Thrust line

[fly boy3]

Hi all, I can understand and used side thrust and down thrust when building models. I have read about the position of the motor being above and rarely below this line in the vertical plane. Is it a fact this thrust line passes through the c/l of the wings or is it a design feature on the plan ? Is the centre line of the fus. a near enough substitute for design thrust line. In practice most of our fire walls are about the same size as our engine mounts so there is not a lot of room for vertical positioning. Thanks

[Peter Miller]

May I suggest that you invest in a copy of Kwrmode's Mechanichs of Flight. First printed in 1930s and stillin print. Amazone an Ebay will have copies.

It explains aerodynamics and angles etc in simple terms

[Peter Jenkins]

FB3 that is a complex question. The position of the CG, in all 3 dimensions, the centres of lift and drag will all affect how much you would need to alter the motor mounting direction in order to minimise trim changes with varying power. That is why the skill of the designer is so key to how well an aircraft flies. Of course, it has to be built accurately as well.

[fly boy3]

Thanks both, I did expect the answer would be beyond me any way. Thanks for book tip Peter will check it out.

[cymaz]

Posted by Peter Miller on 16/11/2020 20:42:52:

May I suggest that you invest in a copy of Kwrmode's Mechanichs of Flight. First printed in 1930s and stillin print. Amazone an Ebay will have copies.

It explains aerodynamics and angles etc in simple terms

Kermode also wrote , Flight without Formulae , another essential read.

[Robin Colbourne]

In simple terms, when a wing generates lift, it must also generate drag. The wing is therefore the pivot point about which the engine thrust acts. Imagine having an engine and propeller mounted on the seat of a swing. The thrust will pull the seat but the hanging point for the swing resists it, so the engine will pull the seat forwards and the hang points will turn this into an arc upwards.

If the wing and tail and engine are all in line, then no down or up thrust is likely to be required. Moving the engine thrust line above the wing, it will pivot around the wing, pulling the nose down, and if the engine is below the wing it will again pivot around the wing and pull the nose up.  Inclining the engine in the opposite direction helps restore equilibrium.

 

Edited By Robin Colbourne on 17/11/2020 20:30:48

[Peter Miller]

Posted by cymaz on 16/11/2020 22:43:21:

Posted by Peter Miller on 16/11/2020 20:42:52:

May I suggest that you invest in a copy of Kwrmode's Mechanichs of Flight. First printed in 1930s and stillin print. Amazone an Ebay will have copies.

It explains aerodynamics and angles etc in simple terms

Kermode also wrote , Flight without Formulae , another essential read.

Sorry.I actualy meant Flight without formulae. Same book withpout the maths an the one I sear by

[Gary Manuel]

Flight without Formulae looks interesting.

Page 1 to 85 are available to download HERE as a taster but that only gets you to straight and level flight. Ive just ordered a paperback copy of edition 4 from Amazon for £3.99 + £2.80 p+p. That should keep me quiet for a while.

[Robin Colbourne]

I too will vouch for 'Flight without Formulae'. It is a 'must read'.

Alfred Cotterill Kermode was the first Director of Education at RAF Halton. Presumably he wrote his books in terms easy enough to be understood by the Halton 'Brats' or apprentices. Kermode Hall at RAF Halton is named in his memory. Air Vice Marshal Alfred Cotterill Kermode

[Geoff S]

Posted by Robin Colbourne on 17/11/2020 23:43:44:

I too will vouch for 'Flight without Formulae'. It is a 'must read'.

Alfred Cotterill Kermode was the first Director of Education at RAF Halton. Presumably he wrote his books in terms easy enough to be understood by the Halton 'Brats' or apprentices. Kermode Hall at RAF Halton is named in his memory. Air Vice Marshal Alfred Cotterill Kermode

Hey, I object to that! My little brother (well he is 13 years younger than I am!) was a so-called 'brat' at Halton and he isn't an idiot!

I'm sure Kermode's book is excellent but my go to book on aerodynamics simplified is Alasdair Sutherland's 'Basic Aeronautics for Modellers' which is an easy read and related to our modelling needs.

Geoff

[Peter Jenkins]

Posted by Robin Colbourne on 17/11/2020 20:00:47:

In simple terms, when a wing generates lift, it must also generate drag. The wing is therefore the pivot point about which the engine thrust acts. Imagine having an engine and propeller mounted on the seat of a swing. The thrust will pull the seat but the hanging point for the swing resists it, so the engine will pull the seat forwards and the hang points will turn this into an arc upwards.

If the wing and tail and engine are all in line, then no down or up thrust is likely to be required. Moving the engine thrust line above the wing, it will pivot around the wing, pulling the nose down, and if the engine is below the wing it will again pivot around the wing and pull the nose up. Inclining the engine in the opposite direction helps restore equilibrium.

Edited By Robin Colbourne on 17/11/2020 20:30:48

The important point to remember is that as speed increases drag increases as the square of speed so a trimmed condition is only absolutely right for one speed and weight of the aircraft. For specialist aerobatic aircraft, the design is such that the aircraft is designed to be as neutral as possible and it will tolerate a slightly wider speed range for a given trim condition. Generally speaking, the power setting and attitude are the key to getting a perfectly trimmed aircraft at a given speed,

[PatMc]

Posted by Peter Jenkins on 18/11/2020 12:45:58:

Posted by Robin Colbourne on 17/11/2020 20:00:47:

In simple terms, when a wing generates lift, it must also generate drag. The wing is therefore the pivot point about which the engine thrust acts. Imagine having an engine and propeller mounted on the seat of a swing. The thrust will pull the seat but the hanging point for the swing resists it, so the engine will pull the seat forwards and the hang points will turn this into an arc upwards.

If the wing and tail and engine are all in line, then no down or up thrust is likely to be required. Moving the engine thrust line above the wing, it will pivot around the wing, pulling the nose down, and if the engine is below the wing it will again pivot around the wing and pull the nose up. Inclining the engine in the opposite direction helps restore equilibrium.

Edited By Robin Colbourne on 17/11/2020 20:30:48

The important point to remember is that as speed increases drag increases as the square of speed so a trimmed condition is only absolutely right for one speed and weight of the aircraft. For specialist aerobatic aircraft, the design is such that the aircraft is designed to be as neutral as possible and it will tolerate a slightly wider speed range for a given trim condition. Generally speaking, the power setting and attitude are the key to getting a perfectly trimmed aircraft at a given speed,

Peter, I think the magnitude of the increase in drag has little relevance in this context. To fly in equilibrium drag is equal to thrust. Thrust has therefore increased by the same magnitude as the drag & if there is no trim change they are both forces are effective from the same respective points.

However the model is no longer in equilibrium as an increase in speed has caused an increase in lift from the wing which is not countered by the same magnitude of lift being generated by the (less efficient) tailplane. This imbalance will cause the nose to rise & is the reason why a higher tailplane : wing area improves stability during speed changes.

[thunderstreak.keith]

so how does it work with a jetex50 fitted underneath a k/kraft super sabre? or is it just trial and error? i only ask as i have just built one and fitted a jetex

[Peter Jenkins]

It will tend to pitch up unless you have some downthrust built into the installation.

More to the point, where do you get the Jetex pellets from? Somewhere, I have a Jetex motor from oh about 1964 but no pellets to run it. Do tell.