Ball park over the fence speed calcs

[piperboy84]

What mental math calcs do you more experienced guys use for calculating short final approach speeds based on weight under gross when the strips a bit tight ?

[Above The Clouds]

1.3 x the stall speed for the configuration you are in.

[Genghis the Engineer]

The speed in the POH multiplied by the square root of (weight / MTOW).

G

[Pace]

Piperboy

1.3 x stall is just a number for a typical aircraft more draggy aircraft may require more more slippy aircraft less.
Its a figure to allow you enough energy to transit from a descent profile to a flare and landing and is the minimum figure quoted by the regulators

To be really accurate try flying with the ASI covered up and using an AOA gauge if any light aircraft have them :ok:
G mentioned one aircraft which had a lot of drag where the POH mentioned 1.6 plus

Pace

[piperboy84]

Pace, I actually looked at putting an aoa on the plane, there is a big push by the FAA for GA aircraft to have them fitted. I was going to wait and see if Garmin integrate there new AOA unit to show on the Garmin pilot app, like they are doing with the flightstream 210 GNS 430 setup. The current certified units seem like a helluva lot of money.

Back on subject:

So if you guys had a aircraft in this case a Maule MX7 - 180 a with a gross weight of 2400lbs and as flown on the day a weight of 1800lbs and the published VSo is 47 MPH at gross, what ball park over the fence (short final) speed would you use for a short field landing. I understand the ASI is inaccurate as all hell at thos speeds but I just was interested to see how my calcs stack up compared to what other posters would fly it at

[Mach Jump]

If you reduce the weight, you reduce the lift required to support it.

As the lift increases as the square of the speed, it follows that the speed required to support a reduced weight falls with the square root of the weight. eg.

With a max Ldg weight of 2000 lbs, and a POH threshold speed of 100kt.

Reduction in weight of 10% ie. 1800 lbs would allow a reduction in speed to the square root of 0.9, ie. 0.95 of 100kt =95kt.

Reduction in weight of 20% ie. 1600 lbs would allow a reduction in speed to the square root of 0.8, ie. 0.89 of 100kt =89kt.

In this particular case, and Genghis will probably beat me up for this, one might observe that, as a rule of thumb, each 10% (200 lbs) reduction in weight will allow a 5kt reduction in threshold speed.


MJ:ok:

[piperboy84]

My ball park based on

2400 gw
1800 actual
47 mph published VS0 at gross (as per POH)

My Calcs:

600lbs under gross , knock of 1mph for every 80lbs under gross (recommended by my old instructor DHH up here in Scotland.

600/80 = 7.5 rounded down to 7 mph
VSO of 47mph minus the 7mph for weight adjustment = new VSo of 40mph
40 Mph x 1.25 ( short field approach) = 50mph short final speed.

[Mach Jump]

I would have made it 52, ie. 1mph less threshold speed for every 75 lbs below 2400, as a rule of thumb. So not all that different.


MJ:ok:

[Chuck Ellsworth]

How do you people hold an indicated airspeed to an accuracy of one or two MPH?

The airspeed needle is wider than that.

[Mach Jump]

How do you people hold an indicated airspeed to an accuracy of one or two MPH?

We didn't say we could fly it, Chuck. ;) Only that we had some way of working out what it's supposed to be.


MJ:ok:

[Pilot DAR]

I regularly fly into tight places, both runway, and water. If I really have to get in tight, I won't be looking at the ASI much, if at all. Similarly, for those aircraft I fly equipped with AoA, I never look at it, I'm too busy paying attention to where I'm going out the windshield.

The sight picture, relative motion of the aircraft in the vertical plane, and the feel of the plane tell me what I need to know to fly my best short approach.

If I were to refer to an ASI for a short approach, the relationship of IAS to CAS would begin to be a factor, at speeds that slow, and with the precise reference to speed being suggested. You'd best have the instrument calibration record too, as ASI's can regularly be out a few MPH/knots either way.

Having a speed worked out with the formula, based upon a CAS stall speed, and then referring to it in IAS, without consideration of instrument error could easily have you 5 MPH/knots too fast or too slow. For GA aircraft, best to just focus on flying the plane with good ground reference, and feel.

This is one of my tight landings, and I did not go eyes in at all, preferring to focus on the sight picture. This aircraft is equipped with an Alpha Systems AoA, which works really well, but aside from setting it up, I have never bothered to use it.

lgn3N12rTTk

[DeltaV]

If I really have to get in tight, I won't be looking at the ASI much, if at all.
Amen.

Lovely strip.
I've not seen image stabilisation like that before. It's just a pity it didn't capture the actual touchdown and the Beta range turnaround was interesting. Good video, though.

[Above The Clouds]

I thought the question by the OP was a "Ball Park Figure" not a mental gymnastics test on finals.
As said by Pilot DAR, you shouldn't be looking in at the ASI or AoA by the time you are over the hedge, by then its sight picture, relative motion of the aircraft in the vertical plane looking out of the windshield, and the feel of the aircraft, this applies to all types of aircraft.

[Shaggy Sheep Driver]

Good case for an AoA indicator. AoA is what the wing knows about, not speed, and the wing is what we fly. That we don't have a direct AoA readout is a major flaw in the design of most aircraft, especially aerobatic ones.

[Pace]

I use the AOA for high level climb in the Citation and a multitude of other things. It can tell you so much it is an amazing piece of Kit and does mean you are flying the wing and not numbers.

Light aircraft never seem to have one maybe on cost? Are there low cost units for Light GA? Pilot Dar! great video

Pace

[Above The Clouds]

Good case for an AoA indicator. AoA is what the wing knows about, not speed, and the wing is what we fly. That we don't have a direct AoA readout is a major flaw in the design of most aircraft



I use the AOA for high level climb in the Citation and a multitude of other things. It can tell you so much it is an amazing piece of Kit and does mean you are flying the wing and not numbers.


Does buffet during aeros or high energy manoeuvring not give the best indication of AoA then you don't have to be looking in the cockpit ?

Using FLC, CLB Mode, VPATH etc these modes are using inputs from AoA, equally climbing using CAS or Mach published speeds will give you the best AoA to produce the best coefficient of lift, your airspeed indicator is measuring calibrated airspeed (CAS), which is proportional to the square root of ½ρV2 therefore indirectly is a measurement of the AoA, in other words you use them all together to get the best results.

Back on topic.

As Pilot DAR stated earlier and I fully agree by the time you are coming over the fence during the final stages of a landing the last place you should be looking is in the cockpit at the ASI or AoA, in fact when I land my light aircraft or the 100,000Lb bizjet I couldn't tell you what the speed is during the final stages of the landing because I am looking out of the window.

Going back to the OP question, "Ball park over the fence speeds" then 1.3 x CAS stall speed ( for most light aircraft, OP aircraft is a Maule ) in whatever landing configuration you have chosen will give you the safest approach speed, once approaching over the fence you should be looking for the correct sight picture and relative motion of the aircraft in the vertical plane as the ASI and AoA are really irrelevant at this stage during the landing regardless of wether its a 10,000ft runway or 250 meter grass strip.

[Pace]

ATC

Even using the AOA you will not be flying it to the ground :E The question is is 1.3 an accurate margin for all aircraft?

Ghengis mentioned one aircraft where the POH recommended 1.64 as it was a very draggy aircraft where 1.3 was not sufficient.

On the flip side a very slippery aircraft and 1.3 maybe too high a figure and maybe 1.2 would suffice

The AOA is a much more accurate indicator than the Airspeed indicator no one is saying at some point you do not transition from the the gauges to feel and outside references only that at what speed your VREF do you make that transition

Pace

[Above The Clouds]

The question is is 1.3 an accurate margin for all aircraft?

Ghengis mentioned one aircraft where the POH recommended 1.64 as it was a very draggy aircraft where 1.3 was not sufficient.

On the flip side a very slippery aircraft and 1.3 maybe too high a figure and maybe 1.2 would suffice


I would say you answered your own question, use the AFM.

If no information is given then 1.3 is a good starting point.


The AOA is a much more accurate indicator than the Airspeed indicator


That would depend on the type of system fitted.

[Pace]

ATC

1.3 is a man made figure and I think this is what the OP is getting at. The .3 being a margin in IAS above the stall in a given configuration to allow enough energy to go from a descent profile to a reduced descent profile and a flare with the throttle closed.

This margin should give enough so that you land at or near the stall.
Gs example of 1.64 means that that particular aircraft is so draggy that it requires a bigger margin.

A very slippery aircraft may mean that at 1.3 there is too great a margin.
The only way to better those fixed figures is to do a flight test on a particular aircraft, work out the correct stall figures in different configs for that particular aircraft and using 1.3 as a baseline experiment above and below that figure

A good AOA gauge is far more accurate than the ASI.

Pace

[Above The Clouds]

Pace
A very slippery aircraft may mean that at 1.3 there is too great a margin.
The only way to better those fixed figures is to do a flight test on a particular aircraft, work out the correct stall figures in different configs for that particular aircraft and using 1.3 as a baseline experiment above and below that figure

Sorry am I missing something I thought that is what I said.


If no information is given then 1.3 is a good starting point.

[Flyingmac]

I know when I'm flying too slowly on short final. Nowhere near the stall, but the sink rate gets silly. Can't see how an AoA indicator will better my Seat of Pants approach.

[Shaggy Sheep Driver]

If no information is given then 1.3 is a good starting point.

Indeed, but given the absence of an AoA indicator, how on earth do you know what IAS 1.3 VS is?

And no, seat of the pants won't tell you.

[Above The Clouds]

SSD
Indeed, but given the absence of an AoA indicator, how on earth do you know what IAS 1.3 VS is?


So you are saying that you do not know your aircraft stalling speed to be able to times it by 1.3 therefore giving you a minimum approach speed.:eek:

[Pull what]

I know when I'm flying too slowly on short final. Nowhere near the stall, but the sink rate gets silly. Can't see how an AoA indicator will better my Seat of Pants approach.

That has to be one of the most silly statements Ive heard since an instructor I was checking out told me that you should always aim for the first (nearest) displaced threshold arrow.

This margin should give enough so that you land at or near the stall.

Where on earth do you get that from, you dont plan to land an aircraft at the stall, you plan to cross the threshold at a calculated margin above the stall which after after normal maneuvering and attitude changes allows you to touch down still above the stall speed but less than the threshold speed


Gs example of 1.64 means that that particular aircraft is so draggy that it requires a bigger margin.

A very slippery aircraft may mean that at 1.3 there is too great a margin.

Slippery? I only know of one aircraft that crossed the threshold at 1.2 VS and that was the Bristol Freighter and one that had a published speed greater than 1.3 at the threshold and that was the Aztec

[Shaggy Sheep Driver]

So you are saying that you do not know your aircraft stalling speed to be able to times it by 1.3 therefore giving you a minimum approach speed.

Neither do you. Because there is no such thing as 'stalling speed', only 'stalling AoA'. If you are a qualified pilot, you really should know that.

[Monocock]

If you are flying that closely by numbers, you ought not to be going into really tight strips.

Your arse should tell you how things are going once you are over the hedge, not your ASI.

[Pull what]

Your arse should tell you how things are going once you are over the hedge, not your ASI.

At that stage I always take over from 'The Arse' and fly it myself or call, "go around".

[Genghis the Engineer]

There certainly is such a thing as stalling speed SSD, it's just that unlike AoA, it's dependent upon weight, configuration, deceleration rate and g. Stalling AoA is only dependent upon configuration and deceleration rate.

Draggy / slippery is really not the issue here. The aircraft I know with Vref>>1.3Vs tend to have high induced drag, low inertia. In other words, as you roundout and flare, there's a lot of drag acting against not very much momentum. The result is rapid deceleration.

A motorglider is slippery - yes. But also they're heavier than most - probably all - microlights, and with that long slender wing, have very low induced drag. So, the speed bleeds off very gradually, and thus a relatively low approach speed can be appropriate. By comparison, most microlights have low mass, already low approach speeds (remembering that kinetic energy goes with the square of speed, so reducing approach speed to 80% reduces kinetic energy to 64%), and a short fat wing which tends to generate a lot of induced drag at high AoA.

I would guess that the Bristol Freighter was similar to a motorglider - ***** heavy, with a fairly long and efficiently lifting wing. Not slippery however, definitely not slippery !

G

[Shaggy Sheep Driver]

Genghis - you eloquently point out that there is no such thing as stalling speed for all practical purposes. Sure, a speed at which an aeroplane at a given loading (G and weight) will stall can be calculated if you have all the data, which you don't.

An AoA indicator gives an unequivocal direct simple read-out of how close to the stalling angle you are.

Agree with Mono that the next best thing to an AoA indicator is 'your arse', or more correctly the 'feel' of the aeroplane. If you are familiar with the type, and esepcially if you aerobat it, you will know how the controls feel just before it departs (this assumes you are creeping up to the stalling angle gradually, as one would be on approach; a sudden hard pitch up might put you beyond the stalling angle with no warning signs).

[Above The Clouds]

SSD
Sure, a speed at which an aeroplane at a given loading (G and weight) will stall can be calculated if you have all the data, which you don't.


What other data do you want for a light aircraft, the calculated stall speed is given for most light aircraft at max weight, in unaccelerated flight 1g, in a given configuration, clean and with flaps extended, those are the bench marks.
Below max weight then you will have to go out and experiment to find out what the stall speed is at lower weights but to be honest why bother on a small aircraft when there is probably only a few knots difference, thereafter you can times it by 1.3 for an approx. approach speed.


An AoA indicator gives an unequivocal direct simple read-out of how close to the stalling angle you are.


When you have an AoA fitted to your chippy and you are side slipping in to that grass strip tell me if your above statement is true.

The majority of light aircraft stall spin accidents in the circuit happen around the turn on to base leg and final approach or short finals, generally when the workload is high for the low time or out of currency pilot. The reason, because they were not monitoring the airspeed, i.e not looking at the ASI or due to workload not registering what it was telling them.

So lets add another gauge AoA for the same pilot, do you really think this is now going to get anymore attention in the same situation ? an AoA is nothing new they have been around for years since the Navy developed them for carrier landings but unless the pilot has the ability to actually fly and monitor the situation developing around him/her then no amount of instruments are going to help.

[Cows getting bigger]

I can't remember the last time I confirmed I was at Vref when "over the fence" on a SEP performance landing.

Perhaps I'm just old fashioned? :bored:

[Above The Clouds]

Cows getting bigger
I can't remember the last time I confirmed I was at Vref when "over the fence" on a SEP performance landing.


:D:D:D:D couldn't agree more, but the children of the magenta line have found a new toy, an AoA gauge.

[Shaggy Sheep Driver]

What other data do you want for a light aircraft, the calculated stall speed is given for most light aircraft at max weight, in unaccelerated flight 1g, in a given configuration, clean and with flaps extended, those are the bench marks.

....And then the driver, flying it solo with not much fuel in the tanks, adds 15% for the wife and kids. Could explain all those busted nose wheels!

[Genghis the Engineer]

I can't remember the last time I confirmed I was at Vref when "over the fence" on a SEP performance landing.

Perhaps I'm just old fashioned? :bored:


I tend to monitor airspeed, because in a typical year I maybe fly a dozen types, and don't trust myself to get the pitch attitude right when I'm seldom all that familiar with the model. Airspeed, with a little bit of maths, IS a useful guidance method. Most of the time we're also in relatively unaccelerated flight, so wings level stall speed is a simple function of weight.

Also whilst 95%+ of aeroplanes don't have IAOA available, 99%+ of aeroplanes do have IAS. We use what's there. Oh yes, and I'd guess that less than 50% of IAOA systems have been subjected to rigorous flight testing leading to proper guidance and training material on their use.

G

[Above The Clouds]

Genghis the Engineer
Oh yes, and I'd guess that less than 50% of IAOA systems have been subjected to rigorous flight testing leading to proper guidance and training material on their use.

And of course lets wait and see how many incidents there are going to be when Garmin bring out another computer generated magenta line in the form of AoA and the devoted start following it religiously :D

[9 lives]

I've flown AoA indicators in several types, from C-150 to Piper Cheyenne. It was great in the Cheyenne, as that type is less forgiving of seat of the pants flying - just fly numbers. But lighter aircraft may reward the sensitive butt with good feedback. In any case, letting your eyes go back in to the panel at the critical moments of the final approach probably creates a much greater risk than flying the approach a bit off speed.

I know when I'm flying too slowly on short final. Nowhere near the stall, but the sink rate gets silly. Can't see how an AoA indicator will better my Seat of Pants approach.

This can certainly be a serious situation on some types, which are heavy and draggy, with lots of lift. Cessna floatplane amphibians being an example. You can fly a final approach at about the 1.3Vs, and with the AoA indicating okay, but in a condition that only a lot of power at the bottom is going to fix. The errant pilot has let the plane slow too much to enable a flare from a steep approach on the combination of speed and altitude alone.

This can happen with a poorly set up approach to a short landing area over an obstruction. An unaware pilot will cross the obstruction going down, at a suitable airspeed, but with a rate of descent which is much too great to arrest at the bottom with the aircraft's reserve of energy alone. For these approaches, you either retain the required excess energy as speed all the way down final, or carry lots of power down final (which has the dangers associated with an engine failure - assured crash). Several amphibians I fly have a power off stall speed about 55 MPH. For a power off landing, I'll train a glide speed of 80. You can glide at 65, and the aircraft is quite happy, but when you pull to flare, you'll get an instant stall horn, and no arrest of descent rate - crunch! One of these planes has the newest AoA, which I set up as new. In 80 hours of flying it, I have never referred to it, I'm too busy watching where I'm going out the windshield.

For the GA type aircraft, you have to know the plane, hone your seat of the pants and hand&feet skills and fly them that way first. Magenta lines and instrument readings only when the aircraft is already being well flown!

[Above The Clouds]

Quote:
What other data do you want for a light aircraft, the calculated stall speed is given for most light aircraft at max weight, in unaccelerated flight 1g, in a given configuration, clean and with flaps extended, those are the bench marks.


SSD
And then the driver, flying it solo with not much fuel in the tanks, adds 15% for the wife and kids. Could explain all those busted nose wheels!


That does not answer my question, what other data is it you require. As I stated earlier figures in light aircraft AFM's are based on max weight, if you need to find out the stalling speed at lower weights then you have to go and fly the aircraft and stall it.

[Shaggy Sheep Driver]

I'd want an AoA indicator principally for aerobatics, where one can stall at any IAS (or not). But it would be useful to glance at on approach as well.

Someone said of IAS "we use what we have". Crikey, talk about begging the question! What we 'have' can be changed, you know!

[Pull what]

if you need to find out the stalling speed at lower weights then you have to go and fly the aircraft and stall it.

No you dont, it can be mathematically worked out in the same way as you can work out the stall speed in a level turn at various angle of bank due to the wing loading increase

[Above The Clouds]

Pull What, yes of course you can, I was using the KISS principle for some.

[Pull what]

KISS! I like to keep it simple for the lawyers too so will stick to the threshold being a last check of airspeed for landing.

I like Bob Hoobers phillosphy of," you can do anything in an aircraft providing you have the airspeed". Should he ever change that to attitude and the feel of your arse, I may consider changing to the knucklehead way of doing it.

[Shaggy Sheep Driver]

I like Bob Hoobers phillosphy of," you can do anything in an aircraft providing you have the airspeed".

Bob Hoover is a great pilot and I am in full agreement with his statement. The missing word is, perhaps "...the appropriate airspeed".

Not too much.

Not too little.

And on approach, or when aerobatting, the appropriate airspeed is often best achieved by flying the correct AoA.

Reminds me of the Trident captain (a Yorkshireman) doing a 'Westcott Snatch' (direct off Westcott at maybe FL100 to final for the 09s) who, when asked by the controller if he was "all right for speed and height" (meaning 'aren't you a bit high and fast for this?') received the reply "Aye. We have an abundance of both!".

[9 lives]

I really appreciate Bob Hoover's immense contribution to our understanding flying, and am hardly qualified to second guess him, but perhaps what he said can be expressed:

"you can do anything in an aircraft providing you have the airspeed".

Too much speed can require management, but of course we know that.

The reliance on AoA to assure that the "speed" will be right on approach is okay for most landings, as long as the pilot realizes that certain approaches could require more margin than that provided by adherence to an AoA. For my experience in single engined GA aircraft, if a pilot needs an AoA to safely fly a good approach, that pilot needs more mentored experience on type.

The aftermarket AoA systems are very good, but may not subject to the certification standards for function and reliability. A bumped probe will not attract attention during a walk around, but will surely result in errant information - could you tell you were getting wrong AoA information on final? And when would you most likely be really trying to tuck the plane into a tight landing area? During a forced approach - at which point you may have turned off the master in accordance with established procedure, and you'll no longer have the AoA indication. Does anyone put a cover over the AoA probe, as they do a pitot tube cover? Could you misalign the probe putting the cover on?

Future designs of aircraft may include AoA as a part of the type design, and I hope, indication which is very "eyes out" for ease of use. Indicators as far out the nose as they can be placed, and somehow operational during a forced approach with systems secured. Still, pilot will have to be aware of factors associated with very steep approaches, where the AoA could "trick" the pilot into thinking the approach was okay, and yet leave inadequate reserve of airplane energy to safely execute a larger than normal flare.

There is no substitute for just getting a good feel for the plane in different approach configurations, and possibly referring to the ASI occasionally.

[abgd]

As a thought, you could set up a binary AoA indicator (i.e. a stall-warner) to sound at the ideal approach AoA.