66 mph CAS
66 mph CAS converted to IAS =
60 mph IAS target speed for base leg

And for our 1.2 Vso target speed for final approach using those same two methods:


First, using Indicated Airspeed only:
41 mph IAS stall speed x 1.2 = 49 mph IAS target speed for final approach

Now using Imeson's
Calibrated Airspeed method:
51 mph IAS stall speed x 1.2 = 61 mph CAS
61 mph CAS converted to IAS = 53 mph IAS target speed


Using Calibrated Airspeed (which is the "real" airspeed), we would fly our base leg at 60 mph IAS, and our final approach at 53 mph IAS. Those speeds give us the actual 30% and 20% margin over the actual stall speeds.

If we incorrectly used IAS only in our calculations, we would fly base leg at 53 mph IAS, and final approach at 49 mph IAS. Our margin above the actual stall speed would be substantially smaller than we believe it to be.

Using a 30º bank for our base-to-final turn, our stall speed would go up 8% to 55 CAS (~47 IAS). A 53 mph IAS, my margin above the stall speed is somewhat smaller than I was expecting... (22% for base, and 12% on final)

But let's say I increase the bank angle to 45º in that turn... Now the stall speed increases by 19% to 61 CAS (~53 IAS), and at 53 mph IAS, I have no safety margin at all... The slightest increase in back pressure or bank angle will stall the airplane. Here I thought I had plenty of safety margin (30% above stall speed), but I actually have none.

As I said, your airplane is different than mine, so your calculations could be dramatically different. I know of one airplane that stalls at 34 mph CAS, but the airspeed indicator shows "zero" airspeed well before stalling. If you're trying to fly 1.3 VSO based purely on the ASI indication in that airplane, you're going to be in a world of hurt... As many others have said before me, you have to develop a "feel" for your airplane at MCA, and that may require covering the ASI up completely for a few hours while you practice slow flight and then progress to pattern work. I found CC Pocock's book Bush and Mountain Flying was pretty helpful in getting comfortable doing that, and heartily recommend it. He also teaches bush and mountain flying
 
Got it. Thanks Jim. That's exactly what I was trying to figure out at the beginning of this thread. But I take the point about flying the approach with visual clues and feel and not relying on IAS.

I'm experimenting with different power settings and approach angles. Ideally I'm looking to get to a 10 degree approach angle (way above 3 deg standard or 4.5 deg Imeson suggests) to give me more runway when coming over my trees.

At about 1,500rpm a little above MCA the approach is pretty straightforward but far too flat. I'm going to try somewhere in the 1,200rpm range with small adjustments to stabilise the approach.

But before I do that I have to work out different fpm descents and wing-to-horizon angles at different power settings at MCA.
 
..At about 1,500rpm a little above MCA the approach is pretty straightforward but far too flat. I'm going to try somewhere in the 1,200rpm range with small adjustments to stabilize the approach.
Try closing the throttle all the way and just gliding in. It works well.
 
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