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CAS vs IAS

jsbc

MEMBER
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.
 
Should add that 44mph GS for stall is corrected for altitude. It was actually 48mph at 5,000 DA.
 
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.
 
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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.
 
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.
 
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
 
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
 
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
 
CAS is numbers in a book. IAS is reality, what you live with..........

With due respect,

Jack
 
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.
 
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
 
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.


Sent from my iPhone using Tapatalk
 
Open the window. When you feel your shirt sleeves "billowing" from a back draft, you're too slow.
 
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.
 
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...
 
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.
 
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
 
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.
 
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
 
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
 
.... the plane doesn't really break and barely buffets in the stall just mushes into an 800fpm descent and stays there.....

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.
 
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.
 
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
 
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.
 
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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