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).

[/URL][/IMG]

Cheers
Nige

AOA?

Nigel,
What is the reason you are choosing to not use AOA?
Cheers,
Jim

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

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,

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.

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.

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..

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!

esco said:

....your wing throws its lift rearward....

Click to expand...

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

David Paule said:

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

Click to expand...

A simple change of coordinate system.

(for David & Bill)

Gents:

Thank you both for the clarification.

I don't know that AOPA Pilot will take a letter on a topic from the November issue, but I'll pass on your comments to the original author. Another example of the unlooked-for value of VAF!

And now, back to Nigel's original question.

Landing Distance vs Vref

A couple of days ago I put a DGPS in my 8 to record a series of landings varying Vref from approximately 1.5 Vs to 1.1 Vs with the results shown in the graph below. I used runway 24 at California City, L71 which has a 0.9% upslope (which is shown as the black line on the graph). The winds were calm. The aircraft was between 1540 and 1600 lbs. and the cg 80.2". I did all the landings with 40 degrees of flap, and in this configuration my 1g power off stall is 50 KIAS. I flew a back side technique for each approach using pitch attitude for airspeed and power to maintain about a 3.5 degree final approach angle. The aircraft has a constant speed prop and I had it set full fine for each approach. For all landings I used what I would consider 'moderate braking', in the interests of not putting the prop into the runway, flat spotting the tires and general mechanical sympathy. I was after consistent rather than max effort and shortest distance (you could surely do better with some commitment). For the wheel landings I braked with the tail up.

Ldg #1. Vref 76 KIAS. A fast wheel landing with power to idle at ~30' AGL with a long float. Unpredictable touchdown point.
Ldg #2. Vref 74 KIAS. Fast wheel landing with power to idle at ~25'.
Ldg #3. Vref 72 KIAS. Wheel landing with power to idle at ~20'. Acft floats 400' past aim point before touching down.
Ldg #4. Vref 70 KIAS. Tail low wheel landing with 350' float.
Ldg #5. Vref 68 KIAS. Fast 3 point landing, float before touchdown, aircraft skips along runway on touchdown before solid contact.
Ldg #6. Vref 66 KIAS. Over rotate on flare and slight balloon, prior to 3 point landing.
Ldg #7. Vref 64 KIAS. Power to idle just prior to flare, little float. AoA warning prior to touchdown.
Ldg #8. Vref 62 KIAS. Small power changes required for constant approach angle. Slightly less flare effect, much little float. AoA warning during flare.
Ldg #9. Vref 60 KIAS. Frequent power changes required for flight path control. Power to idle as flare begins, no float. Timing and rate of flare noticeably more critical.
Ldg #10. Ref 58 KIAS. AoA warning on final approach. No float even with low and assertive pitch attitude change for flare to 3 point attitude. Little margin for error.
Ldg #11. Ref 56 KIAS. Frequent and occasionally large power changes required to prevent excessive descent rate on final approach. AoA warning on during final approach. Aggressive flare with late power reduction required to arrest rate of descent. Slightly tail low on ground contact. No margin or error or environmental conditions.

The blindingly obvious results of this test?
Lower approach speeds result in shorter landing distances, in this case about 40' per knot of approach speed.
Approach speeds of 70 KIAS or 1,4 Vs result in large float and unpredictable touch down point. The good news is that there is plenty of time to finesse touch down attitude and correct for drift. The bad news is that you burn a lot of runway and the touch down point is not very predictable, which is fine if you have a long runway.
Approach speeds slower than 65 KIAS or 1.3 Vs, where flight path stability is negative, require very close attention to rate of descent and power to prevent excessive rates of descent.
Approach speeds slower than 60 KIAS or 1.2 Vs have very little margin for error and the risk of a heavy landing increase.
Doing a series of landings one after the other with only a small change is a good way to get to know how sensitive the system is.
Based on this I've settled on a Vref of 68 KIAS for wheel landings and 64 KIAS for 3 point landings. Each is a little faster than absolutely necessary but reasonably predictable and with some margin at the expense of stopping distance. Next job is to go and do a few of each to get an accurate measure on the distance required and correlate with AoA.
Cheers
Nige
[/URL][/IMG]

don't forget to adjust those speeds for different weights.

Is it worth it?

I know the theoretical answer is yes, but what's the practical answer? Are we over complicating the situation and solving a problem that doesn't exist?

Is there enough weight change for the standard 2 seat RVs to be worth varying approach speeds with weight?

A light RV-8 is ~1400lbs at landing, and MLW is 1800lbs. That's a difference of 22%. A King Air has different Vref speeds based on weight, but it's landing weights vary 33% or more. Move into a 737 and it will be higher again.

Approaching at an appropriate speed for 1800lbs when the plane weighs 1400lbs will cause floating slightly more, but the landing distance will still be shorter.

I'd be interested in seeing the quantitative and qualitative results of the same test flown at 1800 to 1750lbs

David Z said:

I know the theoretical answer is yes, but what's the practical answer? Are we over complicating the situation and solving a problem that doesn't exist?

Is there enough weight change for the standard 2 seat RVs to be worth varying approach speeds with weight?

A light RV-8 is ~1400lbs at landing, and MLW is 1800lbs. That's a difference of 22%.

Click to expand...

A 22% weight difference is a 10-11% difference in stall speed. That is significant. If you go out and do some detailed testing because you want to nail down the speed at which you get good repeatable landings, then 6-7 kts is going to mean something. If you want the same L/d in the flare each time, so you don't either float or go kerplop, then you have to have a consistent aoa. Aoa changes with the square root of weight.

If you are always landing on a 4000 ft runway and don't mind floating then you may not care. But if you are trying to get into a tight strip with obstacles it might make a big difference. I know of RVs that have been wrecked landing too long and fast. Plus if you always land at the same aoa the aircraft behaviour will be more predictable and your landings will be sweeter and who doesn't love that feeling.

While I agree with you in theory, in reality a 6-7 kt difference in approach speed would be 60 to 67 kts (examples only). To set a flat 65kts Vref would be a good safe compromise in the name of simplicity.

If you are trying to squeeze into a 1200' strip, then you are operating closer to the limit and added complexity is required to fit the unusual circumstance. More testing and practice is probably in order. Also maybe try 1.2 Vso instead of 1.3 Vso as well. Or a good AoA.

In the bush pilot world, pilots often fly by feel. Intentionally fly approaches to short strips so it feels like landing 9 and 10 is described above. It also results in more bent airplanes.

Lots of valuable information here but as Paul said FP vrs CS matters.

I always use 40 flaps, stall is 51 KIAS with about half a load of fuel. I have Catto FP prop.

If i make approach at 75 knots (1.4 Vs) and go into flare with it, the airplane floats forever using lots of runway.

What works best is 65 or 60 on calm day, float is minimal. And of course I do not float along waiting for stalled touch down, it's wheel it on every time.

Nigel,

Thank you very much for sharing this extremely detailed analysis along with graphs.

Your illustrations and explanations really help me better understand the relationship of air speed and rollout performance.

While I like the comfort and control that 1.4 final approach provides, I know in order to become more proficient I need to experiment with slowing it down some so as to achieve a shorter rollout and use up less runway like said in a previous post.

I don't have an AoA and use the ASI. I think an AoA could be very useful if placed appropriately so one could see it while looking outside (glare shield).

It would be really interesting to see a graph like those you've presented that include AoA information.

Thanks again. You do awesome work!

AoA, Stick Position & Pitch Attitude for Charlie

Charlie,
Here are a couple of the landings charted individually to show the AoA, Pitch Attitude and Longitudinal Stick Position.

Not sure is you are familiar with the Dynon AoA system but it uses differential pitot pressure to determine an AoA (1 port faces straight ahead the other faces down at about 45 deg, as the AoA changes the ratio of the two pressures changes). It is not a directly measured angle but a scale that tells you at what percentage of the maximum AoA you are at. So 100% represents stall AoA. There are some limitations to this system. The first being you have to calibrate it, the second being it does not allow for multiple calibration tables to allow for the different stalling AoA with different flap configurations. I calibrated mine with 40 deg flap.

The pitch attitude comes from the ADAHRS. It is generally pretty good but it does suffer from some errors when accelerating or decelerating longitudinally. I see this on takeoff and you can also see it in the very end of the landing roll when braking is heaviest. The aircraft is at about 11 deg nose up in the 3 point attitude, there is no way it goes higher than that on the ground when nearly stopped so the blip up to about 15 deg is an instrument error.

The stick position comes from a string potentiometer that I installed on one of the pitch bell cranks, it is read as a sensor by the engine monitor. 100% is full aft, 0% is full forward.

The data was only sampled at 1 Hz so it is artificially smoothed and some of the higher frequency control inputs are lost but the general direction and rate is still there. You can read the graphs by imagining the aircraft is flying from right to left. The vertical lines represent a distance from where the aircraft came to a full stop and by default also represent a point in time.

[/URL][/IMG]

[/URL][/IMG]

[/URL][/IMG]

[/URL][/IMG]

Cheers
Nige