True Air Speed Vs Indicated Air Speed

[pbehn]

Looks pretty good save 'TAS = speed over the ground". TAS is that speed which the airframe/wing 'sees'. Otherwise a 90kt headwind is uninteresting.

A very learned religious scholar opined a few years ago that claiming the earth rotated was ridiculous. If the equator was rotating at around 1,000 MPH then aircraft would never be able to land, simples.

A question for pilots. I once saw a Lancaster flying overhead coming in to land and obviously labouring into a head wind, but I was stood on the ground and it was obviously being buffeted by turbulent air. If you are at high altitude, flying above cloud in a strong but smooth headwind, do you even know without modern navigation aids?

 

[drgondog]

A very learned religious scholar opined a few years ago that claiming the earth rotated was ridiculous. If the equator was rotating at around 1,000 MPH then aircraft would never be able to land, simples.

A question for pilots. I once saw a Lancaster flying overhead coming in to land and obviously labouring into a head wind, but I was stood on the ground and it was obviously being buffeted by turbulent air. If you are at high altitude, flying above cloud in a strong but smooth headwind, do you even know without modern navigation aids?

No. That is why fundamental principles of navigation (time-ground speed- course-waypoints/sun position), particularly when no decent frame of reference for ground speed exists. I would hate to make a living flying long distances of water without nav aids... like amelia earhart.

 

[Engineman]

A very learned religious scholar opined a few years ago that claiming the earth rotated was ridiculous. If the equator was rotating at around 1,000 MPH then aircraft would never be able to land, simples.

A question for pilots. I once saw a Lancaster flying overhead coming in to land and obviously labouring into a head wind, but I was stood on the ground and it was obviously being buffeted by turbulent air. If you are at high altitude, flying above cloud in a strong but smooth headwind, do you even know without modern navigation aids?

Hi. I do not want to be scientific here, but I have quite a bit of worldwide pilot navigation experience, so here are a few experiences!
At high altitude with limited vis, you have little perspective of movement, except if you manoeuvre or approach clouds or encounter turbulence. You can get 200Kts+ smooth Jetsream and not notice it. Of course, at say about 33,000ft you probably travel at 280KCIAS/ .83M/ 485TAS so drift angles are usually small. However, you can see the drift if you look carefully and navigation can be done with Astral sextant and even with things like smoke floats at low level.
Modern navigation? Depends what "modern" you mean? Radio navaids go back to the 1930's and can be interpreted for the solution of the triangle of velocities navigation and triangulation of position fixing, although they are more commonly used for more simple point to point navigation.
Inertial navigation is a great thing that can also give wind velocity readout. Present civil systems use GPS, and coupled with INS provide all the civil 3D navigation you could want! There are other clever ways as well!
Going back to your question, without navaids of some sort (visual, electronic, etc) you cannot sense a smooth wind in flight.

Eng

 

[MIflyer]

flying above cloud in a strong but smooth headwind

My HS physics teacher was a USN aviator stationed at San Diego. He took off one day, heading East, and observed cloud cover appeared soon after takeoff and obscured the ground. He checked weather at his destination airport; they reported all clear. When he arrived at the point where the airport should be, there were still overcast clouds below. He was baffled, checked the weather at stations north and south of where he should have been and found they were also reporting clear. Finally, two hours after taking off, he descended through the clouds and was astonished to come out into the clear over the ocean, flew East for 10 min or so and landed at San Diego. At altitude he had gotten into a headwind that gave him a net negative 5 mph speed and blown him West, back over the Pacific, with the usual California Marine Layer underneath.

 

[SaparotRob]

… Otherwise a 90kt headwind is uninteresting.

Unless you're tacking against it.

 

[Engineman]

Unless you're tacking against it.

Yes, and a 200Kts headwind is pretty awful! Often, it is possible to minimise adverse Jetstream headwinds by avoiding their generally narrow path. Global Jetstream tracking and prediction over 12 to 24hr periods is very accurate, although you can be caught out by exceptions. Most sensible long-range aviators plan with computer planning tools that optimise the route for advantageous wind. However, sometimes you cannot beat the situation and have to work with the winds you have. In truth, long range flights almost always fly with minimum fuel +5% or so, relying on the overall wind forecast to not be worse than that, or you may have to divert. A by-product of winds can be Turbulence, and again this is predicted.
At high altitude severe Clear Air Turbulence is avoided where forecast.
There are many tales of aviators with wind and weather problems, and I would advise anyone flying anywhere to always get the best forecasts for their journey!


Eng

 

[SaparotRob]

I meant on a twelve foot catboat.

 

[drgondog]

Yes, and a 200Kts headwind is pretty awful! Often, it is possible to minimise adverse Jetstream headwinds by avoiding their generally narrow path. Global Jetstream tracking and prediction over 12 to 24hr periods is very accurate, although you can be caught out by exceptions. Most sensible long-range aviators plan with computer planning tools that optimise the route for advantageous wind. However, sometimes you cannot beat the situation and have to work with the winds you have. In truth, long range flights almost always fly with minimum fuel +5% or so, relying on the overall wind forecast to not be worse than that, or you may have to divert. A by-product of winds can be Turbulence, and again this is predicted.
At high altitude severe Clear Air Turbulence is avoided where forecast.
There are many tales of aviators with wind and weather problems, and I would advise anyone flying anywhere to always get the best forecasts for their journey!


Eng

Slightly 'off topic' but your narrative points directly to the use of Combat Radius mileage as a 'guide', particularly at high altitude conditions where the jetstreams thrive. Many WWII escort fighters were forced to land in occupied territory or ditch when the unanticipated (and unsensed) headwind gobbled up the fuel reserve.

 

[Engineman]

Slightly 'off topic' but your narrative points directly to the use of Combat Radius mileage as a 'guide', particularly at high altitude conditions where the jetstreams thrive. Many WWII escort fighters were forced to land in occupied territory or ditch when the unanticipated (and unsensed) headwind gobbled up the fuel reserve.

Correct. The study of upper winds and Meteorology in detail for Aviation was strongly pushed by military imperative in WW2. Beyond that, the pressures on Civil flying and aviation safety also support the global weather forecasting that we have today.

Eng

 

[MIflyer]

In the book written by the 100BG lead navigator, he describes that on his first mission as lead he was roused out of bed when the real lead Nav did not show up, only finding out when he got to the B-17 that he was the lead. They were hitting a target in Norway and cloud cover obscured the ground with the radio operator unable to get any bearings. Suddenly the clouds cleared up and he looked down to see a smoke screen being generated down below. Then he realized that they were approaching the target, a fact he would not have recognized if the Germans had not tried to conceal it; they were two hours early! A ferocious tailwind had thrown off their timing. They hit the target and then he advised the pilot that going back the same way would be a bad idea and recommended they just fly out to the coast and head over to Scotland. They picked up some flak, when led him to realize they were over the German U-boat base at Narvik. One B-17 had an engine shot out but he now had a great fix on their position. Two Ju-88's then attacked and the B-17's shot both of them down. He did not need to provide any navigation after that, since Scotland was their typical training mission and pilotage would get them home. He figured they would send hm home in disgrace but they were so delighted they gave him a medal and made him permanent lead nav. terrible news, since he would finally have to learn how to navigate.

So, just think, TWO HOURS EARLY to the target due to unforecast winds.

 

[drgondog]

thinkin target was Heroya 7-43. Very odd tailwind South to North,

 

[MIflyer]

thinkin target was Heroya 7-43. Very odd tailwind South to North,

He said that they told them the target was a destroyer base but in reality they were hitting the German heavy water plant. And that strong headwind East to West over San Diego was an odd one too.

 

[Engineman]

In the book written by the 100BG lead navigator, he describes that on his first mission as lead he was roused out of bed when the real lead Nav did not show up, only finding out when he got to the B-17 that he was the lead. They were hitting a target in Norway and cloud cover obscured the ground with the radio operator unable to get any bearings. Suddenly the clouds cleared up and he looked down to see a smoke screen being generated down below. Then he realized that they were approaching the target, a fact he would not have recognized if the Germans had not tried to conceal it; they were two hours early! A ferocious tailwind had thrown off their timing. They hit the target and then he advised the pilot that going back the same way would be a bad idea and recommended they just fly out to the coast and head over to Scotland. They picked up some flak, when led him to realize they were over the German U-boat base at Narvik. One B-17 had an engine shot out but he now had a great fix on their position. Two Ju-88's then attacked and the B-17's shot both of them down. He did not need to provide any navigation after that, since Scotland was their typical training mission and pilotage would get them home. He figured they would send hm home in disgrace but they were so delighted they gave him a medal and made him permanent lead nav. terrible news, since he would finally have to learn how to navigate.

So, just think, TWO HOURS EARLY to the target due to unforecast winds.

So, Heroya and Telemark both within about 500Nautical miles from Cambridge area. B17G would do about 250Nautical miles an hour (kt) still-air groundspeed at 25,000' so about 2 hours outbound still-air. A 250 kt tailwind would give 500kt G/S, so about 1 hour so absolute max time saved possible, about 1 hour. However, winds vary a lot in these parts, but coming from a usual South Westerly direction. Max likely Jetstream at about 25,000' here would be 150 kts, so G/S 400kts, time saved would be 48 mins.
Difficult to see 2 hours saved on a 500 Nm outbound trip.
Narvik is an unlikely point on a way home from Telemark, it is about 650Nm North of Telemark. However, Bergen is a more likely point on a return to Scotland and it was a U-Boat base.
Maybe that was it?

If the route out had gone (a very long way round) to Northern Scotland before turning direct East to Telemark, it would be about 650Nm, taking about 2.6 hours in still air. The unlikely 250kt tailwind all the way would give 1.3 hours elapsed. So that would be 1.3 hours saved.
Bear in mind that a 250kt Headwind on the way back might have meant zero G/S.

Overall, I think it is possible that they got over 100kts tailwind outbound and were nearly an hour early, and over 100kts headwind on the way back and the homebound route took about 2 hrs longer.

Eng

 

[pbehn]

In the book written by the 100BG lead navigator, he describes that on his first mission as lead he was roused out of bed when the real lead Nav did not show up, only finding out when he got to the B-17 that he was the lead. They were hitting a target in Norway and cloud cover obscured the ground with the radio operator unable to get any bearings. Suddenly the clouds cleared up and he looked down to see a smoke screen being generated down below. Then he realized that they were approaching the target, a fact he would not have recognized if the Germans had not tried to conceal it; they were two hours early! A ferocious tailwind had thrown off their timing. They hit the target and then he advised the pilot that going back the same way would be a bad idea and recommended they just fly out to the coast and head over to Scotland. They picked up some flak, when led him to realize they were over the German U-boat base at Narvik. One B-17 had an engine shot out but he now had a great fix on their position. Two Ju-88's then attacked and the B-17's shot both of them down. He did not need to provide any navigation after that, since Scotland was their typical training mission and pilotage would get them home. He figured they would send hm home in disgrace but they were so delighted they gave him a medal and made him permanent lead nav. terrible news, since he would finally have to learn how to navigate.

So, just think, TWO HOURS EARLY to the target due to unforecast winds.

For years I flew from Teesside to Schipol and on to Hanover and return. Flying to Schipol the plane was in the landing system there while still on the ground at Teesside or Hanover. Flying out of Schipol to Teesside sometimes was longer or shorter due to winds. One time it was ten minutes early, on a flight of around 300 miles. One time going to Hanover the pilot said he expected to arrive early. It is a "hop" of just over 200 miles and despite the plane obviously being on a low cruise setting, the engines hardly made a noise, we arrived 10 minutes early, on a flight that was only scheduled for 40 minutes from push back to docking and "taking it easy". Flying around in heavily laden bombers in those conditions makes me surprised any got back at all on some raids.

 

[MIflyer]

Overall, I think it is possible that they got over 100kts tailwind outbound and were nearly an hour early, and over 100kts headwind on the way back and the homebound route took about 2 hrs longer.

Well, they might not not have flown direct to the target but did some feinting. And typical loaded cruise for a B-17 was about 180 mph and they would have needed time for climb at a lower speed as well as assembling the formation.

By the way, by getting there early they accidentally arrived during lunch, when the Norweigan civilian workforce was at lunch, and only a few of them were killed, which not only pleased the civilians but also impressed everyone in Europe with the USAAF's ability to hit a target in daylight at such a long distance and do it so they arrived at lunch.

I recall reading in one book, "Combat Crew" about a mission when they were flying what in reality was a B-17E that did not have the "Tokyo Tanks" installed. The group leader seemed to be oblivious that not all the B-17's had as much gas on board as he did. When they got close to GB on the way back things were so desperate that the pilot broke formation and headed for a small fighter airfield he knew of. Several other B-17's followed them, presumably figuring he knew a place to land. They made it down Okay but if they had tried to go back to home base with the rest of the group they would not have.

 

[Engineman]

Well, they might not not have flown direct to the target but did some feinting. And typical loaded cruise for a B-17 was about 180 mph and they would have needed time for climb at a lower speed as well as assembling the formation.

By the way, by getting there early they accidentally arrived during lunch, when the Norweigan civilian workforce was at lunch, and only a few of them were killed, which not only pleased the civilians but also impressed everyone in Europe with the USAAF's ability to hit a target in daylight at such a long distance and do it so they arrived at lunch.

I recall reading in one book, "Combat Crew" about a mission when they were flying what in reality was a B-17E that did not have the "Tokyo Tanks" installed. The group leader seemed to be oblivious that not all the B-17's had as much gas on board as he did. When they got close to GB on the way back things were so desperate that the pilot broke formation and headed for a small fighter airfield he knew of. Several other B-17's followed them, presumably figuring he knew a place to land. They made it down Okay but if they had tried to go back to home base with the rest of the group they would not have.

Yes, I think we are in the ballpark with 250kts TAS @ 25,000'. The form-up and slow climb would also reduce the time with the extra groundspeed from the tailwind, which mainly occurs at altitude, so that would reduce the possible gain in time against the plan. The direct route from Cambridge to Telemark is way out in the North Sea and my distance is generous anyway (to the advantage of the time gained) so without the real plan, I have allowed for some feints. Mind you, maybe they did go to Narvik? As I show, an enormous detour to Northern Scotland would still make gaining even 1.3 hours faster than a still-air plan unlikely with a 250kt tailwind, and note, they would not get home against that 250kt headwind!

Eng

 

[MIflyer]

Well, the tailwind was not present during the trip back, which is why the navigator recommended cutting over to the coast. 250 kts ( 285 mph) TAS at 25K sounds way too high. The B-17's flat out top speed was right about 300 MPH and they would be way below that, 180 mph commonly cited. I recall a WW2 Hump crew chief telling me that when a C-54 headed back to the US from India it was doing only a little over 100 mph at the start of the trip, speed increasing as they burned off fuel.

I'll have to see if I can find the book.

 

[Engineman]

Well, the tailwind was not present during the trip back, which is why the navigator recommended cutting over to the coast. 250 kts ( 285 mph) TAS at 25K sounds way too high. The B-17's flat out top speed was right about 300 MPH and they would be way below that, 180 mph commonly cited. I recall a WW2 Hump crew chief telling me that when a C-54 headed back to the US from India it was doing only a little over 100 mph at the start of the trip, speed increasing as they burned off fuel.

I'll have to see if I can find the book.

Yes, the original info from the book would be great.
The B17G Performance data does list a whole load of different speeds and performances that include "Normal power" highspeed cruise speed of 245kn TAS @25,000'. That gives an average speed of 214kn over the whole 595nm Combat radius on that profile according to the tables (still air) . However, tables also show cruise at many other slower speeds, including your quoted slower speed of 182, again given as an average speed.
So take your pick. If you use the 182TAS as an average, you get 2.75 hours for the 500nm sector still air. To be 2 hours early would require 666kn G/S with 485kn tailwind.
If you look at the map of what we are talking about, going back to Scotland is the closest route home (well, to friendly territory) and possibly could have/did reduce the headwind. But, if they didn't know what the winds were, it could also have made it worse.

Eng

 

[drgondog]

Well, the tailwind was not present during the trip back, which is why the navigator recommended cutting over to the coast. 250 kts ( 285 mph) TAS at 25K sounds way too high. The B-17's flat out top speed was right about 300 MPH and they would be way below that, 180 mph commonly cited. I recall a WW2 Hump crew chief telling me that when a C-54 headed back to the US from India it was doing only a little over 100 mph at the start of the trip, speed increasing as they burned off fuel.

I'll have to see if I can find the book.

I agree too high. SOP for Wing Leader was to cruise at 150 IAS MPH when at 25K, B-24 cruised at 180 IAS.

The reason for the low band cruise was to permit the older, tired B-17s to keep up without blowing an engine.

Most if not all anecdotal recollections are referenced to IAS in mph

 

[Engineman]

I agree too high. SOP for Wing Leader was to cruise at 150 IAS MPH when at 25K, B-24 cruised at 180 IAS.

The reason for the low band cruise was to permit the older, tired B-17s to keep up without blowing an engine.

Most if not all anecdotal recollections are referenced to IAS in mph

You Guys!
Too high for what? Too high for that mission (little info was given), Too high for normal missions? Go look at the B17G performance data. So yes, OK then, 150mph IAS @25K is 224mph TAS, or 195knots! Faster than the 182knots we looked at!
I have given both the high speed option and the lower speed cruise quoted by the OP and both options in the performance tables. It doesn't fit. You can't save 2 hours.

Eng