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  #1  
Old 01-27-2016, 10:41 PM
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nigelspeedy nigelspeedy is offline
 
Join Date: Dec 2008
Location: Tehachapi, CA
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Default Vref as a function of Vs

Howdy All,

So after about 100 hours in my 8 I figure its about time I learnt how to land it. My cunning plan is to do an experiment starting with fast approach speeds and gradually reduce them until I find out what is safe, predictable and has some margin for gusts/turbulence/poor handling.

So I realize that your particular aircrafts approach speed will change with weight, altitude, the temperature and a host of instrument and position errors, so to keep it simple for my simple farm boy brain what is the the '"last look" number you see on short final as a function of your stall speed in that configuration? So do you like your airspeed to be 1.1 or 1.4 or 1.x? times the stall speed?

To get things going I did a flight path stability experiment. My 1g power off stall speed with 40 deg flap is about 50 KIAS so 55 is 1.1 Vs, 60 is 1.2 Vs etc. Keeping power constant I varied airspeed and noted the rate of descent and plotted that as well as the resulting flight path angle (gamma).

[IMG][/IMG]

Cheers
Nige
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  #2  
Old 01-27-2016, 11:40 PM
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jdearborn jdearborn is offline
 
Join Date: Nov 2011
Location: Kennesaw GA
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Default AOA?

Nigel,
What is the reason you are choosing to not use AOA?
Cheers,
Jim
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  #3  
Old 01-28-2016, 02:12 PM
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nigelspeedy nigelspeedy is offline
 
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Location: Tehachapi, CA
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Default Speed vs AoA

My Dynon Skyview has AoA but on a 7" display its not that attention getting. I really like the audio though. Down the road I may see what the AoA indication is at a good speed and then start using the AoA more, or perhaps do the landing experiment and use the AoA increments.
Cheers
Nige
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  #4  
Old 01-28-2016, 02:32 PM
mdevans9 mdevans9 is offline
 
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Location: Hockessin,Delaware
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Default 1.4vso

Is what I use, and the number that I believe Vans recommends. The drag rise and resultant sink rate at lower speeds can be high. Increasing the "traditional" approach speed of 1.3vso to 1.4vso makes things more comfortable. Conversely, adding speed above 1.4vso can add to your workload, as the airplane wants to continue flying.

So, I use 70kias until starting to round out. If my last peek shows 65kias, I don't worry about it - slower, I would. In all cases, carrying a very slight amount of power into the flare seems to help me. But, using 70kias is just fine for power off approaches, but be prepared as the descent rate is pretty high. So, any slower at the bottom and you will give up some margin for error.

Just my experience. ymmv.

Cheers,
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  #5  
Old 01-29-2016, 01:16 AM
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RV10inOz RV10inOz is offline
 
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Default

70 knots in an 8 seems high to me, I use that +/- a couple depending on weight in the -10.

When in any of the 6/7/8 models I find 65 is more than enough and a calibrated AOA is part of your scan/audio.

Vs x 1.3 is used widely for good reason. And it works.

YMMV.
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  #6  
Old 01-29-2016, 10:39 AM
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Ironflight Ironflight is offline
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Default

Neatwork Nigel! I forget if you have a fixed pitch of constant speed, but that really makes a huge difference in the way theydecelerate in the flair, and many of us that have been flying the -8 with C/S for a long time add just a touch of power in the flair to prevent the bottom from dropping out since it slows down so quick at idle.

It's been a long time since I looked at the ASI on final (becasue I fly AoA), but I usually trim about 75, and th elast time I look at airspeed is to make sure I am in the white arc for flap deployment.
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  #7  
Old 01-29-2016, 11:29 AM
pvalovich pvalovich is offline
 
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Default Landing an 8A with CS Prop

I hold 70 kts in the pattern - 75 if gusty or a lot of cross wind. Try for power off approaches but usually end up using a bit of power turning final, especially if there is any nose up input coming out of the turn. Always aim for touchdown just past the threshold, training for when I really have to put it in short. One final airspeed check crossing the threshold. If 70 kts or higher, power to idle and a careful nose up flare referencing the far end of the runway. If less than 70 kts, I pay a lot of attention to power management to get back in the box and accept a longer touchdown point..
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  #8  
Old 01-29-2016, 02:41 PM
esco esco is offline
 
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Location: SoCal
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Default Full Flaps, final: 1.3 Vso

Nige:

To your question, my "last look" speed on short final, w/ full flaps, is 1.3 Vso.

Initially, this was due to the law of primacy: it was what I trained at and on in Cessna products.

Later, it was because the RV-9A didn't float, or drop, excessively at that speed.

More recently, I came across the theory, and it makes sense to my pea brain:

"...Runway length, however, is not the reason for crossing the runway threshold at or below 1.3 VS. Higher ?over the threshold? approach speeds increase the probability that you?ll float, balloon, porpoise, or drift while attempting to land. Crossing the threshold at or below 1.3 VS means that the typical general aviation airplane experiences increasing drag, not decreasing drag, during the roundout and landing flare...

Let?s examine why this is, using the flaps-up approach model for simplicity. With flaps up, the airplane?s best L/D speed (best glide speed) is found at the bottom of its total drag curve. As you?ll recall from ground school, the total drag curve is the combination of a decreasing parasite drag curve (as airspeed decreases) and an increasing induced drag curve (as airspeed continues to decrease). In a generic Cessna 172, the best glide speed is approximately 66 knots calibrated airspeed (KCAS). The flaps-up stall speed at maximum weight is 49 KCAS. Approaching at 1.3 VS with flaps up results in an airspeed of 64 KCAS. This puts you near the bottom of the total drag curve. Big surprise? Not really.

At 64 KCAS, as you increase the angle of attack during the roundout for landing, your wing throws its lift rearward, and induced drag increases (yes, ground effect reduces induced drag a bit, but induced drag still increases overall). You decelerate as a result. Your chance of floating during the roundout diminishes because of the relatively quick decrease in your airspeed.

Imagine what happens if you cross the threshold at 80 KCAS (16 knots faster than best glide speed). During the roundout, the parasite drag decreases faster than induced drag increases as you decelerate from 80 to 66 KCAS. This means that you initially don?t slow down as quickly as you?d like, which ultimately increases your chance of floating during landing, and running off the end of the runway. This is one instance where having insufficient salt actually increases your blood pressure. While this example reflects a flaps-up condition, the same principle (albeit with different airspeeds) also applies to landing with full flaps..."

See the rest of Rod Machado's column in the November issue of AOPA Pilot:

YMMV!
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  #9  
Old 01-29-2016, 03:12 PM
David Paule David Paule is offline
 
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Quote:
Originally Posted by esco View Post
....your wing throws its lift rearward....
Lift is a force perpendicular to the relative wind - that's its definition. As your angle of attack increases and you are still moving parallel to the runway, the lift is still vertical relative to your flight path or the ground. Since the airplane is pointing up somewhat, that means that the lift vector is pointing forward a bit, with respect to the wing chord.

Drag is the force parallel to the relative wind. So at a higher angle of attack, it has a force vector that's perpendicular to the wing, as well as its force component that's aft.

We often think of lift being perpendicular to the wing and drag being parallel to the wing, but that's only an idealized notion for low angles of attack.

Dave
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  #10  
Old 01-29-2016, 07:31 PM
BillL BillL is offline
 
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Default

Quote:
Originally Posted by David Paule View Post
Lift is a force perpendicular to the relative wind - that's its definition. As your angle of attack increases and you are still moving parallel to the runway, the lift is still vertical relative to your flight path or the ground. Since the airplane is pointing up somewhat, that means that the lift vector is pointing forward a bit, with respect to the wing chord.

Drag is the force parallel to the relative wind. So at a higher angle of attack, it has a force vector that's perpendicular to the wing, as well as its force component that's aft.

We often think of lift being perpendicular to the wing and drag being parallel to the wing, but that's only an idealized notion for low angles of attack.

Dave
A simple change of coordinate system.
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