B-36 early rotation
 

Videos of B-36 takeoffs show them rotating well before liftoff begins, rolling for many seconds on the main gear with the nose raised until it flies off in that attitude. It's hard to imagine how that wouldn't add significant drag and lengthen the takeoff run. Does anyone know why this practice was used?

The concept of ‘rotation’ at a pre-determined speed didn’t come along until later. The method you describe was the norm for large and small aircraft with tricycle undercarriage at that time.

Hence the Comet accidents. I saw a T-33 do exactly that at Duxford a few years back. Off the end of the runway. Aircraft destroyed, but occupants OK.

With all wheels on the ground it may not generate enough lift because of the angle of incidence.
Rotate to an angle of attack that will generate lift.
By the way, in large jets Vr and Vlo (lift off) are noticeably different.

https://www.boeing.com/commercial/aeromagazine/articles/qtr_02_09/pdfs/AERO_Q209_article04.pdf

Here is a decent example:

https://cimg2.ibsrv.net/gimg/pprune....ba9374c971.gif

With some aircraft, all wheels on the ground may not provide a positive angle of attack so it's necessary to lift the nosewheel when you have sufficient elevator authority in order to generate lift; many a time I watched Illyushin IL 18s depart from Glasgow and the nosewheel was always lifted well before takeoff; likewise FRCs for the JP Mk5 say 'nosewheel off at 85kt, rotate at 95kt'.

Low angle of attack with all wheels on the ground make sense. The B-36 wing angle of incidence isn't easily found but it did seem to sit nose low on the ground.

B2N2: Good point about difference between rotation and liftoff speeds with modern airliners. Nice white paper from Boeing - thanks. But it's also true that the rate of acceleration is a lot higher now so the period between rotation and liftoff is only a few seconds.

Jhieminga: As you suggest, it does seem to reflect a general evolution in takeoff technique over the years from "raise the nose early and hold the attitude while speed increases to liftoff" vs. "keep the nose on the ground for most of the ground roll and then rotate at a constant pitch rate (2-3 deg/sec) while accelerating to final liftoff speed." A 1961 video of a later model Comet takeoff still reflects the early nose lift with only a slow rotation to liftoff after quite some further acceleration. Recent videos of DC-6 takeoffs seem to show all wheels on the ground until Vr.

Probably type specific as well, noting Chevvron's reference.

Thanks, all -

Keeping the nose gear firmly on the ground until a certain speed is required by many types for directional control reasons.
The Viscount was fairly notorious for this - many having been lost in asymmetric power training accidents where the nose was lifted too early. I suspect the DC-6 would be similar.

i was the FISO in the tower on that day at Duxford. Got my attention!! Very fortunate outcome.

On an ATCO Fam flight in a Trident, the captain explained to me that as there was some standing water on the runway, he would do a 'skim' takeoff, lifting the nosewheel early so as to stop any spray from it entering the outer engines and possibly triggering a flameout.

In the 1930s KLM bought a single Stearman-Hammond Y so that the pilots could learn how to take off and land an aircraft with a third wheel at the front. Up until that time, taildraggers were the norm and with these, your wings have a positive angle of attack from the start of the take off run. As elevators become effective long before lift off speed, and as the unprepared surfaces of those days could be troublesome for nose gear legs, the practice of lifting the nose wheel and letting the aircraft fly itself off came about. There are some drawbacks though:
- You will be lifting off at the minimum flying speed, making every speed excursion within those first minutes hazardous. One little gust could ruin your day.
- As there is a lot of variation possible in attitude, the take off distance needed would vary a lot from take off to take off.
- Lifting the nose too high could leave you in a stalled attitude on the ground (as the Comet would show in its early days).
As a result of this, the concept of rotation at a pre-determined speed was developed. This technique led to a lot more precision in calculating needed runway lengths and allowed a margin of speed over the stall speed at lift off. It also helps with directional control during crosswind situations as mentioned above.

An interesting side effect is that we now see private pilots on Cessnas and Pipers who will tell you that their trusty steed has a 'rotation speed'. On a light GA type, I feel that the 'lift the nose early' technique is still the best way to take off, and that a rotation speed is absolute nonsense, but it's difficult to get this message across when every Cessna pilot thinks he's a Boeing pilot in training. But that's a different discussion....

chevvron,

The VC10 had chines on the nosewheel tyres which were designed specifically to deflect the spray so that it spread out sideways well below the engine intakes. We were instructed not to raise the nose like your Trident pilot did as there was no need and that it was important to conform to the normal rotation speed for the exact reason that jhieminga has posted above. I seem to remember there was a training film showing how effective these chines were.

Look at the ‘negative’ stance of for instance a Gulfstream IV.
Without a positive rotation it will just go off the end of the runway at 200mph.


https://cimg0.ibsrv.net/gimg/pprune....de5d0c26b.jpeg

Back in the 1960's BOAC used to operate a once per week Comet 4 service to my home port which at the time was a typical military runway made up of precast hexagonal concrete blocks. The Comet was the only commercial jet using this runway and used to employ early nose wheel lifting on take off similar to the B36. I was told this was to reduce damage to the runway (and possibly the aircraft) caused by the jet efflux throwing up debris from the gaps between the concrete blocks. It was interesting to see the Comet barreling down the runway with its nose gear in the air until it gently flew off. Once fully airborne the aircraft would rotate quickly to its normal climb attitude.

I found an old Pathe film about B36s visiting the UK. There are three takeoffs in the film: The first two are at a large stage just before takeoff but the third seems to be a modern rotation about 20 knots before unsticking.

Wow that is an absolute gem! Proper B-36 noise too!