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Jetstream: Flight Tests (incl. Spiral Dive)  
User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Posted (15 years 1 month 1 week 4 days 12 hours ago) and read 3835 times:


I know I am probably annoying some people if I post a second trip report about the same flights, but only 5 or so people bothered to read the first one, and I thought maybe a different title and a better report structure might help, so here it goes:

First, I will cover some theory explaining what the flights were all about. If you do not want to read this, I have marked where the actual report about the actual flights begins with a set of ****s.

The 2-day course we took is run by Cranfield University, using their flying laboratory G-NFLC. Here is a photo of the interior. Notice the guy facing the camera - he was our instructor in two of the three flights.

As you can see, it is a comfy little plane with 10 passenger seats, 1 instructor seat and for everyone's pleasure there are PTV's installed. Unfortunately, I could not switch the channel to a movie programme, and so I just had to content myself with data about our flights....

Anyway: The first flight was all about drag measurement. So how is drag measured in an aircraft? Well, if the measurements are to be taken during steady, constant-velocity flight, it is fully sufficient to measure the thrust of the turboprop engines. This part of the course was basically about the aircraft performance course we had had last year, and it wasn't the most interesting bit.

The second flight is mainly about curved flight.

The third part is, from an airplane dynamics point of view, the most important: The different reactions the plane has to certain perturbations are measured:

Short period oscillation (Pitch):
When a plane is perturbed (disturbed away) from the steady (trimmed) state, and provided it is statically stable, it will attempt to return to the state it is trimmed to be in. Hence, if the pitch is increased for a very short moment, and the controls are released, the plane will immediately head back down again - and overshoot a little and come back up etc. etc. The result is a short period oscillation, that doesn't go through more than 1 or 2 periods before being dampened out.

Dutch Roll:
The Dutch Roll is the result of a perturbation in yaw. It is essentially the equivalent of the short period for yaw perturbations. However, as there are fewer and smaller vertical surfaces than horizontal ones, it does not stop quite as soon as the pitch oscillation. More interestingly, there is the secondary effect of roll: When the plane yaws to one side, the wing on the other side generates a bit more lift, and hence a roll motion is started. When the plane yaws back to the other side, the wing generates less lift and the roll changes direction. Hence, the yaw oscillation causes a roll oscillation as well. In a passenger aircraft, you can recognise that the plane if flying a dutch roll if you watch the wingtips and they trace an elliptic shape across the horizon. However, it is not too likely that you will see this a lot, as the pilot will suppress any such motions quite soon.

The phugoid is the long-period oscillation in pitch. You might know that any vibration system has several resonant frequencies. Anologously, the dynamic system "aircraft" has different oscillation periods that are caused by perturbations. If the perturbation in pitch is big enough / lasts long enough, the plane will not experience the short-period oscillation described above, but a phugoid motion. This oscillation has a very long period (40-50 seconds) and it does not die down very quickly - it can continue for several minutes easily. You will probably not notice this much in a passenger flight, as pilots will automatically suppress this oscillation.


We were quite lucky, to begin with. IC was going to send out 2 groups of students to Cranfield at the end of March. However, the first group got delayed until May because of "technical problems". We were told we had a 50:50 chance of being allowed to fly, and luckily, the course took place on 29/30th March as planned.

After we had been given some lectures covering all the theory I have just mentioned (very briefly) above, we were ready to fly. We were given a safety briefing for all three flights, which included some complicated instruction on the topic of "how to tind and open your sickbags". (These were at the side, not at the seat in fron of you, and they were packaged in a tiny paper bag. So any weakly-stomached people would have to
a) grab the little paper bag from its hiding place under the window,
b) take the thin plastic sickbag out of the paper bag and
c) open the plastic bag, which anyone who has ever tried to open a supermarket plastic bag can tell you, is not easy if you're in a hurry)

Finally, we were also told the nature of the technical problem: A window had broken, and as there are no longer any spare parts being produced, they had to buy a military Jetstream window and certify it to fly on the civilian version first.... Bureaucracy....

Then, rather eagerly, we boarded for the first flight. Everyone had headphones, as the laboratory Jetstream is not equipped with any means to silence the cabin - they would weigh too much. We could hear all the tower communications, and the pilot going through his checklist. As we taxied towards the runway, he went through loads of items, and it was quite funny to hear this for the first time...

After lining up, the pilot applied full thrust before releasing the breaks, and off we went - I never thought little turboprops could accelerate that well! The first experiment consisted mainly of flying at different heights at different constant velocities and measuring the thrust. So it wasn't the most exciting of experiments, but it was a fun half hour nevertheless. We soon returned to the airfield and landed. This is a bit of an experience: Sitting in the last row and looking through the pilot's windows is really a weird feeling. First, you only see the ground and the runway heading straight for you, then, just before touchdown, the nose goes up and all you see is sky.... It really is an odd angle to be watching the landing from... When I left the plane I grinned like a moron, happy, happy happy - yes, I find flying fun, always! My fellow students remarked upon this -even a relatively unexciting flight like that one got me grinning...

The second experiment was to be the most feelable. The flight was scheduled to take place right after breakfast on the second day, and the experiment was to be about curving flight. It sounded harmless enough, until we were told that it involved flying curves at up to 2G normal accelerations! Well, I had been very careful not to eat much during breakfast - the last thing I wanted is to go through all the hassle with the sickbags....

We departed once again, and after we reached the chosen altitude, we started flying curves and circles with ever-decreasing radii. At first, it felt not much more noticeable than an airliner flight, but after each measurement point was taken, the pilot tightened the curve, and it got increasingly hard to lift up the hand to the monitor where we had to take measurements. The feeling in the head is also quite weird, it basically feels as if your skull was a big rock pressing your brain down... OK, so 2G isn't much, I suppose, but it was far more than I had ever experienced - I had never been on a rollercoaster ride before... I would rate the feeling as 20 times higher than the forward acceleration you feel in a 737 on the runway. It was definitely a weird experience, but nowhere near as weird as getting back out of that acceleration: Suddenly, it felt as if the world dropped and your brain was about to go up and push the skull away. Well, the sensation was not painful, but it is hard to describe it for someone as inexperienced at descriptive language as I am...

The second landing was also worthwhile, and for the first time I noticed those 4 red and white lights by the side of the runway that show the pilot whether he is too high or too low. It was fun watching those lights change from 3 red to 3 white and back before sttling on 2/2.

The third flight was to be the one covering the various oscillations described above, plus the extra bonus of a spiral dive. Once again, we departed (just before lunch) and headed for a nice place with free airspace. The first modes we flew were the short-period pitch oscillation. In flight, this really isn't noticable: It feels pretty much like a bump in the road, and not more. The second period is really not noticeable at all.

Then, we flew the dutch roll. This is a noticeable mode, especially if you watch the wingtips tracing the ellipse against the horizon. It is a left-and-right see-saw experience that is not really very strong, just as if someone was slowly shaking you around a bit. We then tried the manoeuvre again in landing configuration (flaps and gear out), which made it a bit stronger but otherwise no big difference occurred.

The third flight mode to be tested was the phugoid. In level flight, it feels like an odd up-and-down motion, and you do notice the phugoid - if the pilot doesn't stop it. This wave motion continued for several minutes getting weaker only very gradually.

However, all that was to be different in climb: When we also measured the phugoid in climbing flight, we slowly realised that this was an unstable phugoid: It got stronger and stronger with every period. The up and down became absolutely fantastic, until we shot up at the sky at incredible pitch angles (it felt like 60 degrees) ind ultra-low velocities (70 knots or so), only to then shoot back down at very high velocities.... It is truly incredible. Especially noticeable was the odd fact that we could not stall: The stall speed usually is around 100-110 knots, and yet we flew slower and straight at the sky without stalling.
We were told this is due to the angle of attack of the wings towards the airflow being constant - i.e. the flow never separates. So while in level flight, if you fly at such a low velocity, your angle of attack needs to be really big to generate enough lift to hold you up there, the phugoid basically shot us up with so much momentum that we were "flying" like a stone thrown up, and not by the lift force of the wing, and hence the angle of attack of the wing could remain constant and we did not stall. Well, we were given this information AFTER we had experienced the phugoid, and I remember sitting there thinking "We're gonna stall! We're gonna STALL" for a few moments....

The final manoeuvre to be flown was the spiral dive. In our lectures we had been told numerous ways to die in a spiral dive, and the instructors really stressed how fatal this mode could be. Very calming, huh?
So when the pilot inclined the plane to the left and down, we eagerly awaited this initial state to grow worse and worse (i.e. steeper left curve, dive), but: Nothing happened. The plane just flew along a bit and eventually the pilot levelled it again.
Then, we tried the same thing to the right, and sure enough: The bank increased and the dive got steeper and faster... we continued for a few more seconds until the whining of the air got quite louder than usual before pulling out of it and straightening the plane. We were told this behaviour (not to the left, but to the right was due to the moments the simultaneously rotating propeller exert on the plane. At the peak moment of the spiral dive, I would estimate (by pure gut feeling and guesswork) that the G-forces never exceeded 1.3-1.5 G - so the manoeuvres definitely weren't as gut-wringing as the second flight.

The overall experience was very enjoyable, and if I had to choose which one was my favourite flight and manoeuvre, I'd have to say I really liked the unstable phugoid best - it was really incredibly going up at this immense angle!

I hope you enjoyed this report, and I hope you will forgive me for posting a second report about the same flight, but I did feel this was an interesting experience worth sharing, and maybe I just picked an uninteresting title for the first one!

I would also like to thank our instructors and pilot for the great job they did!



11 replies: All unread, jump to last
User currently offlineB747-437B From , joined Dec 1969, posts, RR:
Reply 1, posted (15 years 1 month 1 week 4 days 8 hours ago) and read 3281 times:

Awesome report. Sure as hell beats the regular stuff we get on here about Johnny going to Grandma's house.

User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 2, posted (15 years 1 month 1 week 4 days 5 hours ago) and read 3289 times:

B747-437B: Glad you enjoyed the report!

I just wanted to add something I had forgot in the explanations section:

The Spiral Dive:
When an aircraft is perturbed away from trimmed state into one that has both bank and negative pitch attitude, what can happen is that the plane does not stabilize itself. Instead, the deviation from trimmed state is constantly increased - i.e. the pertrubation caused an instability. This ever-increasing roll and ever-decreasing pitch attitude is commonly known as spiral dive. It is most perilous to pilots flying under IFR conditions, because they might not notice the initial perturbation. Then, if they do not fully trust their instruments, they lose valuable time while the plane gets more and more out of control until either
a) the wings break off or
b) the pilot pulls out of it, but he was too late and the velocity is so high that pulling up breaks the wings off.
At least, this is what we were told in that rather amusing lecture about "What we are going to do next"...

We were also told that this dynamic instability causes several fatal accidents a year, including the John Kennedy Jr case.... so it is quite an important thing that pilots need to be aware of.



User currently offlineB747-437B From , joined Dec 1969, posts, RR:
Reply 3, posted (15 years 1 month 1 week 3 days 19 hours ago) and read 3265 times:

Ikarus - as someone with a background in Aerospace Engineering, I found this to be fascinating. I would have killed to have had a lab like this to play with! And your technical explanations are very competent too. However, most of the kids in this forum a) dont understand the principles of flight and b) dont care if its not a Boeing or Airbus product. So don't sweat the lack of views/responses - rest assured that those who do care find it very interesting.

User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 4, posted (15 years 1 month 1 week 3 days 14 hours ago) and read 3254 times:

Thanks a lot, B747-437B!

I was actually wondering whether my explanations were correct, as I'm still learning the stuff and I'm a bit insecure about it. Thanks for your posts, they definitely give me a bit more confidence in this report!



User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 5, posted (15 years 1 month 1 week 1 day ago) and read 3249 times:


I have decided to add a picture I took of our Jetstream:

It is currently waiting in the uploading queue, which is where this link leads.



User currently offlinePilot1113 From United States of America, joined Aug 1999, 2333 posts, RR: 11
Reply 6, posted (15 years 4 weeks 1 day 19 hours ago) and read 3227 times:

This was very interesting and very informative! You wrote a well written article here.

There isn't enough money you could pay me to go up and do half of those things and I'm a pilot (albeit the private kind)!

However, it sounded very exciting to say the least! I like the pictures you've added. They really show us what it looked like inside that cramped airplane and what kind of airplane it was.

Thanks for an awesome thread!

- Neil Harrison

User currently offlineGyro From , joined Dec 1969, posts, RR:
Reply 7, posted (15 years 4 weeks 1 day 9 hours ago) and read 3226 times:

Great report Ikarus!!! I must agree that describing the feeling of G's is difficult but boy are they fun!!! If you ever get a chance to go up with a Commercial Pilot ask him for some Lazy 8's or Chandelles. I had to do those a couple of months ago and they are basic commercial maneuvers that pull quite a bit of G's aswell... I've noticed that there is only 1 pilot in the jetstream, is it usually operated by just one?

User currently offlineCapt.Picard From , joined Dec 1969, posts, RR:
Reply 8, posted (15 years 4 weeks 1 day 9 hours ago) and read 3219 times:

Thanks for your report Ikarus;

I enjoyed it tremendously; needless to say I have "archived" it onto Word!! Big grin

I have to experience this unstable phugoid sometime, sounds like great fun!

Will help when I'm training with BA in 2 years....  Smile/happy/getting dizzy Big grin

You guys are too lucky......


User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 9, posted (15 years 4 weeks 1 day 4 hours ago) and read 3206 times:

Wow! No one reads it for 10 days, and then three (?!?!) replies appear! I'm delighted! It's not too boring after all!

Thanks - but the manoeuvres aren't half as bad as they sound. It's just odd and different from the usual stuff... You should really give it a try sometime

Yes, this Jetstream is usually operated by one pilot only. I suppose it's the same for commercial Jetstreams, but I can't say for sure. For takeoff and landing, however, the instructor always sat in the copilots seat...

Capt Picard:
Maybe you should try and get your university to fund such a course as well. You know, have a look at geographical formations, moraines etc. while doing a 2G curve. It generally makes it easier to look out of the window if the window is pointing straight down Big grin



User currently offlineUSAFHummer From United States of America, joined May 2000, 10685 posts, RR: 51
Reply 10, posted (15 years 4 weeks 1 day 1 hour ago) and read 3211 times:

On my one and only Jetstream flight (Jetstream 31 LAX-SAN by Trans States Airlines representing US Express) there were two pilots...


Chief A.net college football stadium self-pic guru
User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 11, posted (15 years 3 weeks 6 days 6 hours ago) and read 3194 times:

I dunno. Maybe they have regulations that on passenger flights, two pilots are needed (in case one of them dies or something)?

We were told two things. First, in our confirmation letter, we were told that this aircraft is exempt from normal CAA flight regulations for passenger transport. Second, they told us before the flight that the plane was designed to be flown by a single pilot. That's all I know, so maybe someone else can shed light on this?



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