View Full Version : Aerobatics cleared in straight wing classic jets
Treble one
23rd Feb 2018, 19:55
As reported here.
The CAA have lifted the ban on aerobatics over land for straight wing classic jets. Full aerobatic displays are now permissible.
http://www.air-shows.org.uk/2018/02/airshow-news-straight-wing-ex-military-jet-aircraft-cleared-to-conduct-full-aerobatic-displays-at-uk-airshows/
Why not swept wing jets for those who have lots of time on them, like me ?
(I admit I couldn't do it now).
Have they defined what they mean by "straight wing"? Equal taper? Tapered about the 25% chord line? Anything from straight LE to straight TE?
PDR
Good question.
Maybe they mean a wing where the mean chord line is normal to the longitudinal fuselage axis.
Good question.
Maybe they mean a wing where the mean chord line is normal to the longitudinal fuselage axis.
That would be almost none of them then! Sweep is normally defined at the quarter-chord line so the mean chord line is normally swept forwards on a straight-winged aerplane, but some people see aircraft like the Fouga Magister as straight-winged (straight TE), and some even see the F18, 22, 35 as straight winged (TE swept forwards).
And what about this swept-forward winged jet doing it's aerobatic display:
http://www.youtube.com/watch?v=rB1QzzjX1B0
I think what I'm getting at is that the swept/straight thing isn't really the discriminatory they should be looking at. I'd rather see them review and approve specific types, or better still specific aeroplanes as supported by a specific maintenance organisation using specific procedures and flown through a specific routine by a specific pilot.
PDR
LOMCEVAK
24th Feb 2018, 14:22
There are three fundamental differences between straight and swept wing aircraft with respect to displaying:
1. Induced drag. A swept wing has much higher induced drag than a straight wing. There are two implications from this. First, during flat, turning manoeuvres there is a much greater potential for speed loss with a swept wing than for a straight wing. Secondly, there will be much greater variability of apex height and airspeed (energy) in looping manoeuvres with a swept wing aircraft than with a straight wing aircraft. From a given pull-up airspeed the apex height for a straight wing aircraft is reasonably consistent. However, with a swept wing there is a large range of heights that can be achieved, and it is easily possible to apex below a height from which a safe pull-through may be flown. Therefore, it is essential that a 'gate height' philosophy must be used when flying looping manoeuvres in a swept wing aircraft.
2. Stall Angle of Attack. A swept wing stalls at a much higher AoA than a straight wing. Therefore, when flying at maximum instantaneous pitch rate there is a large difference between pitch attitude and flightpath which can generate deceptive visual cues during a loop pull-out. Also, very few historic swept wing aircraft are cleared for intentional stalling and may depart if a full stall is encountered.
3. Swept wing aircraft typically fly at a higher airspeed which results in a larger turn and loop radius. This makes it much more challenging with respect to the required horizontal and vertical airspace.
What defines an aircraft as having swept wing characteristics needs to be considered on a case-by-case basis using suitably knowledgeable experts; a mathematical definition is not necessarily appropriate. In many ways, displaying straight wing jets such as the Jet Provost has far more commonality with powerful piston warbirds than with aircraft such as the Gnat or Hunter - except that the directional control aspects are easier than with piston aircraft!
Rgds
L
I agree with you. I did say "maybe".
During my short time at the CAA I was surprised by the number of people who were not pilots.
There are three fundamental differences between straight and swept wing aircraft with respect to displaying:
1. Induced drag. A swept wing has much higher induced drag than a straight wing. There are two implications from this. First, during flat, turning manoeuvres there is a much greater potential for speed loss with a swept wing than for a straight wing. Secondly, there will be much greater variability of apex height and airspeed (energy) in looping manoeuvres with a swept wing aircraft than with a straight wing aircraft. From a given pull-up airspeed the apex height for a straight wing aircraft is reasonably consistent. However, with a swept wing there is a large range of heights that can be achieved, and it is easily possible to apex below a height from which a safe pull-through may be flown. Therefore, it is essential that a 'gate height' philosophy must be used when flying looping manoeuvres in a swept wing aircraft.
2. Stall Angle of Attack. A swept wing stalls at a much higher AoA than a straight wing. Therefore, when flying at maximum instantaneous pitch rate there is a large difference between pitch attitude and flightpath which can generate deceptive visual cues during a loop pull-out. Also, very few historic swept wing aircraft are cleared for intentional stalling and may depart if a full stall is encountered.
3. Swept wing aircraft typically fly at a higher airspeed which results in a larger turn and loop radius. This makes it much more challenging with respect to the required horizontal and vertical airspace.
What defines an aircraft as having swept wing characteristics needs to be considered on a case-by-case basis using suitably knowledgeable experts; a mathematical definition is not necessarily appropriate.
Yes, agree with all that (I think it's what I meant in my last paragraph). On a pedantic point I think your item (2) is more related to low aspect ratio than sweep per se - the description fits the handling of a T-38, but not a 747 even though the 747 has greater sweep.
PDR
Basil
24th Feb 2018, 16:22
PDR1, Thanks for the Blanik video. Impressive rolling manoeuvres; couldn't see how much rudder was involved. Used a lot of brake; wonder if the jet has a thrust lever or if it's an ON/OFF machine.
LOMCEVAK, Thank you for that clear and concise differences briefing. I've only flown the JP and haven't tried aeros in a swept wing jet transport ;)
Basil
24th Feb 2018, 16:28
On a pedantic point I think your item (2) is more related to low aspect ratio than sweep per se - the description fits the handling of a T-38, but not a 747 even though the 747 has greater sweep.
Is that AoA related to the general air mass or to the air close to the wing which (I understand) has a downwash component caused by the proximity of the wing? (Induced downwash?)
Forgive me if my aerodynamic knowledge is a little dated cf modern thinking.
... and haven't tried aeros in a swept wing jet transport ;)
Well it would make for less tedium while holding in the stack...
:E
PDR
Mogwi
24th Feb 2018, 16:59
Have barrel rolled a 747 (in the sim!) and the 707 was famously pictured inverted over the "Boing" factory in SEA during a similar manoeuvre.
LOMCEVAK
24th Feb 2018, 18:44
Mogwi,
747 - you and me both!
PDR1,
I think that you mean my previous point 1 relating to induced drag, and in that you would be absolutely correct; it is a function of aspect ratio. I was being a little too generic in discussing in the terms of the thread. However, related to my point 2, the CL vs alpha curve and AoA max is a function of 1/4 chord sweep angle and wing section, not aspect ratio.
Mogwi,
747 - you and me both!
PDR1,
I think that you mean my previous point 1 relating to induced drag, and in that you would be absolutely correct; it is a function of aspect ratio. I was being a little too generic in discussing in the terms of the thread. However, related to my point 2, the CL vs alpha curve and AoA max is a function of 1/4 chord sweep angle and wing section, not aspect ratio.
Hmmm... we may have to agree to disagree on this. I was under the impression that stalling angle increases as AR decreases, and this source (https://surjeetyadav.wordpress.com/2014/01/28/aspect-ratio-and-its-effect/) (among others) seems to agree with me, providing this illustration:
https://surjeetyadav.files.wordpress.com/2014/01/ar-stall-aoa-rc-airplane.jpg
PDR
LOMCEVAK
25th Feb 2018, 14:27
PDR1,
Point taken; I was just considering the effect of sweep angle on a given wing and, for a given wing, increasing sweep angle reduces AR. I was considering taking a given straight wing and sweeping it back ie. maintaining the same area (excepting the wetted area consideration within the fuselage). In the graph that you have presented, how is the AR varied, changes in wing area or change in wingspan? Also, to maintain a constant CL max whilst varying AR would require many changes to the wing planform and, possibly, wing section. Overall, it is not simple and, to go back to the thread, each type needs consideration on a case by case basis as to which category it should be in for displaying.
Rgds
L
Arclite01
26th Feb 2018, 09:02
Several things WRT to the Blanik Video:
1. The jet engine seems to make a lot of noise but the thrust doesn’t appear to be huge
2. Because the speed is slow I would have though the amount of rudder required wouldn’t differ much from that applied as a normal glider (say on the winch launch or aerotow). Thrust is certainly down the centerline of the airframe and there is no torque to worry about…………..
3. The Blanik has very nicely harmonized controls anyway but she will drop a wing at the stall
4. What a pity they didn’t make it mid-wing rather than high wing as the vis would have been so much better. But it’s still a very elegant looking aeroplane and nicely flown in the video.
5. Not sure venue in the video is great though – high crop and power wires – Hmmmmm
I wonder what sort of licence you need to fly that then ?
Arc
PDR1,
In the graph that you have presented, how is the AR varied, changes in wing area or change in wingspan?
I don't know (it's not my graph!) but I suspect neither. I think it is presenting how the lift slope of an ideal wing will be different for different AR.
Also, to maintain a constant CL max whilst varying AR would require many changes to the wing planform and, possibly, wing section. Overall, it is not simple
Again, Im guessing - I suspect the graph presents all AR curves with the same roll-off value of Cl to indicate that the relationship being described is only valid in the "approvimates to straight" part of the lift-slope curve-line.
...and, to go back to the thread, each type needs consideration on a case by case basis as to which category it should be in for displaying.
Couldn't agree more, as I said in my earlier post. Different aeroplanes have very different handling and complexity and they should all be assessed individually. I'm led to believe (after asking the late Eric Brown) that the Me163 had very simple systems and very benign handling, but I'm not sure I'd consider approving it for a public display with a 300hr PPL-IR in the seat unles she/he had dozens of hours on the type. But I'd be happy to approve a 300hr PPL for displaying (say) a Cassutt at low-level if 150 of those hours had been on-type and ~100 of them had been at low level (ie racing)*.
PDR
* these numbers are based on what I understand to be possible from my possibly false memory of a conversation with Robin Voice about 25 years ago
Several things WRT to the Blanik Video:
[excellent points snipped]
Absolutely. I was just pointing out that it would fall within the category "swept wing jets" if the category didn't have any specification beyond the words in the title.
PDR
BEagle
26th Feb 2018, 12:30
The CAA has released SN-2018/001, the scope of which is to restrict the operation of ex-military jet aircraft at flying displays. It can be read here: http://publicapps.caa.co.uk/docs/33/SafetyNotice2018001.pdf .
On the topic of aspect ratio and sweepback, though never good enough to have been a TP (how I wish...), my A2 notes suggest that although sweepback reduces aspect ratio, the same wing when swept behaves as a low cambered, low aspect ratio wing with reduced LE diameter - the latter being the main cause of a lower CLmax, which occurs at a lower value of α than for a 'straight' wing.
Very useful for my A2 on the Bulldog, this swept wing stuff....:hmm:
Edited to add Reduced LE diameter plus surface roughness results in a lower stalling α - which is something people who fly aircraft with stall warning devices must remember when flying in icing conditions, as the aeroplane may stall before the stall warner alerts the pilot.
POBJOY
26th Feb 2018, 12:50
It is absolutely NOTHING to do with the aircraft but all to do with the person flying it, the current experience level, and performing a sequence that is 'safe for the conditions at the time'. In a high performance Jet or warbird the public hear the noise, sense the power, and see the speed, and do not really want to have it disappearing way up in the sky for no real effect. There are clearly defined rules for displays, and the pilots allowed to operate, but if someone chooses not to stay within those limits it is no fault of the machine or the system.
Airshows have stated rules and conditions with the ANO covering the wider area, but a poor pilot decision is just that; the aircraft very rarely is the guilty party.
Treble one
26th Feb 2018, 13:38
PDR1, Thanks for the Blanik video. Impressive rolling manoeuvres; couldn't see how much rudder was involved. Used a lot of brake; wonder if the jet has a thrust lever or if it's an ON/OFF machine.
LOMCEVAK, Thank you for that clear and concise differences briefing. I've only flown the JP and haven't tried aeros in a swept wing jet transport ;)
Basil, you should find the C-27J Spartan a suitable mount if you want to have a go at aeros in a transport aircraft. The chaps at the RSV in the Italian AF do some unspeakable things with it....rolls, loops, nearly choked on my pork pie first time I saw them....
roving
26th Feb 2018, 16:51
Here you go Treble one
https://www.youtube.com/watch?v=5uU7neiDJk0
POBJOY
26th Feb 2018, 17:44
That's a proper display bar none.
Fareastdriver
26th Feb 2018, 18:46
You can see why they put Chianti in baskets with handles to tie them down.
Treble one
28th Feb 2018, 21:02
Here you go Treble one
https://www.youtube.com/watch?v=5uU7neiDJk0
Thank you roving-first saw this display at RIAT through a long lens, standing next to the old man. When the pilot looped it, we both looked at each other as to confirm we'd actually seen him do it.
Then he rolled it. Twice. The knife edge pass is pretty impressive too.
Hats of to the team at the RSV.