Gliding with dead engine?

[rammjaeger]

I am interested in that what Germans call the "Gleitzahl" (= engl. "gliding number"?). Let us assume the motor of an aircraft went dead (lack of fuel or whatever the reason is). The airplane is in 1000 meters altitude. Which distance can the plane sail from this altitude? I have a German source for WWI reporting the number was 13000 meters (Gleitzahl=13) for a not specified German type. Some modern gliders achieve a "Gleitzahl" of 40+ or even 60+.

Is anybody able to guess or report the similar numbers for different types of WWI-planes? Maybe the replica builders have some experience here?

Every plane is different. Drag, weight, airfoil shape and CG condition all affect the glide performance of an aircraft. Even two aircraft of the same model can differ greatly in their glide performance, so it isn't really realistic to assign a glide performance number to an aircraft type.

[AchimEngels]

Rammjaeger,

The "Gleitzahl" is not a number that describes how far an aircraft can glide without the engine running.

It is merrily a number that is used to describe the quality of a wing in dependance to air density and speed. It is calculated by dividing the drag of the airfoil by the size of the lift it produces.

The quotient "Rücktrieb/Auftrieb" can be depicted in a coordinate system. In calculations in those days it was displayed as "E".

That way the real meaning of the "Gleitzahl" is to show how good the wing is and differs in each flying situation. As a matter of fact the smaller the number is the better the performance of the airfoil or better to say the wing is. The numbers of lift and drag are achieved by wind tunnel tests of a model wing. The main propose of this number is to figure out the usefullness of the wing for a specific propose (i.e. fast climbing at a high angle of incidence, slow loose of height at low airspeeds etc.)

The main character of an wing is that it not only produces drag when moved through the air, but also another force that is able to overcome the gravity acting on the airframe.

Anyway, with your question you have to be more precise. Are you talking about the "Gleitzahl" of the wing or the "Gleitzahl" of the aircraft.

These two differ, since for understandable reason the drag of the entire airframe is larger than that of the wing alone.

As you can see by the above an aircraft has different "Gleitzahlen" which depend on air density, speed, angle of incidence and so on.

Back in those days it was a common practice to use the "Gleitzahl" to describe the quality of the wing. I do not know what your source was, but anything as high as 13 I would consider to be of bad quality. It would be a different thing if in todays aerodynamics the use of the word "Gleitzahl" is something different. Up to now I have personally only dealt with aerodynamics as calculated back then.

Below I have attached the "Gleitzahl" diagram of the Fokker Triplane wing airfoil as it was obtained by the wind tunnel testings at Göttingen.

Although I have not provided a listing of Gleitzahlen so far with my answer here, I could well send you copies of the Göttingen Airfoil diagrams that have been been published with the Technische Berichte, or better yet a copy of my book on this.

Achim

[rammjaeger]

Thank you, Achim!
Hi, hi - I had already expected that this question will raise many technical implications!

I was simply reading in one of von Koerbers "adventure books" (Feldflieger and der Front). He reported - and at least he was aviator himself - a not named plane type would be able to sail a distance 13 times bigger than its altitude if the motor is out.

I was searching for a technical term for this ratio and fell over this text at a homepage dealing with modern gliders:

"Ein weiteres wichtiges Stichwort im Segelfluß ist die Gleitzahl. Sie gibt an, wie weit ein Segelflugzeug gleiten kann. Heute werden Gleitzahlen über 60 erreicht (60 bedeutet, daß ein Segelflugzeug aus 1000m Höhe über Grund 60 km weit gleiten kann). Diese Gleitzahlen sind allerdings der Offenen Klasse vorbehalten. Die 15m-Segler der Renn- und Standardklasse erreichen eine Gleitzahl von ca. 43 - Tendenz langsam steigend."

I don´t know about the correctness of the use of the term "Gleitzahl" on this page but I am interested in just this ratio. It is of some importance because the traditional claim is that Fokker-Eindeckers was not allowed to cross the lines. Different to this claim I saw some drawings showing the so-called "Fokker-Linie" and this line was not the same as the front line. In an altitude of 2000+ or so meters the Fokker was allowed to cross the lines a bit because a gliding back to the own front was implied there.

VBR
Rammjaeger

[rammjaeger]

BTW I order your complete book set later because I have still to make considerable payments for earlier ordered very expensive main sources of WWI aviation.

[AchimEngels]

Rammjaeger,

I do not think he was refering to the technical term of the "Gleitzahl" as used by the scientists back in WWI.

Maybe he was reffering to some modern term describing what he said, but I am not aware of this.

Anyway, your question is quite interesting and I would also to know much more about it since I have not yet found any source that makes any useful statement of the behaviour of different aircraft when the engine is turned off. I think this is due to the fact that indeed every aircraft is different as pointed out by Rex Feral.

If you find more, please let me know.

Achim

[Dean]

I would think that a glide ratio of 13:1 would be somewhat "optimistic" for a WW1 crate.

I had a brief discussion with Tom Glaeser on this very subject last Armistice Day. *Tom flies a 7/8ths scale Nieuport 11 with the Kansas City Dawn Patrol.

He said that if you lose power in one of those planes, you don't glide very far. *"Just look straight down ahead of you, because that's where you're going to land.", he said. *Granted, the KC Dawn Patrol seldom flies much higher than a thousand meters altitude, but I don't think it would make much difference. *

The drag from all of the struts and wires is tremendous. *He said that if you were flying along with full power, then chopped throttle that it felt like someone in an automobile putting the brakes on hard. *He said it would really push him into the straps.

That much drag surely can't help the glide ratio at all.

Best regards,
Dean

[EdStevens]

I've read a few times (though I can't quote sources unfortunately) that it was not unusual to be able to glide one mile per 1,000 feet of altitude with a dead engine. I seem to think this was for a Pup or something similar, which admittedly has a pretty light wing loading. My brain is not up to transferring this to kilometres per 1,000 meters.

[Dan_San_Abbott]

Achim:
* * *If I am not mistaken you divide the lift by the drag, this is the performance ratio. *For example the last parachute I designed had an L/D of 5.12:1 which in parachutes is not too shabby. *High performance soaring gliders have an L/D of 50 :1 or better. I think an L/D ratio 13:1 would be a stretch for WW1 aircraft. I would think the L/D ratio would be in the range 6-8 :1. The drag factor on most WW1 aircraft was quite high.
* * * * * * * * * * * * * * Blue skies,
* * * * * * * * * * * * * * * Dan-San

[Dean]

I've read a few times (though I can't quote sources unfortunately) that it was not unusual to be able to glide one mile per 1,000 feet of altitude with a dead engine. I seem to think this was for a Pup or something similar, which admittedly has a pretty light wing loading. My brain is not up to transferring this to kilometres per 1,000 meters.

That would be about a 5.28:1 glide ratio. *Sounds about right.

If it was that poor for a Pup, I wonder what it would have been for a Spad 13??!!! *

Best regards,
Dean