Nosewheel braking trials?
 

I would have thought that with the anti-skid and dynamic stability advances in recent years (as used particularly in road vehicles) that nosewheel braking would become an interesting means to dramatically improve take-off and landing performance. Is there much interest in this currently?

Yes there is interest but there are a couple of big problems.

The relativly small NG has limited space for the brakes themself. To provide a reasonable amount of braking performance without overheating requires a large thermal mass. This would seriously affect the size and weight of the NG to get reasonable performance. Look at the amount of brake pack available on the MG of a typical aircraft and imagine how much of that practically would be able to be packaged in the NG. Not much.

Structurally the size and weight of the NG also needs to increase to accomodate the additional braking forces - not a huge issue in itself.

There are performance advantages, but these are severely limited by these factors.


I think I remember reading once that a 727 option was with NG braking, but was rarely used and even then only in a 'suplemental' capacity when full MG braking was used.

I have seen 727's equipped, you can find a few today. Usually private conversions to get every ounce of stopping power when landing on small island strips.

After the 727's go at it the application for commercial purposes has been abandoned.

Since most of the aircraft weight is on the main gear, a nose wheel brake will generally not justify its weight.

You need to consider weight transfer onto the front wheel under braking.

By way of illustration, try stopping a motorbike with just the rear brake..

The amount of weight transfer on the nose gear during braking is function of center of gravity height proportionally to the wheel base. On a motorbike it is certainly very high, not to mention that the center of gravity is not near the rear wheel to start with.

On a light prop aircraft with a relatively tall gear, nose gear weight fraction might go from 15% static to 30% at 0.3g deceleration (unless you pull on the stick to create more elevator drag and transfer weight back to the mains) but on a typical commercial aircraft, braking deceleration will shift weight on nose gear by only a few percent (8% static to 11% at 0.4g deceleration to give a number).

So the nose gear is 7-9 times less effective at braking than the main gear, and the nose wheel brake weight is better invested elsewhere.

I mean it is not a bad idea, it is looked at regularly but the math just does not work out except for a few particular situations.

my coursenotes say that nosegear braking (on 727) is speed restricted 40 to 80 kts and when brake pedals are depressed by 50% of their travel
so i assume that you have to be facing a sticky situation to be braking that hard
should that read 80 to 40kts?


gs

Thanks - interesting figures. I'm particularly interested in VLJs (eg CJ1, Phenom 100, etc) which don't have the luxury of thrust reversers.

My "best guesstimate", based on no more than flying them, plus a lot of motorcycle crashing (!) is that some sort of nosewheel braking, with stability control, might add a lot to landing performance in the wet. Perhaps not so much an issue with the CJ range, as they have low undercarriage and long wheelbase, but (for example) the Phenom 100 is relatively high with a relatively short wheelbase compared to CJ.

The Phenom 100 has indeed a relatively tall landing gear and a back of the envelope quick calculation suggests 13% static to 26% at 0.4g.

When you design a new aircraft you do so by complying with a set of design constraints such as maximum speed and landing distance. The best design is the one that blends the best design criterias at the lowest weight and cost. Tools exist that can quantify those things with a high level of accuracy.

The landing distance criteria can be met by reducing wing loading (increasing wing area), by increasing lift coefficient (increasing complexity of flap system) or by increasing the performance of the braking system (anti-lock for example).

Or you could add a nose wheel brake.

Now consider the weight and cost of the nose wheel brake and compare that with the weight and cost of increasing wing area a little bit, or going for a slotted flap system.

What design engineers find is that the nose wheel brake is always the heaviest and most expensive solution. So much so that it is a technical solution that is not normaly being looked at during the conceptual design stage.

Where a nosewheel brake may be considered would be as retrofit to an existing aircraft or a badly screwed up design, where there are no simpler solution to reduce landing distance since the wing cannot be modified without going through an extensive (re)certification program.

In short, the nose wheel brake is not the lightest and least expensive path to the design solution, so that path is simply not taken.

have had no problems with the front wheel on my a321, I did have an issue with the left wing wheel, burst upon landing

Saab fitted it to the Grippen. Very effective.

The Gripen has a nose gear brake because the canard applies a significant downforce during landing deceleration. This is a unique case.

While Googling for an image I stumbled upon a seven years old and identical thread on Pprune http://www.pprune.org/archive/index.php/t-95014.html

Hawkers fitted a nose wheel brake to one Hunter back in the early 60s (the two seater G-APUX) it made little difference just after touchdown but really bit at low speeds. Overall the landing run was reduced by approx the same amount as if the tail chute was popped - which was interesting. The tail chute was favoured though as it also worked on slippery runways.

Nose-wheel tractor + brake?
 

Nose wheel braking on Mig fighters
 

Lots of MiG variants : MiG 21, MiG 27 have Nose wheel braking. you select them on for take off and landing but keep it off for taxy.