Any of those sounding familiar?<\/p>\n
![\"Tower](\"http:\/\/www.fortyfivehours.co.uk\/wp-content\/uploads\/2016\/05\/Tower_Farm_Final.jpg\")
<\/a>Tower Farm: Final Approach<\/p><\/div>\n
When I started farm strip flying I did a lot of training and practice on getting my final approach airspeeds down from the Cessna 172S typical approach speed of 65 knots, down to 50 knots.<\/p>\n
Kinetic energy increases as the square of velocity<\/span> (ground speed).\u00a0 So if you arrive at the landing point at 55 knots (just 10% higher), you will increase your landing distance by 21%<\/strong> or put simply:<\/p>\n Less ground speed equals less energy to convert on landing, thus a shorter stopping distance.<\/p><\/blockquote>\n The above gives you the rationale for why I aim for a final approach of 50-55 knots depending on the wind (higher if the wind requires it until potentially the strip becomes un-landable and we should go somewhere else today – but you did your homework about the likely max surface winds before we took off right?).<\/em><\/p>\n Once stable and down to approx. 50ft of height remaining, I’m looking to bring that speed down to ~48 knots (i.e. sub-50 but +40 with margin [POH Vs0 to Vs1 speed]).\u00a0 This is still ~14% slower then you might expect for a threshold speed.<\/p>\n A short diversion into Stalling<\/strong><\/p>\n This article isn’t meant to be a detailed look at stalling, but it would be incomplete to talk about slower approach speeds without briefly touching on stalling.<\/p>\n Your first reaction to the above speeds will either be “wow that’s slow<\/em>“, or possibly “but the POH says…<\/em>”\u00a0\u00a0 The POH on a Cessna 172S says it’ll fly (i.e. won’t stall):<\/p>\n So the plane is not going to stall (according to the book of words).<\/p>\n It’s worth pausing again here, I’m talking about airspeeds and stalling.\u00a0 You might be screaming “critical angle of attack<\/strong>” at the computer screen \ud83d\ude41\u00a0\u00a0 True, a wing stalls when it exceeds critical angle of attack, it can technically be made to stall at any airspeed.\u00a0\u00a0 You won’t find that angle published anywhere though, the POH will only talk about indicated airspeed and for a controlled and gentle approach, airspeed can become a very close approximation.<\/p>\n Finally on stalling:<\/p>\n So no prizes for guessing what I aim to do when farm strip flying:\u00a0\u00a0 I take at most 1 passenger, I want enough fuel to have safe margin, but if half tanks gives me 1hr of contingency then that’s plenty.\u00a0\u00a0 The Vs speeds are for Max Weight, Full Forward CoG so if we reduce the weight and shift the CoG backwards then we can expect to get better performance (i.e. lower stall speed) then is quoted in the book of words and that gives us better safety margin.<\/p>\n Landing, Lift Force and Dynamic Pressure<\/strong><\/p>\n Landing happens because the plane isn’t producing enough lift to hold it in the air or balance the weight vector if you prefer – remember your force couples.<\/p>\n Airplane Forces<\/p><\/div>\n So when the plane stops generating enough lift, eventually you hit the ground, hopefully at a gentle rate of descent:<\/p>\n Lift = Coefficient of Lift * Dynamic Pressure * Size of Wing<\/p>\n<\/blockquote>\n So increase the Coefficient of Lift, and all other things being the same you increase the lift.<\/p>\n Dynamic Pressure changes as the square of Airspeed<\/p>\n<\/blockquote>\n Assuming your air density is the same, at 50 knots if the dynamic pressure is 8 pounds per square foot, then at 100 knots the dynamic pressure will be 34 pounds per square foot. \u00a0 Now go back to the Lift Equation above, even without changing the Coefficient of Lift, if you have twice the airspeed you have A LOT more Lift<\/span>.<\/span><\/p>\n Real world examples are better to get your head round then pure equations, so let’s take a slow approach of 50 knots<\/strong> indicated airspeed and a fast approach of 75 knots <\/strong>(Let’s assume the plane has a 174 sq foot of wing [Cessna 172S] and we have ISA sea level standard air density\u00a01.225 kg\/m3<\/sup>, it doesn’t matter what we pick here really because all we’re saying is “it’s the same plane in the same conditions except for speed”<\/em>).<\/p>\n Dynamic Pressure<\/a> = 0.5 * Air Density [kg\/m3<\/sup>] * Airspeed ^ 2 [m\/s]<\/p>\n Lift<\/a> = Coefficient of Lift * Dynamic Pressure * Size of Wing<\/p>\n Because all we care about for now is the effect of being fast, lets consider an ~5 degree angle of attack, which on a typical light aircraft will give us a Coefficient of Lift of about 1<\/span> which is a nice round number, you can find this in most text books on lift, but as I said, it doesn’t really matter because both examples use it as a constant.<\/p>\n The basic empty weight of a Cessna 172SP is around\u00a0 1700 pounds, if you’re landing with 20 US Gallons of fuel left (which is 2 hrs worth, so we’re not light here) and the pilot weight is about 140 lbs (10 stone).\u00a0 Then this plane isn’t going to land, it’s going to Climb away!!\u00a0\u00a0 The only way to get the plane on the ground would be to reduce speed or lower the nose to reduce the Coefficient of Lift.\u00a0\u00a0 By comparison you can see that at 50 knots the Cessna is going to be descending.<\/p>\n What’s interesting about the math above, is that at 75 knots for a low weight Cessna 172, you’d need to cut the coefficient of lift by about half before it’d descend<\/em>.<\/p>\n That’s very relevant to the next part.<\/span><\/p>\n A final note about coefficients of lift, I’m assuming above that both planes are in the same flap configuration for a final approach.\u00a0 Flaps change the Coefficient of Lift so a plane flying with its flaps fully retracted will have a lower coefficient of lift, for the same angle of attack, then a plane with flaps fully extended thus the amount of lift generated when flaps are fully retracted will be lower.\u00a0\u00a0 If you’re learning to fly, perhaps this helps you understand why ‘flapless’ landings are done at a higher airspeed.<\/em><\/p>\n Ballooning Bringing us back to the root focus of this article:\u00a0\u00a0 Ballooning and Ground Effect<\/span><\/p>\n Ballooning typically occurs because you were descending, then you pull back on the controls in the flare:\u00a0 Increasing the angle of attack of the wing thus increasing the Coefficient of Lift on the wing (you can see from the above math you can also balloon by adding airspeed [gust of wind \/ adding excess power]).<\/em><\/p>\n If you’re airspeed is high, then as demonstrated above, this increase in Coefficient of Lift is going to stop the plane descending and quickly start making it climb.<\/p>\n The danger now is that airspeed will drop off as you climb and that will make the amount of lift drop off like a brick (note the amount of lift between 50kts and 75kts above AND that climb performance is a function of excess power)<\/em>.\u00a0\u00a0 As you lose lift quickly you’ll start to descend rapidly, the angle of attack is now high because your relative flight path is towards the ground but the wings are pointing up (giving a high angle of attack).\u00a0\u00a0 You now risk entering a Quasi-Stall condition or worse a full stall.\u00a0 Pointing the nose at the runway (“pushing the plane on to the runway”) isn’t a brilliant idea as you’re maybe only 70ft above the runway.\u00a0 You’re burning up landing distance while this is all going on so even if you do get it sorted out, you might have used 150-200m of runway!\u00a0 Can you still land in the runway left?\u00a0 Hitting the hedge at end of the runway is still a crash….\u00a0 Thus why in training it will be drilled into you that in the event of a balloon, apply full power and go-around immediately.<\/p>\n I want to emphasise here that at a high airspeed, it doesn’t take much in flare error (amount of<\/em> change in elevator control<\/em>) to make it all go wrong because you have so much lift to begin with.\u00a0 So you might get away with it on a couple of circuits, but the next time you try, just a degree or two more angle of attack from the wings will make a massive difference to the lift force.\u00a0 Thus approaching at the correct airspeed gives you some safety margin on your elevator control when beginning to flare and stop the plane jumping into an attempt to climb when it doesn’t have the power to do so.<\/p>\n Remember also that control forces are less responsive at slow airspeeds so it’s inherently harder to make the plane suddenly change its angle of attack to an extent that will cause a balloon if the final approach airspeed is right.<\/p>\n Ground Effect<\/strong><\/p>\n So perhaps I’ve convinced you of two things:<\/p>\n Quite simply if we want to stop in the shortest distance possible, we want to do everything possible to get the airspeed down without stalling the wings – stalling is bad!<\/p>\n Easton Maudit Final 50ft<\/p><\/div>\n You can get 48 knots in a Cessna 172S fairly easy on approach, as your airspeed drops to 40 you’re going to very likely hear the Stall Warner begin to screech intermittently.\u00a0\u00a0 Doing this in gusting winds is a really bad idea because while power + attitude = performance, the performance isn’t instantaneous so gusting could stall the wing.<\/p>\n We therefore want to ‘transition’ the airspeed from 48 knots to 40 knots and have some margin.\u00a0\u00a0 With practice and a precise landing for when to transition, Ground Effect will let you achieve slower airspeeds over the threshold safely.<\/p>\n Ground Effect works by reducing the induced drag of the wing when in close proximity to the ground.\u00a0\u00a0 It’s equivalent to suddenly increasing the wing span and as we saw from our math above – if we increase the wing size, we get more lift for the same airspeed, therefore letting the plane fly slower for the same lift as it would generate at higher airspeeds outside of ground effect.<\/p>\n Change of wing span, what?<\/span><\/p>\n This is where you often read that ground effect reduces induced drag thus why it works, job done.\u00a0\u00a0 True, ground effect reduces induced drag, but I just said it’s like increasing the wing span, what’s wing span got to do with anything?\u00a0 Well wing size is really all you have in the lift equation to play with, if airspeed is constant then dynamic pressure will be constant and the lift coefficient will be constant because angle of attack is constant.\u00a0 So we’re left with an equivalent change of wing span.\u00a0 It’s outside of the scope of this article, but if you want to get into the detail, go look at the equation for induced drag<\/a> and you’ll see that for a basic aerofoil shape\/wing, wing size is all that controls it if everything else is constant.\u00a0\u00a0 But suffice to say don’t just think about the fact your changing the airflow behind the plane, but you’re also changing the ability for vortices to destroy the airflow above the wing thus you have more wing (effectively) for the same plane, much as if you had a bigger wing…..but without the weight.<\/em><\/p>\n Ground effect begins to occur when the plane is about one wingspan<\/span> from the ground and increases effectiveness the closer you are to the ground.<\/p>\n A Cessna 172S with a wing Span of 11m (36ft) thus enters ground effect just after the threshold speed typically quoted for 50ft.\u00a0\u00a0 We can then begin to transition from Vs1 (48knots) speed to Vs0 speed (40knots), in the knowledge that Ground Effect is about to kick in and ensure the wings continue to generate lift (i.e. don’t stall) as we land.<\/p>\n It is important to note ground effect works by reducing drag, but drag helps us decelerate the plane to achieve best stopping distance.\u00a0\u00a0 We can’t avoid ground effect on landing, so reducing power and airspeed as we enter ground effect helps achieve best landing distance performance (stopping in the shortest distance possible).<\/p>\n Final Thoughts on Safety<\/strong><\/p>\n I’ve talked a lot about and perhaps convinced you with some equations to try slowing down your final approach airspeeds.\u00a0\u00a0 Too much speed in untrained hands when driving a car is dangerous.\u00a0 Equally, flying slow speeds in a plane when inexperienced and especially if combined with a challenging runway that will in itself present challenges and distractions is dangerous.\u00a0 Always seek proper training, if you’re new to short strips and slow approaches, find an experienced farm strip instructor and go do some flying with them first!<\/em><\/p>\n Oh and use the airspeed the runway requires, because stalling 20ft short of a one mile long runway is always going to look bad and suggest things to investigators like you were trying to show off for no reason.\u00a0\u00a0 The aim of the game is to fly safely and to manage the risks!<\/em><\/p>\n","protected":false},"excerpt":{"rendered":" When you’re learning to land there’s a couple of things that typically go wrong and perhaps you can relate to these experiences: The ground starts getting very big out the window so there’s a natural tendency to try and avoid hitting it.\u00a0\u00a0 Your instructor is probably telling you “Keep it coming down, keep it coming […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[3],"tags":[],"_links":{"self":[{"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/2381"}],"collection":[{"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2381"}],"version-history":[{"count":37,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/2381\/revisions"}],"predecessor-version":[{"id":2421,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/2381\/revisions\/2421"}],"wp:attachment":[{"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2381"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2381"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2381"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\n
\n
<\/a>\n
\n
\n
\n
\n
\n
\n
\n
\n
\n
\n<\/strong><\/p>\n\n
<\/a>