"pushing" in a power-off glide

[palgia]

Hello all,

I have a question regarding the notion of "pushing" down to extend a glide. As far as I knew, the only way to obtain the max power off glide range was to stick to the best glide speed (eventually adjusted for weight, wind, bank, density, ect).

However after flying in a Piper cub with a fellow instructor, he argued that when trying to extend the glide to clear an obstacle, you should "push down" pointing the aircraft straight towards the obstruction and then pull-up at the last minute in a zoom climb to clear the obstacle. When pushing down, the aircraft will be flying at a speed higher than best glide.

To me the above reasoning does not make sense. If you fly at the speed that gives you [L/D]max you should get the most efficient convesion of altitude into range. Any speed higher or lower than best glide should decrease your glide range. (hence the name best glide:rolleyes: )
Furthermore, any time you trade airspeed for altitude (and vice versa) you always lose energy since the conversion is not 100% efficient. Therefore pushing down to increase speed only to have to have to pull up again later seems like something you wouldn't want to do.


The same instructor claims that glider pilots regularly use this technique to extend their glide a little further and clear an obstacle. He calls it "learning how to push". I only have some 30 flights in a glider, 10 being solo, but I don't recall any of my instructors saying anything about "pushing".

Could somebody shed some light on this topic?
Am I missing something here?
Has anyone else heard of this practice? Is it only an aviation myth or is it actually true? (hopefully the explanation is more convincing though)

Could it be that this presumebly-incorrect notion of "pushing" to extend the glide has its roots in the fact that it may be necessary to "push" the nose down in the event of an engine failure after takeoff in order to accelerate to best glide speed and maintain an adequate stall protection margin? (assuming your climb speed was below best glide speed) Some people might have misunderstood this and believe that pushing down will extend their glide range.... just a speculation.

Thanks in advance for your replies.

palgia

[BlueRedGreen]

Palgia,

I do not have an answer to your question but I would like to add my support for your theory. 'Pushing' sounds like driving your car as fast as you can to get to a garage before the little remaining petrol runs out!

I will be interested to hear a definitive answer to this one!

BRG

[Loose rivets]

Sounds odd. Perhaps he means in an scenario where you are committed to going over that particular obstacle and this would be a way of knowing beforehand if you had sufficient kinetic energy.

Most times, height is money in the bank...unless you're on fire of course.

[Rainboe]

Sounds rubbish. The only justification can be to give you the flexibility of converting speed to altitude- at minimum drag speed you don't have a lot of option to climb. But to extend to max range, you sit there at minimum drag speed with a slight increase for headwind.

[Kilo-club SNA]

After flying gliders for a few years and instructing GA for another few years this sounds like a lot of B/S. It would be nice to hear how he responds to the arguments you have posted here.

There are a few good reasons to push the stick forward (safety, wind etc) but none of them extend the glide angle ie distance traveled.

Confront him! Before he teaches someone else to do this!

[828a]

Palgia;

Crop dusters know that if one undershoots an engine failed forced landing to the point that a fence or similar obstruction will not be cleared then if the aircraft is positively forced onto the ground and made to bounce this action may possibly save the day. Extreme measures for sure but something happens to give more lift. 828a.

[ft]

There's a slight benefit to be had by going into ground effect. If you time it exactly right, you could probably clear an obstacle marginally farther away by timing your entry into ground effect before the obstacle and then pulling up before crossing it.

The chance of getting it right would be next to nil though, IMO. The risk of ending up in the obstacle at a higher speed or stalling out on the pullup would be significant.

Going into GE to save the day is not an uncommon tale in glider pilot folklore.

[palgia]

Thank you for all your replies.

Looks like I'm not the only one who has a problem with this idea of pushing...

I will definately confront this instructor and hopefully clarify his/our ideas.:ok:

Thanks again.

pagia

[one dot right]

erm,actually ,before everyone shoots this instructor down in flames,he has a point.
By pushing down you take the aircraft through the wind gradient,
converting the potential energy you had at altitude to kinetic energy at ground level,minus the headwind component,which in some circumstances will enable you to pull up over the obstacle rather than gliding into it!
It's not the ground effect that helps,but getting out of the headwind!

[JeePilot]

I have also had this 'technique' taught to me but never bothered believing it for the reasons stated in the first few posts.

[Mobieus1]

Palgia,
You are quite correct in deviateing from the best glide speed will reduce your range.
I have had flying in ground effect demostrated to me by my instructor at the time in a glider and flying at very low level the speed pegged itself at 87knots and showed no sign of slowing. Suspect it has something to do with the induced drag disappearing. I was told that some pilots do it to recover back to a airfield if they are low on height and need to cover a extra few miles. I modern glider (very low profile drag) can get up to over a hundred knots diving from about 300ft!
Could a Pup do that? I would'nt do in a spam can that is for sure. What would happen if you stalled trying to get over the obstacle?

[lhr_slots]

Actually, what the instructor said was correct for a glider because you fly the circuit SLOWER than best glide speed.

If you are too high, you should pull back/reduce speed and this increases your glide angle (because you are slower than best glide). You don't want to push the nose down if you are high, because you would then come in too fast. The glider may not have enough drag (even with spoilers) to dissipate the extra energy and as soon as you hit ground effect you'd just keep going off the end of the airfield!

So, if you pull back to avoid landing long, you push to avoid landing short. But you are pushing to increase speed to your max glide speed (plus a bit for a headwind of course), not to dive in at a faster speed.

The second possiblity is the idea of clearing a fence by getting into ground effect early with some extra speed. Get within half a wingspan of the ground and use the extra kinetic energy to maintain that height. Drag is much reduced in ground effect, far more obviousy than in a power 'plane, mostly due to the longer wingspan. This only works if the area in front of the airfield is flat & clear of course.

Having said all of that, I don't think it applies to a dead stick landing in a power airplane because you would already be flying at your best glide speed, so pushing or pulling will increase your glide angle.

[TURIN]

Have to agree with onedotright.

I have been stupid enough to get my self in a situation where on my turn into wind the best glide would put me in to a dry stone wall!
Admitedly it was in a hang-glider but the principle is the same. Pulling on speed (controls on a flexwing are reversed compared to a 3 axis aircraft) and aiming for the wall gave me enough energy to 'zoom' over the wall and land (not very gracefully I admit) safely.

Wind gradient, however small is a big factor in such aircraft.:ok: :O

[DFC]

When making an emergency landing it is always better to land and then crash than to crash and then land.

This is a last desperate attempt to avoid hitting the hedge on the way into a field.

If one is flying at the best glide speed (adjusted for the conditions) then flying faster or slower will always reduce the distance covered.

When one is faced with a situation where the best glide speed is going to put the aircraft into the hedge and it isn't possible to adjust the configuration (reduce flap, retract the gear etc) then the following desperate measure may avoid passing through a hedge at flying speed;

Lower the nose and move the aim point to just short of the hedge thus gaining some airspeed. At an appropriate point raise the nose and "float" the aircraft over the hedge and once clear lower the nose asap to try and avoid the stall.

The landing will never be pritty (if it is then one probably did not need to use the desperate measure in the first place) but it is better from a personal survival point of view than hitting a solid object in the air at speed!

Engine failure in the mountains could also benifit from the above if the "float" was used to clear a ridge thus placing the aircraft at some height above a valley floor.

Regards,

DFC

[Wizofoz]

DFC et al,

I'm a glider and "self launcher" (B737) pilot.

I don't agree that pushing will ever extend your glide or help clear obstacles, UNLESS you are flying into wind and the wind strenghth decreases with altitude.

Flying at best L/D will always provide the longest still air glide. Flying a speed adjusted for wind will always provide the longest glide in the prevailing conditions. "Pushing" and increasing speed will reduce the L/D and shorten the net glide.

HOWEVER, if the requirement is to clear an obstacle, and the glide distance past the obstacle is not an issue, a short duration "Pull" will temporarily reduce the sink rate and glide angle. Once stable at the lower speed the aircraft will resume a stable glide at a steeper angle.

So I think the "Push" is of no value, but the "pull" may be.

[Miserlou]

This pushing technique is only effective for a short dive into ground effect and if you are flying a draggy type of aircraft the effect will be very slight. If you have to maintain the extra speed in the dive you've started too high and won't get the full potential benefit.

You also make use of the fact that in the ballistic bit of the aircraft may be flying flying slower than the 1g stall but, having a lower angle of attack, you don't stall.

If you make it over the obstacle, you then have to hope that you still have sufficient energy to flare, so you can see how 'last ditch' the technique is.

It is fun though!

[828a]

Wizfox;



Stretching the glide is something you should have been taught not to do. Crop dusters who spend their flying days close to the ground recognise there may not be much time to optimise things after an engine failure and accept that the push and" bounce over" technique is far safer than exposing the aircraft to a possible stall as can happen with what you are suggesting. 828a.

[Old Smokey]

A glider pilot friend once told me from her hospital bed that she'd learned one valuable lesson from the crash that put her there.

If you want to go UP, pull the stick back, if you want to go DOWN, pull it back even further.