Any other Nerds or just me? (Airspeed calcs)

[Bob - S50]

For those who don't want to read long entries, this is about airspeed on final.  I have started using yet another speed (involving mental math) and wonder if anybody else is doing something similar.

When we first got the J I flew 80 KIAS on final... too fast.

Then I started slowing to 75 KIAS, still floated forever.

Then I tried 70 KIAS, still float more than I like.

Here comes the nerd part.  Our POH only gives stall speed for max gross weight; full flaps it is 55 KIAS.  It does not give stall speeds at any other weight.  So I got out my trusty calculator app and did some figger'n.  Since lift is proportional to the square of the velocity, I used the formula W2/W1 = V2^2/V1^2.  Since I knew W1= 2740 and V1 = 55 I could enter any weight and come up with the stall speed.  I then took the stall speed at each weight and multiplied by 1.3 to come up with a target speed (no gusts) on final.

Bottom line, I now use the following:  58 KIAS + 1.5 x (fuel+ payload in hundreds).

So for example.  If I come back to the airport solo with 10 gallons on board that's about 190# for me and all my gear + about 60# for fuel = 250# total.  I round that up to 300.  3 x 1.5 = 4.5 so I would use 63 KIAS on final.  If I come back with 30 gallons, and 400# of people and bags I would use 58 + 1.5 x (400 + 180)/100.  Call it 58 + 1.5 x 600 = 58 + 9 = 67 KIAS on final.

So far I'm much happier with my landings.

So is it just me?

[carusoam]

Don Kaye provides a chart with an explanation along with a video.  It may be helpful to combine his thinking with yours.

Dissipating mechanical and potential energy in a well defined, stable way can be a pretty complex discussion.

Pilots have been trying to simplify this down to airspeed, and MP for a century.  Attitude, altitude and vertical speed add to the complexity.

AOA can add to the simplicity...

One question: Are you able to watch the ASI all the way through the landing procedure?  (Some people are better than others at that level of multitasking.)

The O's POH gives stall speeds with a decimal point.

Best regards,

-a-

Edited October 20, 2015 by carusoam

[Marauder]

For those who don't want to read long entries, this is about airspeed on final.  I have started using yet another speed (involving mental math) and wonder if anybody else is doing something similar.

When we first got the J I flew 80 KIAS on final... too fast.

Then I started slowing to 75 KIAS, still floated forever.

Then I tried 70 KIAS, still float more than I like.

Here comes the nerd part.  Our POH only gives stall speed for max gross weight; full flaps it is 55 KIAS.  It does not give stall speeds at any other weight.  So I got out my trusty calculator app and did some figger'n.  Since lift is proportional to the square of the velocity, I used the formula W2/W1 = V2^2/V1^2.  Since I knew W1= 2740 and V1 = 55 I could enter any weight and come up with the stall speed.  I then took the stall speed at each weight and multiplied by 1.3 to come up with a target speed (no gusts) on final.

Bottom line, I now use the following:  58 KIAS + 1.5 x (fuel+ payload in hundreds).

So for example.  If I come back to the airport solo with 10 gallons on board that's about 190# for me and all my gear + about 60# for fuel = 250# total.  I round that up to 300.  3 x 1.5 = 4.5 so I would use 63 KIAS on final.  If I come back with 30 gallons, and 400# of people and bags I would use 58 + 1.5 x (400 + 180)/100.  Call it 58 + 1.5 x 600 = 58 + 9 = 67 KIAS on final.

So far I'm much happier with my landings.

So is it just me?

I never did the high brow math, but after 24 years of ownership have come to the same conclusion to support your hypothesis Dr. Bob.

I am careful about any potential gust factors and learned although I can fly it at 63 KIAS with a lighter load on short final, I have been surprised a couple of times by unexpected winds. I have found over time that 68 KIAS is a good compromise between float and winds that would sometimes get the stall horn chirping.

I haven't taken the AOA to task yet to validate these findings. But I know the AOA isn't happy sometimes when I am on the lean side of 68.

Sent from my iPad using Tapatalk

[gsxrpilot]

All very cool... except that it would require me to look at the ASI on final. I haven't done that in years in any airplane...

[rbridges]

when Marauder has one of his girlfriends, his stall speed jumps up to 150 knots and he can't figure out why.

[PMcClure]

I like the math and was betting the first response would include AOA!! I was right. 

Forgive a perhaps silly amateur question but what is wrong with a little float, especially as compared to the risk of stall on final? I know we need to practice landing as short as possible and agree airspeed control is a very important part of this. But if you have a 5,000 foot runway why not allow a little float and hold off for a smooth squeaker of a landing? Coming in hot and light can lead to some interesting landings but carrying a few extra knots and holding off the RR for a few hundred feet isn't dangerous is it?

Just for comparison, I use 70knots on short final for light and 75 for "heavy" in my Ovation but can still manage good landings at light and up to 80 knots. 

[rbridges]

I like the math and was betting the first response would include AOA!! I was right. 

Forgive a perhaps silly amateur question but what is wrong with a little float, especially as compared to the risk of stall on final? I know we need to practice landing as short as possible and agree airspeed control is a very important part of this. But if you have a 5,000 foot runway why not allow a little float and hold off for a smooth squeaker of a landing? Coming in hot and light can lead to some interesting landings but carrying a few extra knots and holding off the RR for a few hundred feet isn't dangerous is it?

Just for comparison, I use 70knots on short final for light and 75 for "heavy" in my Ovation but can still manage good landings at light and up to 80 knots. 

I agree.  I probably come in a knot or two faster than I should b/c I typically land at a 5000ft runway.  My original CFI always stressed learning to land in a shorter distance.  He said it would make me more comfortable for some of the short runways I may come across.

[bonal]

this subject has been beaten to death and I'm looking forward to some more beatings as its one of my favorite learning discussions. carry on my sponge is dry.

[PTK]

Math is good!

I strive to td on the same spot on the runway. Home field is about 3800 feet. Comfortably manage the second turn off which is about 1800 feet.

Edited October 20, 2015 by PTK

[PMcClure]

All very cool... except that it would require me to look at the ASI on final. I haven't done that in years in any airplane...

OK, I'll bite. What are you looking at?

[ArtVandelay]

From a J POH:

No need for the complex math...

[gsxrpilot]

OK, I'll bite. What are you looking at?

The runway.  

I just fly an M20C, but it's also worked well for me with Bo's and the Comanche I flew for awhile. Assuming a VFR arrival, my eyes are outside the windows once on downwind and generally stay there.

[Hank]

In my C, I'm looking mostly out the windows, for traffic, for the runway and for spacing. I steal an occasional glance at the ASI and altimeter to make sure I'm where I want to be:  90 mph downwind and base; make the base turn 700-750 agl; turn final at 500 agl; roll wings level at 85 mph, decelerating to 70-75 on short final [70 if light, 75 if heavy]. This is very simple to do, with just a few quick glances to back up the sight picture. My rule of thumb is 75 mph short final, minus 5 mph per 300 lbs under gross.

Obviously if you fly a J and not a C, the numbers will change. Our wings are the same, the J is heavier and the C has more drag, and my 3-blade adds more drag still. Plus I'm in mph and the J is in knots . . . But the principle still applies.

[aviatoreb]

Who needs math - what's math good for anyway?

[Hank]

Who needs math - what's math good for anyway?

That's what I like about my method, it can be done on fingers after you guess at combined weight of remaining fuel, self, passengers and cargo.

[aviatoreb]

That's what I like about my method, it can be done on fingers after you guess at combined weight of remaining fuel, self, passengers and cargo.

But that's still math even if you use your fingers.

[Bob - S50]

Don Kaye provides a chart with an explanation along with a video.  It may be helpful to combine his thinking with yours.

Dissipating mechanical and potential energy in a well defined, stable way can be a pretty complex discussion.

Pilots have been trying to simplify this down to airspeed, and MP for a century.  Attitude, altitude and vertical speed add to the complexity.

AOA can add to the simplicity...

One question: Are you able to watch the ASI all the way through the landing procedure?  (Some people are better than others at that level of multitasking.)

The O's POH gives stall speeds with a decimal point.

Best regards,

-a-

I do not watch the airspeed in the flare.  On final I'm runway and airspeed.  Once I pull the power and start the flare I'm 100% runway.  Been that way in every plane I've ever flown.

[Bob - S50]

I like the math and was betting the first response would include AOA!! I was right. 

Forgive a perhaps silly amateur question but what is wrong with a little float, especially as compared to the risk of stall on final? I know we need to practice landing as short as possible and agree airspeed control is a very important part of this. But if you have a 5,000 foot runway why not allow a little float and hold off for a smooth squeaker of a landing? Coming in hot and light can lead to some interesting landings but carrying a few extra knots and holding off the RR for a few hundred feet isn't dangerous is it?

Just for comparison, I use 70knots on short final for light and 75 for "heavy" in my Ovation but can still manage good landings at light and up to 80 knots. 

Not much wrong with floating if I have enough runway.  However, the longer I float, the longer I have to screw up the landing either due to a balloon from a gust/muscle twitch or touching down too fast due to poor eyeball calibration.  I hate logging 3 landings for one approach!

My home drone is adequate, 3500', but I've flown into San Carlos, CA (SQL) which is 2600'.  If I'm used to floating, things could get sporty if I decide I want to go into something even shorter.  I'd rather be comfortable with my speed on final because I do it the same every time rather than be uncomfortable on final at a short runway because I'm 5 or 10 knots slower than normal.

Flying final at 1.3 times the stall speed is pretty standard and provides a 30% margin of error.  If the stall speed is 50, that would be 65 on final.  I would have to be pretty inattentive to get 15 knots slow on final.  If the winds are gusty, I'll add the gust factor up to a maximum of 15 additional knots.

[Bob - S50]

From a J POH:

(graphic deleted)

No need for the complex math...

Nice.  I see you have the higher weight mod.  Ours is a '78 and that mod is not available.

Like I said, I'm a nerd/geek.  I don't mind the math.  Been doing it all my life.  I even estimate the fuel burn in my head when planning a cross country.  I then use that to ensure the flight log from SkyVector or Lockheed Martin seems reasonable.

Like Dirty Harry, I know my limitations.  I figured out a long time ago that I'm good at understanding something but bad at memorizing.  The less I have to memorize the better.  Understanders like me are good at math and science (except biology) because they are logical and build on previous knowledge.  Memorizers are good at English, geography, and history.  Every item is pretty much unrelated to anything you've already learned.  Even with the chart I would either have to pull it out or memorize too many random numbers, and still do mental math to figure out which weight to use from the chart.  I only have to remember two numbers: 58 and 1.5/100#.  Plus, the Air Force gave me lots of practice.  Can't remember the additive for the T38 but it was something like 155 + 2 KIAS for every 100# over 1000# of fuel(?).  The F106 was 186 plus 2 knots for every 1000# over 6000#.

[steingar]

I like watching the runway more than the ASI, but I sell look over the numbers to make certain it says 70.  I like to float in a boat, not an airplane.  My problem is at this stage of the game I'm coming in low behind the power curve and using the prop to drag it in.  It works and I make smooth short landings, but if I ever loose the engine its gong to hurt.  I'm working on coming in higher with less power, but I am certainly not there yet.

[PMcClure]

I was trained and still look at ASI and the RR on final with a cross check of power and the VSI. Once I chop power and commit to landing, I focus only on the RR. Seems to work well for me. 

[Immelman]

You're doing it right. The whole idea is to be trimmed for and maintain a constant angle of attack on short final. The airspeed indicator is a crude way of indirectly seeing AoA, but as you have determined it depends on weight..

Every large aircraft flying uses a final approach (Vref) speed based on the weight of the airplane at landing.

[ArtVandelay]

Nice.  I see you have the higher weight mod.  Ours is a '78 and that mod is not available.

Like I said, I'm a nerd/geek. 

Actually I don't, I have 78J as well, I have 2 soft copies on my iPad for convenience, a 97 and 81.

[Shadrach]

Here is the formula for calculating stall speed by weight. I listed MGW and 2100lbs for comparison.

2100lbs is light but entirely common weight for me in my M20F when solo.

Vso @MGW 2740lbs = 54 knots = 62mph.

1.2 x 54KIAS=65KTAS = 75mph

1.3 x 54KIAS=70KTAS = 81mph

2100lbs =2100/2740 = 0.766. The Square root of 0.766 = 0.875

0.875 x 54KTAS= 47.25KTAS = 54mph

1.2 x 47.25 KTAS = 56.7KTAS = 65mph

1.3 x 47KTAS = 61.1KTAS = 70mph

If 1.2 x Vso feels uncomfortable, start at 1.3 and try working your way down to 1.2...you'll find what you like. 1.1 is shortfield technique, it's more advanced in terms of proficiency and feels mushy relative to what we're used to.

 

Edited October 21, 2015 by Shadrach

[Hondo]

Stall speed = SQRT(weight/gross weight)SQRT(1/cos(B)), where B is the bank angle.

 

Simple rule of thumb, on final, reduce stall speed by 1 knot per 100 pounds below gross weight.