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Thread: CAS vs IAS

  1. #1

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    CAS vs IAS

    I have a 1957 PA18A and am confused over the CAS vs IAS issue. The POH gives speeds in MCAS. Does that mean that the AI also reads in MCAS? Or does it read in MIAS?

    I thought I had it figured but a, now totally confused on reading Sparky Imeson's book on mountain flying on this subject.

    Can somebody please explain...

    Basically I am trying to figure out Vs. I am getting 38mph indicated and around 44mph GS (averaged over four directions). Is 38 IAS or CAS and if it is IAS how do I convert to CAS?

    Thanks.

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    Should add that 44mph GS for stall is corrected for altitude. It was actually 48mph at 5,000 DA.

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    mvivion's Avatar
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    Quote Originally Posted by jsbc View Post
    Should add that 44mph GS for stall is corrected for altitude. It was actually 48mph at 5,000 DA.
    How were you determining ground speed?

    MTV

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    Indicated airspeed is just that. “Indicated” it is what you see on the instrument.

    Calibrated airspeed is the record of the performance when the aircraft was certified with a calibrated airspeed system. This usually means a gimbled pitot tube that is extended into clean air and a calibrated airspeed indicator that is almost twice the size of the one we use for normal flying. The scale is more precise. This give the most accurate values. Most flight manuals are created using the data from flight testing and therefore are CAS. The difference between calibrated and indicated is greatest at low airspeed because the angle of attack affects indicated because the pitot tube is not gimbled and not necessarily in clean air. As the speed increase so does the accuracy of indicated airspeed. CAR 08 spells out the maximum difference allowed between the two values.

    If you want to know the stall speed, don’t worry about ground speed or calibrated airspeed. Just go out and fly it at the weight you normally fly and learn where it stalls. Also keep in mind the speeds in the flight manuals are almost always at gross weight.
    Last edited by Grant; 08-21-2019 at 05:26 PM.
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    Great. Thanks Grant. That makes sense.

    I was using two different apps to determine ground speed. One on an iphone, the other on an iPad and Stratus.

    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.

    I know this should all be done by feel but for someone with only 100 hrs on a cub an airspeed guideline is useful especially close to the ground.

    Are there IAS/CAS calibration charts for cubs. I'm guessing each plane is different.

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    skywagon8a's Avatar
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    Geeeze jsbc, you are to be commended for your due diligence, however it is only a Cub. Cover the instrument panel so that you can't see any instruments and learn to fly the airplane by looking at the wings and nose in relationship to the horizon. There are many visual clues to keep you in the groove.
    N1PA

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    mvivion's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Geeeze jsbc, you are to be commended for your due diligence, however it is only a Cub. Cover the instrument panel so that you can't see any instruments and learn to fly the airplane by looking at the wings and nose in relationship to the horizon. There are many visual clues to keep you in the groove.
    Yup.

    MTV

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    What I’ve recorded: represents a half dozen sessions over time measuring each attitude and speed averaged over 4 compass directions. At 1700# approx GW in a certified SC, 160, borer, 31ABW, pod, stock wing no VGs

    note: my ASI reads 4mph low at cruise attitude (instrument error), this is included in my data. Yours may not have this beginning error

    at cruise 85mph IAS=89 mph CAS
    at 60mph IAS clean = 66mph CAS
    at 60 mph IAS 1 notch= 68 CAS
    At 40 mph IAS 1 notch= 51 CAS
    at 40 mph IAS 2 notches= 58 CAS

    So I figure in my cub I’m flying 17-19mph faster than IAS when landing. Varies a bit with power setting of course. Taken (still taking) a long time to wrap my brain around this when low and slow.

    curious if others have numbers to compare...I’m sure they all a bit different though so I’ll have the salt handy.

    dont go out and use my numbers....generate your own...lotsa variables in rigging/instrument etc...they’re all different

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    Quote Originally Posted by skywagon8a View Post
    Geeeze jsbc, you are to be commended for your due diligence, however it is only a Cub. Cover the instrument panel so that you can't see any instruments and learn to fly the airplane by looking at the wings and nose in relationship to the horizon. There are many visual clues to keep you in the groove.
    As MTV said...yup.


    Sent from my iPhone using Tapatalk

  10. #10
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    CAS is numbers in a book. IAS is reality, what you live with..........

    With due respect,

    Jack
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    Thanks for that 75C. 18mph is a pretty decent error.

    Flying by feel is all very well and something that comes with time. But I'm trying to shave down my approach speed to get into a short mountain strip and want to make sure I understand what is going on.
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    mvivion's Avatar
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    Quote Originally Posted by jsbc View Post
    Thanks for that 75C. 18mph is a pretty decent error.

    Flying by feel is all very well and something that comes with time. But I'm trying to shave down my approach speed to get into a short mountain strip and want to make sure I understand what is going on.
    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.

    MTV
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    And for true short field work, you are looking at 1.1 VSo, not 1.3. The ASI won’t give you that kind of precision, only feel and visual cues will do that.


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    Open the window. When you feel your shirt sleeves "billowing" from a back draft, you're too slow.
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    Gordon Misch's Avatar
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    curious if others have numbers to compare...I’m sure they all a bit different though so I’ll have the salt handy.
    PA-12, Crosswinds STOL, VG's, level flight, altitude around 2000, full flaps, 20 indicated 36 or 37 by GPS, averaged four cardinal directions on a nearly calm day. But like others have said, I don't know or need to know what my touchdown IAS is. All by the feel of the plane and descent profile with IAS somewhere around 40 or a little less. I'll guess touchdown around 35, but that's just a guess and is peculiar to that specific plane.

    Like MTV said, cover the ASI and go fly. That will help. When in Ak my pitot tubing would get water in it and freeze, making the ASI unusable. It worried me at first, but ultimately it improved my skills.
    Gordon

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    Amen

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    Quote Originally Posted by mvivion View Post
    ….forget about CAS vs IAS and go fly the airplane and feel it’s paces.
    This
    Cessna Skywagon-- accept no substitute!

  18. #18
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    Keep it at 60 IAS (or higher) when maneuvering heavy, 50 IAS is fine for final, you can bleed off excess AS, on short final... Forget about all the other crap...
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    Quote Originally Posted by Gordon Misch View Post
    PA-12, Crosswinds STOL, VG's, level flight, altitude around 2000, full flaps, 20 indicated 36 or 37 by GPS, averaged four cardinal directions on a nearly calm day. But like others have said, I don't know or need to know what my touchdown IAS is. All by the feel of the plane and descent profile with IAS somewhere around 40 or a little less. I'll guess touchdown around 35, but that's just a guess and is peculiar to that specific plane.

    Like MTV said, cover the ASI and go fly. That will help. When in Ak my pitot tubing would get water in it and freeze, making the ASI unusable. It worried me at first, but ultimately it improved my skills.
    ^^^ This. FWIW, on a my similar -12, the ASI just starts wagging uselessly (between 0 - 30 mph indicated) as I'm getting slow enough for approach, somewhere around 35 or so by GPS. Based on some time at Willow, at near gross, about 3-5' off the runway, if I hit the threshold (meaning intended touchdown point) at an IAS of 30 mph, I'm going to float for a bit---not a worrisome amount on any strip longer than 700' or so, but a bit. So the trick, I think, is to gradually learn what IAS is a good starting point, and work from there until you know how the plane feels when it's ready to stop flying / touch down. With enough practice....and I'm certainly not always there....one will be able to be at the touchdown point when the airplane "feels" ready or nearly ready to stop flying / touch down without reference to the IAS at any point except entering approach / long final, etc.

    I'd double / triple agree with the previous postings: Covering the ASI gets one familiar with the airplane and how it flies; and if going into a short strip (or one that challenges plane/pilot, even if not that short), looking at the ASI is a unneeded and unhelpful distraction from the focus on getting the plane out of the flying envelope and onto that challenging landing spot.
    Back In Alaska

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    BC12D-4-85's Avatar
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    Make sure your pitot tube to airspeed plumbing is sealed if you're going to ever bother to look at the latter. Slide a small rubber hose tight over the pitot end and roll it up some to create pressure and indicate airspeed. Does the speed stay steady (tight system) or fall back to bottom (leaks)? Simple test and seal plumbing if leaks.

    Gary
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    Ok. Thanks guys for the thoughts. I've done a fair bit of flying around at MCA in the cub, but I'll do some more. I have VGs and the plane doesn't really break and barely buffets in the stall just mushes into an 800fpm descent and stays there. At that time IAS is 38. Pretty easy to keep it straight with the rudders. Have time in a Citabria and Super Decathlon and compared to those the cub is very benign in the stall.

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    BC12D-4-85's Avatar
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    Try provoking a deeper stall at MCA (and simulate a possible sudden wind change) by quickly pulling the remaining nose up elevator at safe altitude to increase AOA. See what it does with and w/o flaps. Rudder is friend.

    Gary

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    mvivion's Avatar
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    Quote Originally Posted by jsbc View Post
    Ok. Thanks guys for the thoughts. I've done a fair bit of flying around at MCA in the cub, but I'll do some more. I have VGs and the plane doesn't really break and barely buffets in the stall just mushes into an 800fpm descent and stays there. At that time IAS is 38. Pretty easy to keep it straight with the rudders. Have time in a Citabria and Super Decathlon and compared to those the cub is very benign in the stall.
    Good for you. You should be developing a good feel for the airplane......you can use that feel to learn to land short.

    Not trying to be a smart ass, but the airspeed indicator in a Cub is pretty useless during landing.

    But keep on working on it, and have fun.

    MTV

  25. #25
    Eddie Foy's Avatar
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    Just remember that stall and rudder (yaw) can equal spin.
    "Put out my hand and touched the face of God!"

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    Quote Originally Posted by jsbc View Post
    .... the plane doesn't really break and barely buffets in the stall just mushes into an 800fpm descent and stays there.....
    Quote Originally Posted by Eddie Foy View Post
    Just remember that stall and rudder (yaw) can equal spin.
    And also remember that for it to spin, it must stall. A spin is a stalled maneuver. If you were to look at the airspeed indicator during the spin (how many of you have actually looked?) you would see a very low number.
    HOWEVER, a mushed "spin" is a spiral, in which the speed can easily build up to a very high number in which you need to be careful during the recovery in order to prevent over stressing the airframe.

    Recovery from a "spin" can be done with less altitude loss than recovery from a "spiral" with less loads applied to the structure.
    N1PA

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    Don't have the experience or ability to add much to what's been said, but will note that my -12 (with VG's etc) stalls (w/o whipstall or other forcing maneuvers) into a falling leaf sort of mush.

    To me, the MCA is not a number on the ASI or any given number at all but is instead the airspeed/configuration that just keeps the aircraft out of that situation: How slow (for any given configuration) can I go (as determined by feel) and still be in control but any change puts me into the falling leaf thing where I can't maintain altitude etc?

    Using that definition, finding MCA involves a very substantial amount of practice up there around 2500 feet.....and, at least in my experience, even when one has learned the appropriate "feel", it is a very perishable skill if not kept up regularly.
    Back In Alaska
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    The one airspeed that no one has mentioned yet is True Airspeed. Get out your whiz wheel and apply it to each one of the above situations. Figure your true airspeed for each and you will find the significant differences between IAS and Groundspeed.
    Here at home (8000 MSL), my Cub has a no wind, GPS touchdown speed of 45 mph. Indicated will be zero. When I take trips to to lower altitude airports that speed gets slower. Get down around OSH altitude, and that speed is around 35 mph.
    Tom

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    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.

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    I try to do all my approach and landing power on at around 1600 RPM. I usually find a way to screw it up a bit so I adjust as needed. Lately I have added slips back into my approach program and that helps. Having power on has several benefits. Lower stall speed, slower touchdown, better tail authority, and go around is just a few hundred RPM away. Adding that burst of power at the end can and does lead to floating and landing long sometimes so the smaller the burst is usually better, but it depends on several factors. Weight, speed, wind, etc.
    DENNY

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    Last edited by CamTom12; 08-24-2019 at 02:40 PM.

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    [QUOTE=mvivion;753115]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
    Last edited by ag-pilot; 08-24-2019 at 10:14 PM. Reason: Another thought
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  33. #33

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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)
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  34. #34
    flyrite's Avatar
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    Quote Originally Posted by jsbc View Post
    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 .
    kinda like water skiing or any other sport, The enjoyment really starts when you get comfortable with your mount.


  35. #35
    BC12D-4-85's Avatar
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    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
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    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.
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  37. #37
    flyrite's Avatar
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    Quote Originally Posted by bob turner View Post
    Most instructors have little experience in either realm. Choose a really good aerobatics instructor,

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    Quote Originally Posted by jsbc View Post
    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)
    50 58
    60 66
    70 75
    80 83


    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 IASstall 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
    Jim Parker
    '65 Champion 7ECA - Flying
    ?? Bearhawk Patrol - Building
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  39. #39

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    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.

  40. #40
    skywagon8a's Avatar
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    Quote Originally Posted by jsbc View Post
    ..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.
    N1PA

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