High speed tail surfaces.
07-23-2005, 08:28 AM
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High speed tail surfaces.
After going to the USAF museum (Dayton Ohio) for about the 8th time
I have a question.
Whey are the tail surfaces on some high speed aircraft so thick. ...x-15 especially. The trailing edge is extremely thick and flat. There is no tapering at all.
I have a couple of theories but I want to know for sure. Thanks
I have a question.Whey are the tail surfaces on some high speed aircraft so thick. ...x-15 especially. The trailing edge is extremely thick and flat. There is no tapering at all.
I have a couple of theories but I want to know for sure. Thanks
07-23-2005, 11:59 AM
#2
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RE: High speed tail surfaces.
There's no air up there! (Where the X-15 flew.)
THe vertical in particular is in the last part of the disturbed flow around the plane, and needed to be truncated to be effective.
THe vertical in particular is in the last part of the disturbed flow around the plane, and needed to be truncated to be effective.
07-23-2005, 01:17 PM
#3
RE: High speed tail surfaces.
OK- I will buy that
Also on the full scale craft such as EDGE a truncated aileron is used .
Some windy tunnel folk claim this is a bad deal -I believe in results and the EDGE wing really works -so what is wrong with the thick blunt ailerons ?
If theorysays it is wrong - SOMETHING is wrong.
Also on the full scale craft such as EDGE a truncated aileron is used .
Some windy tunnel folk claim this is a bad deal -I believe in results and the EDGE wing really works -so what is wrong with the thick blunt ailerons ?
If theorysays it is wrong - SOMETHING is wrong.
07-23-2005, 01:20 PM
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RE: High speed tail surfaces.
You'll also note that they tail airfoils were simple flat wedge shapes with very knife sharp leading edges. It's all got to do with how the air flows at hypersonic speeds. At subsonic speeds the X15 was a drag bucket that barely could hold itself aloft but at over Mach 1 things started to work a lot better. The shape of the fuselage, wings, tails and airfoils were selected to work at multiple Mach numbers where the shock waves are a key component of both the overall planform and airfoil design. The design is wholly integrated from nose to tail so that all the shock waves work together.
Howzat? Did I pass?
Howzat? Did I pass?
07-23-2005, 03:27 PM
#5
RE: High speed tail surfaces.
DM's excellent point > aero flow at supersonic speeds is a different case.
As far as THEORY being wrong... Improper theory or misapplied theory will probably not be valid but this prove nothing.
Allan
As far as THEORY being wrong... Improper theory or misapplied theory will probably not be valid but this prove nothing.
Allan
07-23-2005, 04:18 PM
#6
RE: High speed tail surfaces.
Prove a negative?
Hypersonic flight is a whole different ballgame
But---
Our models are flying at the bottom of the speed scale .
I meant that the airfoil theories discussed here never seem to include the blunt LE squared TE types widely accepted as excellent aerobatic foils on EDGE/CAP/EXTRA
the high point ? - The leading edge
the trailing edges ? chopped of short and thick-
Curve in airfoil? none
Did none of this make it into the textbooks?
Based on what I see on these planes -coupled with what I have learned as fact--on ultra light models (thin flat plates work best)- I have gone to the lightest thinnest sections which don't flex - and they work great .
Hypersonic flight is a whole different ballgame
But---
Our models are flying at the bottom of the speed scale .
I meant that the airfoil theories discussed here never seem to include the blunt LE squared TE types widely accepted as excellent aerobatic foils on EDGE/CAP/EXTRA
the high point ? - The leading edge
the trailing edges ? chopped of short and thick-
Curve in airfoil? none
Did none of this make it into the textbooks?
Based on what I see on these planes -coupled with what I have learned as fact--on ultra light models (thin flat plates work best)- I have gone to the lightest thinnest sections which don't flex - and they work great .
07-23-2005, 04:58 PM
#7
RE: High speed tail surfaces.
Every aero theory I have seen predicts that a sawn off shape (Kamm theory) is better than a too-round trailing edge. This is certainly consistant with the wind tunnel tests I have been part of too.
The theory holds up, as long as it is not "stretched" too far. At extremely low speeds, theory doesn't (at least shouldn't) be used to attempt to explain air flow. Once airspeed has reached a stable point then the theory is pretty good. Of course at foamy speeds, as you well point out, it really shouldn't be applied. At speeds of 15 mph or more, it holds very well.
There are, I will grant you, a lot of folks who quote theory out of context and without full understanding. You can push anything too far and, if that is your point, I will agree with you.
On the other hand, I have seen many many times where low speed flow, once stablized, is essentially the same as higher speed flow, even with regard to separation points, laminar vs turbulent flow boundary layer, and so forth. When we would run flow visualization on a car, the ink would flow and the tuffs would blow at pretty low speeds.
I am not speaking as an aerodynamicist but as someone who has still spent maybe 900 hours actually working in a wind tunnel. How much tunnel time do you have?
Allan
The theory holds up, as long as it is not "stretched" too far. At extremely low speeds, theory doesn't (at least shouldn't) be used to attempt to explain air flow. Once airspeed has reached a stable point then the theory is pretty good. Of course at foamy speeds, as you well point out, it really shouldn't be applied. At speeds of 15 mph or more, it holds very well.
There are, I will grant you, a lot of folks who quote theory out of context and without full understanding. You can push anything too far and, if that is your point, I will agree with you.
On the other hand, I have seen many many times where low speed flow, once stablized, is essentially the same as higher speed flow, even with regard to separation points, laminar vs turbulent flow boundary layer, and so forth. When we would run flow visualization on a car, the ink would flow and the tuffs would blow at pretty low speeds.
I am not speaking as an aerodynamicist but as someone who has still spent maybe 900 hours actually working in a wind tunnel. How much tunnel time do you have?
Allan
07-23-2005, 06:24 PM
#8
RE: High speed tail surfaces.
Tunnel time
absolutely none.
Never claimed any
Never will
practical application time with many model types -
lots of it.
The EDGE/EXTRA/CAP I spoke of fly in the 50-250 mph area and are full scale machines man carrying .
why are airfoils used on these machines never dicussed?
The really high performance foamies, I know , are simply poo pooed as abberations - by those who don't , won't , can't fly em.
No problem - that's the way the world works.
Cars are a love affair I never gave up - I read every thing I could on the original Kamm back stuff -when they were shaking up the establishment.
The tunnel tests sound interesting -and I do understand a lot of it
When we were really into goin fast -The thing that really interested me was how to get the car to stay on the ground-without down load .
The 1/4 mile boys really cram in the load -but traction is a huge part of that game .
My last model airplane carrier hot rod -- was a Volvo wagon with a warmed over 5.0 Ford engine -
a solid car at speed.
Innocent looking --no?
absolutely none.
Never claimed any
Never will
practical application time with many model types -
lots of it.
The EDGE/EXTRA/CAP I spoke of fly in the 50-250 mph area and are full scale machines man carrying .
why are airfoils used on these machines never dicussed?
The really high performance foamies, I know , are simply poo pooed as abberations - by those who don't , won't , can't fly em.
No problem - that's the way the world works.
Cars are a love affair I never gave up - I read every thing I could on the original Kamm back stuff -when they were shaking up the establishment.
The tunnel tests sound interesting -and I do understand a lot of it
When we were really into goin fast -The thing that really interested me was how to get the car to stay on the ground-without down load .
The 1/4 mile boys really cram in the load -but traction is a huge part of that game .
My last model airplane carrier hot rod -- was a Volvo wagon with a warmed over 5.0 Ford engine -
a solid car at speed.
Innocent looking --no?
07-23-2005, 09:12 PM
#9
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RE: High speed tail surfaces.
Dick, at lower speed like we fly I understand that the SQUARE blunt trailing edges produce less flutter than if they are rounded because the air will peel off the sharp edge more cleanly and then flow over a bit of a turbulent dead zone in the wake of the blunt edge. In effect the turbulence forms the missing trailing edge. That's also the Kamm theory I believe.
Rounded trailing edges can produce an action where the air tends to alternately lick over and down and then flips to over and up and sets up an oscillation much like a flapping flag. A great way to induce flutter of course and a good reason for keeping trailing edges blunt if they can't be sharp edged.
Selig's wind tunnel work from the original Soartech 8 showed that there were very real and significant drag reduction to be had by keeping the trailing edge sharp as opposed to even 2 to 3% square blunt. But of course that sort of advantage is only needed for speed runs with racers, be they powered or glider.
As I recall a flat plate airfoil produces a separation bubble at a very low angle of attack but that bubble then acts like an upper surface curve. It's been a long time but I seem to recall that a flat plate Cl vs Cd curve is very linear with a surprisingly high max lift coefficient but an almost direct drag cost as the lift rises. But as we are seeing on the right type of model that sort of charactaristic can be a good thing. In the case of 3D designs it allows the model to dump speed pretty much on demand while maintaining lift. Not a bad thing at all given the mission.....
Rounded trailing edges can produce an action where the air tends to alternately lick over and down and then flips to over and up and sets up an oscillation much like a flapping flag. A great way to induce flutter of course and a good reason for keeping trailing edges blunt if they can't be sharp edged.
Selig's wind tunnel work from the original Soartech 8 showed that there were very real and significant drag reduction to be had by keeping the trailing edge sharp as opposed to even 2 to 3% square blunt. But of course that sort of advantage is only needed for speed runs with racers, be they powered or glider.
As I recall a flat plate airfoil produces a separation bubble at a very low angle of attack but that bubble then acts like an upper surface curve. It's been a long time but I seem to recall that a flat plate Cl vs Cd curve is very linear with a surprisingly high max lift coefficient but an almost direct drag cost as the lift rises. But as we are seeing on the right type of model that sort of charactaristic can be a good thing. In the case of 3D designs it allows the model to dump speed pretty much on demand while maintaining lift. Not a bad thing at all given the mission.....
