V1-Vr split on 4-engine aircraft

[WhiteKnight]

Why is there normally a significant split between V1 and Vr on a heavy 4 engine Long haul aircraft. On a 2 engine aircraft the split between V1 and Vr on a dry runway is normally not as significant. I understand it has to do with the fact that you loose 25% of thrust vs 50% of thrust. I just don’t understand the exact reason behind. Can someone explain please. Thank you.

[Goldenrivett]

Hi WhiteKnight,

V1 max will be the same for both aircraft as it only depends on braking retardation.
V1 min is different due to different acceleration on 1 or 3 engines to VR. V1 min will be lower for 3 remaining engine aircraft acceleration.

If your chosen V1 is half way between min and max, then there will be a bigger “ split” on the quad.

[FlightDetent]

In addition, the regulatory minimum climb gradients for quads and twins are different. When ASD is not limiting, the take-off speeds will be optimized for that gradient, possibly yielding different results too.

Ref: Getting to Grips w/ Aircraft Performance p.64

[tcasblue]

Thinking back about it now, up to 30 knots sometimes between V1 and Vr on the 747. On the C130....they were always fairly close together. Not sure about the comparison for min V1 vs Vr though.

[FlightDetent]

That would be easily explained by the stopping power of the 3 props.

Checking the book last night, I discovered a short discussion on different regulatory requirements for the ASD case. ​​FAR 25 amendment 42. What's prescribed in it, what applied before and how it was superseeed. First read on the subject.

How would this apply for 767-400 v. s. the Dreamliner? Is the - 400 really pre-amendment​​​​​​? What about the 747-8?

[john_tullamarine]

The thread is getting into more interesting stuff as it progresses.

Re what this and that - to the extent we can reasonably infer, it is necessary to look at the TCDS for a specific Type/Model. This will have the basics of the frozen design standards applicable to the certification and some details re backroom agreements. While one can't be sure that one can infer the whole story, the exercise will give you a pretty good picture of what's what..

See the FAA website https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/Frameset?OpenPage and play a bit there to get to the nitty gritty.

[FlightDetent]

V1 min is different due to different acceleration on 1 or 3 engines to VR. V1 min will be lower for 3 remaining engine aircraft acceleration.Attaching some flash to the bone: V1(min) can be lower for the 4 minus 1 case thanks to the acceleration capacity of the remaining thrust. A twin past V(eng.fail) has 50% thurst remaining but quad retains 50% more than that. :E

@J.T. Escalated quickly, thanks!

[tcasblue]

That would be easily explained by the stopping power of the 3 props.


But no reverse is allowed for calculations. So would it just be the air-braking effect of the prop blades, the way the Dart engine used ground fine for normal landings.

[Sidestick_n_Rudder]

Another thing not mentioned before.

I guess Vmcg may be lower for a quad, compared to a twin of a comparable size, due to a lower thrust generated by each engine (think A340 vs A330, 34000lbs vs 70000lbs per eng.) - this, combined with better acceleration capability may allow for a lower V1. Then again outboard engines on a quad are mounted on a longer arm, I don't know which has a stronger effect on Vmcg...

[FlightDetent]

But no reverse is allowed for calculations. So would it just be the air-braking effect of the prop blades, the way the Dart engine used ground fine for normal landings.The beta range is what I had in mind in an absent moment. Thanks for correcting assumingly the feather / no feather would still play a role. IIRC on wet runways the reversers are credited.

Another question: Could the rules for military a/c be significantly different?

[tcasblue]

The beta range is what I had in mind in an absent moment. Thanks for correcting assumingly the feather / no feather would still play a role. IIRC on wet runways the reversers are credited.

Another question: Could the rules for military a/c be significantly different?

I flew the civilian version, officially an L-382.

[sycamore]

There is a thread on Avn Hist & Nost,by the Boeing TP covering flight-testing of the 747 Dreamlifter...
FD,we operated our mil C-130s generally to equivalent civvy Performance criteria,but could go to Mil Perf.for special tactical cases( authorised by God,or the Devil ),usually for strips/rough fields,contaminated,or in the case of our tankers up to overweight at 175k MAUW....actual performance was always as good or better than planning,except..... on one occasion out of Mountain Home ID. to Goose Bay,full crew plus `trappers/checkers checking other `new trappers`,and 8 pallets of equipment.Summertime in ID. is `fire-season ,and hot, new co-pilot leg,take-off and initial climb were ok,but we seemed to be down on climb perf....so,casually into the perf.graphs,and W&B,asked L/Master to recheck pallet weights..by now `checkers checkers are fully `awake`,get Nav. to revise fuel plan,as I estimate we`re overweight by 10k,tell ATC to revise TOC 2000FT lower than planned,etc,etc.Nav says we`ll make it to G-B, will revise as we go, decide we will not dump fuel,and carry on.. Arrive at G-B,and having requested an off-load and check weigh,it was found we were 8k overweight.Half the pallets had been loaded on one`line`,other half on another...Info passed back to Mountain Home, scales on one line were under-reading...Debiefing in the bar was interesting...!