Airbourne prematurely

[taxistaxing]

Hi all,

I'm based at an airfield with a very long runway, so flying into as many short grass strips as possible to prevent sloppy landing/takeoff technique from creeping in.

I fly a 172 and I find that with 10 degrees of flap set the aircraft feels 'twitchy' almost as soon as the take-off roll starts. This is fine on a hard runway as you can simply hold the nose down and let the speed build and 'rotate' at 50-55kts, but a few times on grass strips I've hit a bump and got airborne at around 40 kts with the stall warner sounding. Other than pushing the nose forward immediately and building speed in the ground effect, is there any effective technique to prevent this? Holding the nose down doesn't seem a good option due to the risk of prop-strike.

Thoughts apprecaited!

[robin]

Holding the nose down doesn't seem a good option due to the risk of prop-strike.



A hell of a lot better than keeping the nose high, though

[Ds3]

New PPL so hardly the voice of experience, however the grass strip I learnt at has a displaced threshold on 24, which starts with an upslope and then a bump just before the runway levels out at the threshold, and you usually hit that at around 45kts when two up in the Robin, when you actually want to be rotating at around 55kts.

The technique I was taught was to accelerate with gentle back pressure to take the weight off the nose - assuming you don't have too much back pressure when you hit the bump you grab a bit of air but it settles back down again quite quickly, then you can rotate as desired.

You certainly don't push the nose down at any point, just manage the level of back pressure to maintain the right attitude. If you're solo and get a good run up the slope you can just get launched off the bump :D

[mad_jock]

Its quite a common issue with some C172 airframes.

I used to just let her fly and accelerate in ground effect as per a short field takeoff.

Don't try and keep her on the deck and don't lower the nose.

That main gear can soak a whole heap punishment the nose wheel can't. I have seen folk drop them from 6ft up onto the deck with spectacular rebounds afterwards no harm apart from pride occured.

[taxistaxing]

Thanks so far.

So the correct approach is light back pressure and then maintain the attitude on the ground or in ground effect until rotation speed...

MJ what about when the stall warner sounds? A few weeks back at I was three up flying out of the grass strip at Shoreham which is very bumpy. We hit a bump and jumped a good few feet in the air and the stall warner sounded. I lowered the nose instinctively and must have come quite close to impacting the nose wheel back on the runway. I guess it's a balance between not stalling and not shoving the nose forward so aggressively that you bump the nose wheel.

I was never really taught these techniques as I learned at Biggin which has miles of tarmac, and have joined a club with an even longer tarmac runway.

[Proteus9]

obviously it varies from aircraft to aircraft, but the stall warner usually goes off a bit before the wing is fully stalled.

[Ralis]

172 short feild takeoff is 10 of flap holding on the brakes full power, release brakes and hold weight off the nose wheel.
one strip i fly out of has a seal taxi way 1/3 from the threshold and will bump you off at 40 odd kts but a slight check forward ( not below horizon ) gather speed and away you go.

[mad_jock]

1. Well to be honest on a C172 they are pretty crap anyway.

2. If it goes off your not actually stalled yet its just a warning your getting close.

3. Ground effect is a bit different to normal clean air. Trick is don't get out of it until you have enough energy.

You don't push the nose forward just hold your current attitude and as you speed up gently reduce the back pressure to maintain your height and when your up to your normal speeds pitch for your climb attitude.

The plane may settle back down but don't worry about it.

If you have never been taught soft fields and perfomance departures its good fun. You can use more flap and then clean up in ground effect as well. Read your POH and see if you can use another stage of flap.

Anyway I think you could have a very enjoyable and productive hour with an instructor doing this sort of thing. As usual go and find an auld fart to do it that have done a bit of flying.

[thing]

172 short feild takeoff is 10 of flap holding on the brakes full power, release brakes and hold weight off the nose wheel.]

Even that varies, in the POH for the 172K and M it says no flaps, just full power on the brakes. Soft field is 10.

[jollyrog]

Think about why the stall warner is sounding and what it's measuring. It's not airspeed, it's angle of attack. Until the AOA is reduced, the warner will continue to sound. But, the warning will usually start 5-10 knots above the stall, so if you're actually flying, the airspeed is increasing and the AOA isn't excessively high, you're moving away from stall conditions, not into them. Just don't pull back any harder, a slight release of pressure should be enough.

[Ralis]

Even that varies, in the POH for the 172K and M it says no flaps, just full power on the brakes. Soft field is 10.

Fair point didnt state for N R models as thats what ive used

holding weight off of the nose wheel for trikes I would think would be a standard for a short feild (allways an exception, glad to hear of some)

[achimha]

I like starting the takeoff roll without flaps and then selecting 10° during the run. This reduces drag and gets you to speed faster.

[taxistaxing]

All good advice thanks. An hour with an instructor might well be a good idea.

For the 172p the POH just says 10 degrees for short field (although the club I fly with like you to use 10 degrees of flap on a 1.8km runway in order to reduce wear on the wheels!). I tend not to as I'd rather have more height, quicker in case of an engine failure where the runway is smooth and length isn't limiting.

It also says the aircraft should cruise at 120kts which is news to me :ugh:

[Halfbaked_Boy]

I like starting the takeoff roll without flaps and then selecting 10° during the run. This reduces drag and gets you to speed faster.

All good fun (used it many times operating from 'questionable' strips) until only one side deploys and you're lying broken amongst hot metal and burning bumble bees!

[Crash one]

Quote:
I like starting the takeoff roll without flaps and then selecting 10° during the run. This reduces drag and gets you to speed faster.
All good fun (used it many times operating from 'questionable' strips) until only one side deploys and you're lying broken amongst hot metal and burning bumble bees!


Or one wheel falls off, or the engine blows up at 50ft, or a wing tank bursts into flames.:ugh:

I use that no flaps till 40kts method. But my flaps are manual & a solid bar between them both.

[The500man]

I used to find the C172 N & P would take forever to accelerate between 45 and 50 kts IAS on grass with flap 10 without lifting the nose wheel. In a way if you get bounced off it's a good thing because it means less drag!

I recently flew a C172 SP and it seemed easier to fly to me than the older models. I don't know if there is any significant difference apart from the fuel injected engine and a lower maximum flap setting?

[dont overfil]

The SP is really capable and goes better than an extra 20-30 HP would suggest. Genuine cruise is 115kts tas at 8usgph Maybe not carrying 40 years worth of crud makes the difference.

As for the cruise on the old ones. More like 120mph. Cessna are a bit more truthful these days.


D.O.

[Pilot DAR]

Hmmm, some poor advice here in my opinion...

Grass runway, possibly not as smooth as the pavement you're used to - keep the nosewheel light the whole takeoff run. As you start your takeoff run with 10 flaps, note the amount of horizon you see above the cowl. Full stick back, add power (don't worry about brakes). After a few feet of ground roll, the nose will come up a bit (generally just meets horizon). When you detect this, release a little back pressure, and just hold it in that attitude no matter what. If you're going over undulating ground, just hold that attitude no matter what. The aircraft is accelerating, and will fly when it is ready. If you get a stall warning, just don't pull any more! You're very slowly releasing back pressure anyway right? - the plane is accelerating, so it is moving away from stall "speed" and critical angle of attack. If it's in the air - great, is that not what you were intending? Give it time. If you try to correct by pushing, you're more likely to bounce the nosewheel off the ground, and it all gets really bad after that. I have watched it.

As for fiddling with flaps during the takeoff roll, if they are electric, forget it, it's a waste of time, and dangerous distraction. A C150/152/172/182/206 will have more favourable pitch control on the ground during takeoff with "takeoff" flap rather than no flap, just do what the flight manual says. Drag increases as a square of the speed, so the drag difference between zero and ten flaps on a 172 in the first 20 knots of the takeoff, is negligible, and, you have lost the positive pitch control those ten flaps give you, when you need it most on rough ground.

There are pilots who reposition manual flaps to augment lift during takeoff. I can't say that it does not work. I can say that the Cessna and Piper flight manuals don't tell you to do that. If you need to ask if you should do it - you should not. That technique will optimize takeoff performance under certain circumstances for a very skilled pilot. 95% of pilots never fly in those circumstances. Doing this has serious downsides if you get it wrong, which you'll have real trouble explaining to your insurance adjuster.

[Pace]

Seneca twins are terrors at getting airborne early off bumpy fields. You have to take the weight off the nose to avoid the nose impacting the bumps and you are stuck with the fact that some of those bumps act as launch pads.
I had a Seneca up and flying at 60kts IAS which is a tiny margin especially if the winds are strong and there is wind shear to add to the equation.
I believe the Jockey Seneca which crashed killing the pilot was out of a horse racing track and dropped a wing.
As MJ says its about letting the speed build while not descending back onto the turf and making small movements with the column.
As speed builds then start pitching again but it is about feel and coaxing the aircraft in the initial stages.
Make sure you are trimmed right too!

Pace

[Katamarino]

I like starting the takeoff roll without flaps and then selecting 10° during the run. This reduces drag and gets you to speed faster.

10 degrees of flap will cause so little extra drag at the low speeds you are doing at the start of a takeoff run that doing this will make no noticeable difference whatsoever. I'd think that all it does it cause extra distraction on the takeoff run, and give you the risk of going through the hedge if you set them wrong or they don't operate.

[Shaggy Sheep Driver]

Pilot DAR gives good advice; much of what preceeds his post in this thread is not so good, and some is awful. Fly attitude, use the flaps as Mr Cessna intended them to be used.. and what's this 'rotate speed' all about? It's a 172 FFS not a 747! When it's ready to fly, it'll fly!

Are today's studes actually taught 'rotate speed' in SEPs? If so, I despair!

[Ds3]

Are today's studes actually taught 'rotate speed' in SEPs? If so, I despair!

Yes, or indeed I was anyway. With the theory being it's an important part of short field technique when runway is limited and gaining speed / height is a priority.

The main runway at my airfield has large trees at the end so you don't want to hang around on the ground too long. I was therefore taught to 'rotate' as soon as safe to do so, i.e. 55kts, build up speed in ground effect, then climb out at optimum rate.

Obviously not such an issue if you have 2km of tarmac in front of you, and you can sit around all day waiting for the plane to decide when it's ready to fly!!

[Shaggy Sheep Driver]

Ds3, your 'rotate speed' will come up on the ASI long after the plane has told you it wants to fly. If you have been taught to have the sensitivity in your hands to hear it, that is.

Or you can simply 'fly by numbers' and sit there staring at the ASI if you have the luxury of miles of tarmac stretching in front of you, and if you have no ambitions to be a pilot rather than an aeroplane driver.

[Ds3]

Don't really understand your post, you seem to be agreeing with me in part but also suggesting an HR200 will tell you it wants to fly long before 55kts.

Obviously it's dependant on circumstances, however the example I used was specific to my experience of getting the Robin up when two up with fuel, off a grass strip with trees at the end. In that example, the minimum IAS it'll sustain flight in ground effect is at around 55kts, certainly not well before, and it requires a little extra backwards movement on the stick to lift up, hence use of the term 'rotate'.

You cannot afford to hang around on the ground too long due to the trees, so it's quite the opposite of being an 'aeroplane driver' having miles of tarmac to stare at the ASI, it's using the best method to get the plane up to speed in as short a distance as possible.

Not sure you what you mean about hearing through your hands though?

[mad_jock]

taxi all getting a bit heated as usual.

Best thing is to go and find an instructor who likes this sort of thing. Personally I love doing this sort of training and it used to be my suggestion for the hour flight. I was lucky having Dornoch just up the road to go and use.

If you give a ruff area somebody who likes doing this sort of training might send you a pm.

Its not actually as difficult or as "dodgy" as it sounds on here.

[thing]

As for the cruise on the old ones. More like 120mph. Cessna are a bit more truthful these days.

Yes, I've always been bemused by these quoted figures. Our 'K' with a 160hp donk cruises at about 103 knots; our 'M' with a 180hp cruises at about 110. Both 75% power at 3,000' fullish tanks and two up. Our 28 with 160hp struggles to do more than 95kts unless you really cane it. Our 152 with the cruise prop goes faster...

[Shaggy Sheep Driver]

Not sure you what you mean about hearing through your hands though?

Precisely my point. Carry on driving!

Some aeroplanes are more 'talkative' than others. To get our Chipmunk off a short muddy field I'd hoink it into the air at the earliest possible point, way before it's ready to fly away, way on the back of the drag curve. Holdng the aeroplane level just above the surface one can 'feel' the CP creeping forward as the speed builds until the aeroplane says "OK, I'm good to go now. You can climb away". And I do.

At no point do I look at the ASI (except to check it's 'alive' and working). It's all done through feel. My eyes are outside assessing runway remaining before the hedge, and the rate we're using it up. I have a 'descison point' on the ground by which I'll abort if the aeroplane hasn't told me it's happy to climb away by then, and of course I'm watching that as well.

Pilot DAR describes a similar technique any C172 pilot will recognise in getting out of short fields.

Your method of waitng for a certain speed to appear then 'rotating' is crude and does not eek the best performance out of the aeroplane. It's OK for long tarmac runways where you don't need to get the aeroplane out of the mud ASAP, and where runway length isn't limiting.

Otherwise; for max T/O performance fly the wing, not the ASI. The aeroplane will tell you, through the stick, what the wing is doing. If you are sensitive enough to hear it.

[Ds3]

Edited to respond to SSD's edit.

So what you meant was 'feel', not 'hear'. Ears hear, hands feel ;)

I still don't think you're particularly disagreeing with me anyway. I never said it wasn't done via feel, just that the Robin picks up at around 55kts, when you hit the bump we were discussing at around 45kts, which is invariably before the plane is ready to fly.

Suggesting I spend the whole take off run staring at the ASI is possibly a touch presumptive. I do everything you have described, but also take the odd, brief glance at the ASI to check where things are at. I find this useful in guaging how close the plane is to telling me when it's ready to fly and one of many factors used in decision making.

Obviously it's also dependant on your level of experience, no doubt if you have 100s of hours you can fly without the aid of any of the instruments if desired, however I think it would be naive to suggest to low hours pilots that they ignore things such as the ASI during take off, as yes they may yet not have built up a suitable level of experience to make judgment calls based solely on the varying feedback through the stick.

[Jan Olieslagers]

The advice I was recently given corresponds largely with DAR's idea.
-) get to know the pitch attitude your plane will sustain in climb
-) start to roll with full stick back
-) as soon as the nosewheel lifts off, release stick to keep the horizon at the optimal pitch, which you have determined before. It may change a bit with gross weight, though.

This way, you lift the delicate nosewheel off the ground as soon as possible, without ever risking a stall. (except perhaps with a gust of tailwind)

Mind you, this advice was given for a microlight, I've no idea if it can be taken to a C172 literally.

NB replace "stick" with "yoke" as appropriate

[Pilot DAR]

Ds3,

As I review my 172RG Flight Manual (I don't have one for a regular 172), to assure that I understand what Cessna expects for speeds before I comment (afterall, they are the experts at this). The normal takeoff says to "Lift the nosewheel at 55KIAS". For the short field takeoff the reference to a speed is replaced by "Maintain slightly tail low attitude". It appears that Cessna (at least for this aircraft) is not requiring any particular speed for a short field takeoff.

Differing years of Cessna flight manuals will have differing phraseology for the same technique, mostly from the evolution of flight manual development and harmonizing the procedures with other types as required.

None the less, you will never go wrong using a short field technique on a longer runway, and it's better for the aircraft. You might have to work a bit more at directional control, but nothing wrong with perfecting your skills that way.

If you are flying a multi crew jet, and the pilot not flying is calling out speeds, then certainly "rotating" at a called speed is appropriate, that's the way those planes were designed to be flown. However, I am critical of a 172 pilot who is "eyes in" on, or close to the ground. . You can feel everything you need to about how the plane is flying, and what it will do next, at those lower speeds. Touch is a sense, as valid as any other, so hone it, and use it along with all the others. One day, out of necessity, you will have to maneuver, with reference to the outside, and you just will not be able to look back at the speed, so you might as well practice.

Of more than 60 light aircraft types I have flown, there is exactly one, which no matter how much attention you pay to the feel of the pitch control, cannot be flown in the extremes by feel alone. The control forces are often null, and sometimes backward. I am not surprised that Canada did not type certify it. That is a Siai Marchetti 1019. Magnificent fun to fly, but you cannot relax...

As said earlier, it is not the "new" pilots I blame for not knowing this stuff, it's the inexperienced instructors. The problem is that many of these instructors don't know that they are inexperienced, so they genuinely think that their way is the right way. They were taught to teach by a curriculum, which did not properly address some aspects of flying. So they don't practice, and the center core of knowledge and technique becomes the full breadth, by a narrowing of the range of experience. Yes, there are many things I will do while flying a plane, which I would never write here. I can do it, and I can mentor it to other pilots, but I can not do it justice in words alone, and I don't want someone screwing up because they tried, 'cause I wrote. Yes, I have many times reselected flaps during takeoffs, but only on 180/185 floatplanes, where the trick really does work, if well co-ordinated. But, just the blanket statement to try it, verges on irresponsible, in the absence of one on one mentoring.

At least in the old "hanger talk" days, there was a chance that when the weather cleared, or the following weekend, the wise skygod would say "c'mon, I'll show you" and you were treated to a real experience. The chat group is a fantastic beginning point in mentoring, but it rapidly falls short when it comes to the follow up of "come with me...".

So some of us here do our best to share our knowledge as best can be done by keyboard, and the odd photo. We can only offer so much though, without being in the plane with you....

[Gertrude the Wombat]

You don't push the nose forward just hold your current attitude
Works for me - don't move the yoke at all. Sometimes settles back on after being thrown into the air, sometimes accelerates in ground effect a few inches up.

Haven't broken a nosewheel leg yet :):confused::D

[Shaggy Sheep Driver]

I think it would be naive to suggest to low hours pilots that they ignore things such as the ASI during take off, as yes they may yet not have built up a suitable level of experience to make judgment calls based solely on the varying feedback through the stick.

Not naive, but perhaps presumptive (I've no idea of your experience level and assumed it higher). I do have a lot of experience with small aeroplanes, mostly but by no means all tailwheel, and short strips and aeros going back decades, but even when I were a neophyte I was aspiring to hear what the aeroplane was telling me rather than driving it by numbers. If you have that aspiration, well and good.

We see a decline in basic flying skills in airline pilots (Colgan Dash 8, AF 447). When I hear folk talk of 'rotate speeds' in SEPs (which is a term not appropriate to those types) I have similar worries about GA.

[Pilot DAR]

Bear in mind Ds3, that the published speeds for any aircraft are selected with a margin, where there is doubt about controllability. One of my project required reducing the best rate of climb speed for a highly modified Caravan by 7 knots. To support that, not only did I have to show the climb performance, but also land back from 50 feet after an engine failure after takeoff, and Fly a complete circuit at 5 knots slower than the book speeds I proposed (so takeoff, circuit approach and land at 75 KIAS). Those two requirements were much more un nerving that the more than dozen spins I had to do in it!

There are margins built in to the numbers. If you get airborne "below" the speed, and are climbing, you will continue to. However, if it quits, your chances of entering a good glide may be very reduced. That's why full flap takeoffs are not approved, not 'cause the plane won't do them!

[Gertrude the Wombat]

That's why full flap takeoffs are not approved, not 'cause the plane won't do them!
If you try to take off with full flaps in a 172 there's also the problem that you might find it quite difficult to stop the thing running off the left hand side of the runway :O

[Pilot DAR]

quite difficult to stop the thing running off the left hand side of the runway

You had both flaps down, right?

[Chuck Ellsworth]

I am waiting for Pilot Dar to point out the obvious in this discussion......

....airspeed indicators have a lag especially in the low airspeed realm so why in Gods name would you look at the airspeed before you are flying???

Airplanes fly based on angle of attack........chasing an airspeed indicator will only create a roller coaster ride.

By the way Pilot Dar.......I have all my Cub project back home....including the AOA parts..

We really enjoyed your visit to our home and look forward to visiting you in the future.

Chuck E. :ok::ok:

[Big Pistons Forever]

I think the biggest problem low time pilots have with bumpy fields is the fact that the bumps tend to bounce the aircraft into the air before it is quite ready to fly. Whe this happens the tendency is to pull back creating an excessive angle of attack which causes the aircraft to stop accelerating and in extreme cases staggering into the air and then stalling.

As Pilot DAR pointed out establish the attitude that just flies the nose wheel off the ground and hold that. If the airplane bounces into the air hold the attitude if it is ready to fly it will stay in the air, if not it will settle back on the ground and continue accelerating until it is ready to fly.

As for the flaps issue. Well the POH for every model of C 172 says use 10 degs for soft/rough fields. It doesn't say mess with them on the takeoff run.

Your attention is IMHO far better used concentrating holding the takeoff run attitude. Also the POH (the book that nobody in the UK flight training scene ever seems to look at :rolleyes:) for the C172M says the target speed for a short field takeoff is 59 kts at 50 feet AGL. That is the number you should look for after the aircraft has broken ground and started climbing away.

[24Carrot]

airspeed indicators have a lag

I was always taught that the ASI gives an almost instantaneous readout of the pitot/static pressure differential. Subject to the various errors, this gives a good measure of the airspeed.

Strictly speaking, I believe it is the actual change in airspeed that lags behind the pitch change.

Mind you, this hair-splitting does not change the logic of what Chuck Ellsworth said!

-----------------
Edited to remove confusing reference to true airspeed.

[Redbird72]

That main gear can soak a whole heap punishment the nose wheel can't. I have seen folk drop them from 6ft up onto the deck with spectacular rebounds afterwards no harm apart from pride occured.

Damn! Prune has eyes everywhere, I thought I'd got away with that one...:{

[taxistaxing]

Glad I've got a bit of a debate going!

Big Pistons, yep that's exactly the problem I've found a couple of times on bumpier strips (and what prompted me to ask the question in the first place). You get bumped up into the air at around 40 knots which is below a safe flying speed. So far I've managed to resist the temptation to pull the stick back - I instinctively push the nose forward on the stall warner largely thanks to my instructors drilling it into me during my ppl. The difficulty with it is the risk of overcorrection and bumping the nose wheel back onto the ground.

I guess the "maintain attitude and accelerate in ground effect" technique is actually what I've been doing albeit without realising it. I'm going to do a few experiments with this on my long tarmac base runway to build my confidence for the strips.

I did my PPL on a PA28 from a paved runway and, rightly or wrongly, I was taught to scan the ASI periodically throughout the ground roll and 'rotate'. The PA28 needed positive back pressure to 'unstick' from the runway, whereas the C172 just wants to fly off itself - so this may be the difference, and why the technique is less appropriate for the 172.

[taxistaxing]

And should also say, there's no way I'd feel comfortable monkeying around with the flap settings during the takeoff roll at my current level of experience. One to leave to the bush pilots for now :=

[The500man]

Looking at the C172S POH it does actually say in the normal procedures section for short field takeoff with flap 10:

6. Elevator control - SLIGHTLY TAIL LOW
7. Climb Airspeed - 56 KIAS (until all obstacles cleared)
8. Wing Flaps - RETRACT SLOWLY (when airspeed is more than 60 KIAS)

For a normal takeoff:

4. Elevator Control - LIFT NOSEWHEEL AT 55 KIAS
5. Climb Airspeed - 70 - 80 KIAS
6. Wing Flaps - RETRACT (at safe altitude)

The expanded soft field procedure:

Soft or rough field takeoffs are performed with 10° flaps by lifting the airplane off the ground as soon as practical in a slightly tail low attitude. If no obstacles are ahead, the airplane should be leveled off immediately to accelerate to a higher climb speed. When departing a soft field with an aft C.G. loading, the elevator trim control should be adjusted towards the nose down direction to give comfortable control wheel forces during the initial climb.

There's actually a surprising amount of other useful information in that POH. Cessna have definitely put a lot of work into making it useful. The C172N POH is not as clear and it doesn't recommend using flaps at all, and it has slightly different target airspeeds.

[Pilot DAR]

at around 40 knots which is below a safe flying speed.

Well, because you have included the word "safe" in that statement, I'm going to agree with you in this context. However, it is certainly possible to have a 172 in sustained flight during takeoff at a slower speed, and fly it away. The challenge is that you can get into very high angles of attack doing it, and you are getting closer to the stall (which is just settling back in this condition), or banging the tail on the ground, which never good. Stop watching the ASI, look out the windshield, and feel what the airplane is telling you, the airspeed is only one part of the whole thing.

The stall warning is just that - a warning. If you are not planning to stall, then don't pull back any more. But, if you're in controlled flight, not settling, and accelerating, you do not need to lower the nose either. Acceleration takes time (or you'd have been pinned into your seat!). So give it time, safely controlling it, and it will fix itself. The super fast correction you're applying 'cause your instructor told you to is a form of over controlling. Yes, a pilot can cause a less good condition of flight instantaneously by control inputs which are poorly timed relative to what the aircraft is going to be doing in a few seconds, and tens of second hence. Patience......

If you are in the air at 40 knots, and returning to earth is inevitable, would you not much rather do so in a flared attitude? Keep the nose where you had it, a little "tail low" (to use Cessna's term). A 172 is amazingly shock absorbent in that attitude. If you contact three point, or worse, nosewheel first, the nose strut, firewall, and perhaps propeller, and engine damage is going be costly.

Yes, practice, but unfortunately you cannot practice uneven runway techniques on a smooth paved runway. You can practice just holding the aircraft attitude and getting airborne at a lower than normal speed, and that's excellent, but anything you do on a smooth runway, which seems to induce an "uneven" surface will get very bad fast. If you're going to practice "tail low" takeoffs in a 172, do so with as forward a C of G as you can accomplish. In a lightly loaded 172M or later, I would expect to be happily in ground effect, accelerating, with maybe just a brief peep of the stall horn, at 40 knots.

[achimha]

Here's the C172N manual (1978):

NORMAL TAKEOFF
1. Wing Flaps -- UP.
2. Carburetor Heat -- COLD.
3. Throttle -- FULL OPEN.
4. Elevator Control -- LIFT NOSE WHEEL (at 55 KIAS).
5. Climb Speed -- 70-80 KIAS.

SHORT FIELD TAKEOFF
1. Wing Flaps -- UP.
2. Carburetor Heat -- COLD.
3. Brakes -- APPLY.
4. Throttle -- FULL OPEN.
5. Mixture -- RICH (above 3000 feet, LEAN to obtain maximum RPM).
6. Brakes -- RELEASE
7. Elevator Control -- SLIGHTLY TAIL LOW.
8. Climb Speed -- 59 KIAS (until all obstacles are cleared).

[cockney steve]

First , let me say, I'm NOT a pilot.! (models don't count :O )

If anyone has any doubts, wait till your lawn needs cutting, then try sprinting across it, whilst pushing a wheelbarrow..repeat , early morning with dewy grass......then try again after it's been cut.

now we've established that there's a LOT of drag from an undercarriage, why all the paranoia about floating a few inches above the surface,in Ground-effect?

the aircraft won't SLAM back down,unless you force it, but it WILL accelerate a damned sight quicker than it does when it's dragging wheels along a ground-surface (paved or otherwise)

[The500man]

What's interesting about the N, is that it says with 10° of flap (for rough field), subtract 5 KIAS from the flaps up takeoff speed. Which if you go and look at section 5 is 50-52 KIAS depending on weight. So 45-47 KIAS.

That sounds about right to me. Accelerating from about 45-50 in an N seemed to take forever to me on grass when I last flew one (as I wrote earlier).