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Best Glide Speed


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Here is another one to get our brains thinking :big_grin:

 

Our aircraft have a best glide speed that we would trim for in the event of an engine failure and would be listed in our POH. We understand that this is the best indicated air speed for our aircraft to travel the furthest distance with an engine failure when we consider the 4 key components of flying - Lift vs Weight and Drag vs thrust.

 

With this in mind would our aircraft best glide speed be different with altitude (example 2,500ft vs 10,000ft) because the air would be thinner and with weight (example 450kg vs 600kg)?

 

 

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My understanding of it is that the lighter air will be offset by the asi reading indicated airspeed instead of true. As for weight, we were taught that a heavier aircraft will glide just as far, but will descend faster, so it must be a higher airspeed.

 

I'm probably all wrong, but thats my 2 cents worth.

 

 

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Another two cents worth and probably worth less than that!

 

The heavier an aircraft and providing it has some altitude it has more potential energy. If it is not moving and high enough the potential energy can be converted to kinetic or useful airspeed energy if high enough and also to overcome energy wasting drag.

 

 

 

Usually, once it runs out of height it has run out of spare energy if the motor is not going and the airspeed is on the point of stall - don't turn back..

 

 

 

Drag is usually proportional to the square of the speed for turbulent flow wings.

 

 

 

Increase the speed by 10% results in extra drag of 21%.

 

 

 

But if the wing and the rest of the airframe has laminar flow characteristics the drag will be directly proportional to the speed not the square of the speed - hence the incredible glide angles of top performance gliders or sailplanes like 4 or more times better than a Jabiru..

 

 

 

In the same airframe the more mass up to a point the faster an a/c can glide but it needs to get rid of excess mass before the stall (sorry landing) to reduce the landing speed - chuck out the water or the pilot at the last moment!

 

 

 

Really high flyers will know that the force of gravity decreases as the square of the distance from the craft to the centre of mass of the earth or the moon or the sun or the stars etc.

 

 

 

Regards

 

 

 

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Guest pelorus32

Love these ones Ian.

 

Vbg is the speed for best glide ratio and comes at the best L/D ratio. Interesting thing is that for a "normal" sort of wing like a Tecnam say best Drag is at around 0 degrees alpha and best lift at around 15 degrees alpha. In other words the two things are fighting each other. There is a point on the curve however - usually at around 3 degrees alpha where you get the best compromise - best L/D.

 

That's a pretty low angle of attack so we are not talking really slow flight.

 

As for the other parts of the question. I reckon Bigglesworth gets it in one for the altitude part.

 

The weight part in my view goes like this. Lift is proportional to the square of the speed and also proportional to alpha (in a more complex way I think). So at a particular speed and alpha we generate a given amount of lift. If we increase the weight of the a/c the a/c will sink faster at that speed and alpha and therefore the alpha will increase and therefore the lift increases but so does the drag. Or we could increase the speed and maintain the original alpha in any case I think that the value for best L/D will fall.

 

Still thinking on that.

 

Mike

 

 

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Guest pelorus32

Further to the above. I just dug out Helmut Reichman and had a look at this problem.

 

He makes the point that at high altitude you will have to fly faster (even if it is the same IAS) for any given lift and therefore the rate of descent is greater with altitude. I suspect that at our operational heights it's really a non issue.

 

Looking at his polar diagrams, and as I'm sure all the glider guys know, best L/D doesn't seem to move much with wing loading (although there is quite a flat section of the curve giving a fair variation - more on that later). BUT L/D at higher speeds is better at higher wing loading.

 

Two things. I'm not sure that our curves have the same "flat spot" leading to a greater area of tangency as the gliders will have and I'm not sure that the same weight speed effect holds.

 

BTW the Reichmann reference is Streckensegelflug from 1978.

 

Mike

 

 

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Guest pelorus32

Common - Not to Say Vulgar - English

 

:;)3: radioactive.gif.1acc918ae505c8835a1c29d9312871c0.gif exclamation.gif.7a55ce2d2271ca43a14cd3ca0997ad91.gif :confused: ;) ;) ;) uh - common English needed for me :big_grin:

Oh alright then... :)

 

L = Lift

 

D = Drag

 

L/D = Lift:Drag ratio expressed as a number so 14 means that you have 14 times more lift than drag and it is roughly equivalent to your glide ratio - so for every 1 mile of height you will glide (in this example) roughly 14 miles distance at best L/D. I suspect that our example Tecnam has a best L/D around 12.

 

Alpha = angle of attack.

 

Wing loading = Take Off Weight in kilos/wing area in square metres. So for any given aircraft it is a function of Take Off Weight.

 

:;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3::;)3:

 

Streckensegelflug = Cross-Country Soaring

 

M:cool:

 

 

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Understand that {start braging} CT has 17:1 glide ratio and laminar flow wing (whatever that means) so CT glides 17ft distance and loses 1ft of altitude at best glide speed of 62kts {end braging} but what about weight and altitude impacts? :big_grin:

 

 

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17:1 glide ratio is pretty good. You could probably thermal on that wing as well.;)

 

Here's my understanding of weight and altitude impacts.

 

Indicated airspeed (IAS) auto compensates for altitude so providing that you fly the plane at the correct indicated airspeed you will be acheiving best glide.

 

Yes you will have a higher true airspeed up higher and you will be losing altitude faster compared to a lower altitude. The POH publishes the best speed indicated and this is the best the aircraft can do.

 

Weight is a different thing. Best glide speed is based on weight. It is my understanding that the best glide speed published in the POH is based on max weight and it is somewhere around 1.4 x Stall speed. ( I stand to be corrected here)

 

Consequently if you were to lower the weight of the aircraft, your best glide speed would reduce accordingly.

 

The sums:

 

Max weight 600kg stall speed = 50knots x 1.4 = 70knots best glide speed.

 

Reduce weight to 450kg stall speed = 45knots x 1.4 = 63knots best glide speed.

 

My 2 cents worth...hope it helps.

 

Regards

 

Phil

 

 

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Guest browng
The sums:Max weight 600kg stall speed = 50knots x 1.4 = 70knots best glide speed.

Reduce weight to 450kg stall speed = 45knots x 1.4 = 63knots best glide speed.

It appears however that the calculation is rather more complex than the above;

 

http://www.mathworks.com/products/aerotb/demos.html?file=/products/demos/shipping/aero/astglide.html

 

My distinctly 'low maths' understanding is that as the weight increases both airspeed and sink rate increase, but their ratio remains the same. Both a heavy and light aircraft will achieve the same best glide ratio, but the heavy one does so at a higher speed.

 

 

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Guest brentc

Next time you are out Ian for an interesting comparison, try gliding at idle at the suggested best glide speed. Let the speed sit there precisely for something like 30 seconds - then watch your VSI until it settles. Next, increase your glide speed and hold it too - watch the VSI until it settles. Try a few different speeds. Your figures on best glide speeds may vary significantly to that of the POH. I tried this a couple of times and found I need to go much faster to reduce the rate of descent. If I used their speed, I sunk out of the sky at a very high descent rate.

 

 

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Guest pelorus32
It appears however that the calculation is rather more complex than the above;http://www.mathworks.com/products/aerotb/demos.html?file=/products/demos/shipping/aero/astglide.html

 

My distinctly 'low maths' understanding is that as the weight increases both airspeed and sink rate increase, but their ratio remains the same. Both a heavy and light aircraft will achieve the same best glide ratio, but the heavy one does so at a higher speed.

That's a nice example. I suspect from a very quick squizz that there is some confusion there around CAS and TAS so we shouldn't without further review accept that it is TAS. I suspect it might be IAS.

 

Regards

 

Mike

 

 

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Guest browng

The other factor to consider is angle of bank. When we say "weight" we actually mean aerodynamic weight, i.e. taking into account load factor.

 

From the RAAus website;

 

"Not all aircraft with an identical glide ratio are affected equally when load factor increases. Just because an aircraft has a good glide ratio does not mean it will perform equally well in a turn, it may lose more height in a turn than an aircraft which has a poorer glide ratio. For example a nice slippery aircraft with a glide ratio of 15 may lose 1000 feet in a 210° turn, whereas a draggy aircraft with a glide ratio of only 8 might lose only 600 feet in a 210° turn."

 

 

 

In a real engine out situation, the effects of load factor in turns may be far greater than that of simple 'weight', and the only way to determine that is experimentation in your particular aircraft. My J3 is virtually unaffected by gentle turns in the glide, my Pitts on the other hand did a falling brick impression at very modest angles of bank at best glide speed.

 

 

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I am like Brent, my minimum rate of sink in the '701 occurs at a higher speed than the POH says is the best glide, so I assume the glide ratio is better at the higher speed too. What the actual number is though is pretty academic as it glides like a brick anyway, about 7 to 1. The space shuttle has a glide ratio of 2.5 to 1 and they manage to land it OK

 

David

 

 

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So it seems to me that the POH figure is lower than it needs to be. If that is the case remember when you want to stretch the glide, push the nose down, not up. Unless you are inverted.

 

Personally I can't control my speed to within one knot, so how close can I be to best glide, and I wrote the POH myself.

 

 

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Guest pelorus32

A bit of a joke with one of the Tecnams is Vbg. I think that the POH was probably translated from Italian and the numbers put through a spreadsheet from m/s to knots. The result is Vbg for that aircraft of 59.4 knots.

 

When you're very bored you ask a fellow pilot what Vbg is for that aircraft and then give them a hard time when they say 60 knots.

 

Small things do amuse small minds!!

 

Mike

 

 

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Hi Browng. Nothing to do with the actual topic but saw your logo and had to say, just got my tailwheel endorsement in the Caboolture Recreational Piper CUB and I enjoyed every minute of it, what a classic aircraft........

 

Sean.

 

 

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Guest browng
Hi Browng. Nothing to do with the actual topic but saw your logo and had to say, just got my tailwheel endorsement in the Caboolture Recreational Piper CUB and I enjoyed every minute of it, what a classic aircraft........Sean.

And theirs is a REAL Cub like mine, if you did your tailwheel in that, you really did it...well done. Question, do they put the student in the 'proper' command seat i.e. rear, or in the front? also, what starting procedure did they teach you?

 

 

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It appears however that the calculation is rather more complex than the above;http://www.mathworks.com/products/aerotb/demos.html?file=/products/demos/shipping/aero/astglide.html

 

My distinctly 'low maths' understanding is that as the weight increases both airspeed and sink rate increase, but their ratio remains the same. Both a heavy and light aircraft will achieve the same best glide ratio, but the heavy one does so at a higher speed.

Yes, That is why competition sailplanes carry water ballast, to fly faster for the same L/D. When the lift drops you drop the water to get a better sink rate.

Phil

 

 

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I am like Brent, my minimum rate of sink in the '701 occurs at a higher speed than the POH says is the best glide, so I assume the glide ratio is better at the higher speed too. What the actual number is though is pretty academic as it glides like a brick anyway, about 7 to 1. The space shuttle has a glide ratio of 2.5 to 1 and they manage to land it OKDavid

The best glide [Vbg] quoted in the POH is with the engine shut down and with the prop stopped which is the minimum drag situation. If the engine is shut down but the prop is windmilling then the prop is generating "reverse thrust" and thus considerable drag in which case the rate of descent at Vbg is greater than you would expect with the quoted L/D.

A similar situation exists in a simulated engine failure situation with the throttle closed and engine idling. The prop's rotational speed will be low but the forward speed at Vbg still quite high thus the resultant blade angle of attack for a fixed pitch prop may be very low and possibly negative. The result is an air brake effect similar to windmilling and an unfavourable L/D. [The prop status is opposite to that existing when starting take-off where high rotational speed is combined with low forward speed to produce an initial high aoa]

 

Thus the quoted Vbg only achieves the optimum L/D when the engine is shut down and the prop is stopped so you can't check it unless you do likewise. My suggestion - unless you want to try shutting down the engine and slowing the aircraft until you can get the prop to stop - accept the speed quoted in the POH or manual as realistic

 

Of course the quoted Vbg is a nil wind speed, for instance it must be increased if bucking a headwind; adding perhaps half the estimated wind speed. Also it is usually quite unwise to attempt to stop a windmilling prop following a real engine failure so the rate of sink at Vbg will be greater than expected.

 

John Brandon

 

 

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And theirs is a REAL Cub like mine, if you did your tailwheel in that, you really did it...well done. Question, do they put the student in the 'proper' command seat i.e. rear, or in the front? also, what starting procedure did they teach you?

Hi Browng, send an email to [email protected] and I'll reply to your address with the info, no probs.

Sean.

 

 

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Guest Flyer40
 

 

Really high flyers will know that the force of gravity decreases as the square of the distance from the craft to the centre of mass of the earth or the moon or the sun or the stars etc.

 

 

 

It's not only mass that decreases at altitude, the gravitational time distortion also decreases. So time itself goes faster at altitude, or should I say less slowed.

 

What does that mean for us pilots? well probably nothing except for the long-haul pilots squeezing 25 hours into their day, but it does affect us indirectly through the GPS system.

 

If Euclidean geometry were used for GPS positioning it wouldn't work because time travels at a different rate for the satellites than it does for us here on the Earth.

 

We have to invoke the work of Mr Einstein to correct for the gravitational distortion of space/time.

 

Thankfully Newtonian physics works at low mass and low velocity. Imagine our W&B and T&D calculations if it didn't.............

 

 

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Guest browng
We have to invoke the work of Mr Einstein to correct for the gravitational distortion of space/time.

Do I need to put that in my checklist now?

 

 

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