Vr speed ?

[Obidiah]

Hi All,

Regarding a STOL twin turbo prop and its nominated Vr speed could someone shed some light on appropriate technique and perhaps deeper reasoning.

Some details:

Vs1 78 kts (6200 kg)
Vs2 71 kts (6200 kg flaps @ T/O)
Vmcg ? <65 kts
Vmca 65 kts
Vr 79 kts
V2 86
Vyse 97 kts

I understand that Vr is established at a speed no less than the lesser of 1.05 X Vmca and no less than 1.10 X Vs2 and that the above numbers reflect this however my question is:

Is it considered appropriate to raise the nose wheel off the runway, where ample runway length is available, at a speed earlier than the nominated Vr speed. By raise the nose I do not mean lift off or unstick the mains but simply to position the nose wheel such that it is in a position a few inches above the runway surface and hold this position (attitude) until the aircraft obtains the stated Vr speed then smoothly increase the back pressure to achieve lift off.

The reason I ask is that I fly with other pilots who all insist on leaving the nose wheel firmly planted on the runway until the magic 79 kts then heave back. Some even leave the elevator control almost against its forward stop until this speed is achieved. This seems to me to be horribly cruel to the nose wheel structure and from observing from the right hand seat I have witnessed nose wheel shimmy on occasion and at all other times felt the pounding from the uneven runway surface driven hard up through the nose leg, it really does seem very hard on the structure. At the final separation velocity of 79 kts the nose wheel is sending pronounced vibration through the nose wheel structure due to slight out of balance issues.

By contrast I prefer to raise the nose wheel slightly above the runway at around 70 kts and then allow the aircraft to accelerate to its lift off speed. By observation this is many times smoother on the aircraft structure, and I am certain this technique will significantly reduce fatigue and damage to the nose wheels associated structure.

The original certification weight saw a range of Vr speed from 69 kts at all weights up to 5217 kg then a linear increase to 74 kts at 5670 kg.

Coming from a background of flying similar weighted aircraft but in the form of Ag aircraft I developed and honed the skill of treating the undercarriage with the utmost respect. I have no doubt that my method is far less demanding on the structure but am I wrong to do this. If I ask my colleagues why they use the technique they do I simply get quoted the book of SOP's no one seems to think outside the box or question their techniques.

Is the establishment of the Vr speed simply a case of one size fits all for a FAR certification criteria?

What is the true and proper definition of Vr ? is it rotation for the purpose of lift off or mains unstick as I understand it to be? can I claim that the act of raising the nose wheel to reduce structural loads but not with the intention of lift off is not the act of rotation ?

Is there something unsafe about my technique?

Am I the only one left that has feel and care for the stucture and therefore obsolete?

Regards All

Obidiah

[dogcharlietree]

By contrast I prefer to raise the nose wheel slightly above the runway at around 70 kts and then allow the aircraft to accelerate to its lift off speed. By observation this is many times smoother on the aircraft structure, and I am certain this technique will significantly reduce fatigue and damage to the nose wheels associated structure.

I can see your reasoning and agree it is probably a lot less fatiguing on the nose gear.
However, I would not do it, for the simple reason that if one engine fails, you would probably be on the grass before you knew it.
With the nosegear on the tarmac and the rudder pedals close to neutral, if an engine fails, you have control over which direction the aircraft will proceed. I doubt, with your technique, that you would recognise, and apply appropriate action before the grass.
When I aviate, I try and lessen the variables. It's much easier then!

[Dan Winterland]

Raising the nose at 70kts as you mention, you are above Vmca. But Vmca assumed 5 degrees of bank into the live engine which you will not be able to acheive on the ground. Do what the SOPs or POH specify, then you are covered.


Also, Vr has to be now lower than 1.1 x Vmu (Velocity minimum unstick - in otherwords, the speed the aircraft will fly off the ground if the nose is held up). It sounds like you may possibly get airborne before this and compromise the Vmu criteria.

[Obidiah]

Thanks Guys,

Just to clarify Dan there is no Vmu stated for this aircraft. The aircraft does not leave the ground prematurely because it is not my intention to do so. It is a very easy handling skill to fly with accuracy.

Dogcharlietree I appreciate your comments regarding maintaining runway directional control however I have no doubt that I could recognise an engine failure early enough and be reducing power to ground idle quick enough to avert much more than a degree or two of change.

My above comments do read with a degree of arrogance, please forgive me, my abilities are fairly measured and I do not consider myself an overconfident type. However with 23 yrs experience and a 5 figure total time and a considerable period of which in the demanding and exacting environment of Aerial Ag flying I feel I am not putting the aircraft in a potential loss of control situation.

I thank you both for your feed back and do agree with your comments on lessening the variables, and the coverage afforded by SOP's.

Regards

[HarleyD]

I believe (without the ability to confirm at this moment) that Vr, however determined is defined as the speed "below which the pilot shall not apply the control input intended to raise the nose wheel clear of the ground" or similar. this means is used during demonstration of compliance with the certification basis and is consequently the only specified method of take off procedure that is supposed to be used.

This of course ignores multi tailwheel but that is another matter altogether.

there is in fact no direct civilian equivalent of a 'tactical STOL' take off as the performance section of the AFM defines the performance and conditions that apply to obtain the maximum certified performance as demonstrated by the TP during demonstration of compliance. The true name is a performance take off and whilst some company SOP's have wide ranging interpretations of STOL procedures, these are usually dated and do not fall within the certification conditions. some PNG ops for instance.

Be careful when operating any aircraft outside the design cert limits, you are the TP now.

I also have a 5 figure total (for what it's worth) and Ag and FT exp over 35 years and the older I get the less I stray outside the defined specs, some of which I have proudly been responsible for defining.

I also ride a motor cycle and despite 35 years on them I still know that it will be something that is outside my experience or that I just didn't see that can get me at any moment.

Vmca applies when the aircraft is still on the ground as well as when airbourne, so take care.... I have lost engines on T/O in M/E tailwheelers and that isn't fun at the wrong moment, also had brake fail on landing in similar. At those times a nose wheel on the ground seems like a good idea.

HD

[Dan Winterland]

Vmu isn't specified on any of the aircraft I have ever flow, but it still exists. It is determined during testing and certification and used to determine speeds such as Vr.

In most case, Vmu will be below Vmca so there's no reason to know what it is.

[Obidiah]

Thanks again fellas

HarleyD thanks in particular, your skills and exploits are known to me, say g'day to GL sometime for me if he's still over that way. Great if you or anyone else could confirm the official definition of Vr, clearly if it is as you stated my technique becomes a mute point if I wish to be legal.

An interesting point, at least to me, which appears not to have been considered in the responses is the fact that Vr is given as 69 kts at weights 983 kg less than our TOW.

This infers that the Vr speed is tracking at least from the point of 5217 kg and upward the requirement for Vr not being less than the certification 1.10 x Vs2

I can not see that the relationship between Vmca (65 kts) is influenced by the weight, certainly no promulgated changes in the AFM/POH

Therefore I am seeing the Vr speed rising in consequence to the increasing Vs2. As stated I am not actually lifting off until the prescribed Vr speed of 79 kts or even a few knots later. Other than an increase in drag coefficient by placing the aircraft in a slightly higher AoA and this is not of great consequence in the situations I am referring as runway available is ample.

If my assumption above is correct is accelerating to 79 kts from a lower weight approved Vr rotation speed of 69 kts through to 79 kts with the nose wheel slightly above touching the runway a cause for concern when considering matters related to the stall?

HarleyD interesting comment on being careful flying an aircraft outside it's design certification envelope, somehow we all become untrained TP's by default in that funny old back n forth game with regard to certification weights and the CASA removal of any requirement for adherence to weight limitations. Or the G10 low Vn envelope inherited from much early lower powered models.

Thanks again for your feed back on this.

[CL300]

do you have to demonstrate Vmu on a FAR/JAR 23 turbo prop ?

That's where you have to start with.
for the purpose of demonstrating Vmu , you have to hold the elevator aft to the stops and note when the A/C lifts off and alter V1 in some conditions per computation.
the rest of the game of the recommended rotation speed (FAR23) is just a way to compute the take Off Roll. ( except at Beechcraft I tend to remember)

my 2 cents

[Obidiah]

CL300

Thanks for that, I will do some reading and see if I can't find out whether Vmu is a requirement for FAR23, I'll take a punt and say it isn't.

Regarding minimum Vmu speed I haven't tried to prematurely lift the aircraft off the runway but my gut instinct is that it could be done a few kts lower than the Vs speed, so perhaps a number of 68 kts at MTOW

The rest of your response eluded to the deeper reasoning of Vr that I am seeking. I have held a gut instinct that once the Vr speed has been placed in front of Vmca/Vmcg (clearly common sense) that the margin in front of the stall speed is some what superfluous.

Provided that separation does not occur until the stipulated Vr speed a safe margin is provided above the stall.

It seems intuitive to me that by stipulating a Vr speed the manufacturer obtains a known aircraft profile through the acceleration stage to lift off and as such is able to give repeatable accurate information on what point their aircraft will reach 50'.

CL300 I hope you add more to this as it appears you know a lot more.

Regards

[Matthew Parsons]

Obidiah, I don't deal with the V speeds that much, but from a more theoretical viewpoint keep in mind that when you aren't producing lift you also are not producing induced drag. Keeping the nose wheel on the runway should minimize the lift from the wings and allow you to use more thrust to accelerate. I also believe this is one of the main reasons why Vr is greater than Vmu.

As far as comparing Vs and Vmu, there are a few conditions that force them to be different. There may be a pitch limit due to the tail striking for Vmu that you wouldn't have for Vs. Ground effect plays a part with Vmu but not with Vs. Also, I believe that Vs is determined with power off and Vmu is at takeoff power (I could be wrong on these ones). Propwash flowing over the wings and vertical component of thrust would lead to differences in Vs and Vmu.

Matthew.

[Obidiah]

keep in mind that when you aren't producing lift you also are not producing induced drag. Keeping the nose wheel on the runway should minimize the lift from the wings and allow you to use more thrust to accelerate.

Thanks for that Matt but I think I covered that one here:

Other than an increase in drag coefficient by placing the aircraft in a slightly higher AoA

As for allowing the use of greater thrust and without resorting to trig explanation whilst the few degree pitch up does indeed from a theoretical point of view reduce forward thrust the actual amount would be imperceivable in reality, and may well be off set by the increased rolling friction due to the extra weight exerted on the mains and the now inclusion of the third tyre on the runway. The increase in induced drag as you pointed out is of the order many times greater in opposing force than than loss of correctly vectored thrust.

Tail strike is not an issue on the type I am referring.

In the scenario I am referring, never is rotation initiated prior to what would be Vmu.

Your point on Vs establishment being power on v off raises an interesting point which I had not considered. As Vs is reduced due to power on then what figure should be used for establishing the 1.10 x Vs to define Vr in the take off configuration?

Interesting point thanks for outlining it.

Interestingly to, the manufacturer has kept their Vr speed at what is the absolute legal minimum as defined by FAR certification throughout the weight speed range. Which begs the question if this certification minimum requirement had been say 1.05 x Vs as it is with Vmca, instead of 1.10 would the manufacturer have listed a lower value for Vr?

[Brian Abraham]

FAR 23.51 Takeoff speeds.

(a) For normal, utility, and acrobatic category airplanes, rotation speed, VR, is the speed at which the pilot makes a control input, with the intention of lifting the airplane out of contact with the runway or water surface.

(1) For multiengine landplanes, VR, must not be less than the greater of 1.05 VMC; or 1.10 VS1
(2) For single-engine landplanes, VR, must not be less than VS1; and

(3) For seaplanes and amphibians taking off from water, VR, may be any speed that is shown to be safe under all reasonably expected conditions, including turbulence and complete failure of the critical engine.

(b) For normal, utility, and acrobatic category airplanes, the speed at 50 feet above the takeoff surface level must not be less than:

(1) or multiengine airplanes, the highest of—

(i) A speed that is shown to be safe for continued flight (or emergency landing, if applicable) under all reasonably expected conditions, including turbulence and complete failure of the critical engine;

(ii) 1.10 VMC; or

(iii) 1.20 VS1.

(2) For single-engine airplanes, the higher of—

(i) A speed that is shown to be safe under all reasonably expected conditions, including turbulence and complete engine failure; or

(ii) 1.20 VS1.

(c) For commuter category airplanes, the following apply:

(l) V1must be established in relation to VEFas follows:

(i) VEFis the calibrated airspeed at which the critical engine is assumed to fail. VEFmust be selected by the applicant but must not be less than 1.05 VMCdetermined under §23.149(b) or, at the option of the applicant, not less than VMCGdetermined under §23.149(f).

(ii) The takeoff decision speed, V1, is the calibrated airspeed on the ground at which, as a result of engine failure or other reasons, the pilot is assumed to have made a decision to continue or discontinue the takeoff. The takeoff decision speed, V1, must be selected by the applicant but must not be less than VEFplus the speed gained with the critical engine inoperative during the time interval between the instant at which the critical engine is failed and the instant at which the pilot recognizes and reacts to the engine failure, as indicated by the pilot's application of the first retarding means during the accelerate-stop determination of §23.55.

(2) The rotation speed, VR, in terms of calibrated airspeed, must be selected by the applicant and must not be less than the greatest of the following:

(i) V1;

(ii) 1.05 VMCdetermined under §23.149(b);

(iii) 1.10 VS1; or

(iv) The speed that allows attaining the initial climb-out speed, V2, before reaching a height of 35 feet above the takeoff surface in accordance with §23.57(c)(2).

(3) For any given set of conditions, such as weight, altitude, temperature, and configuration, a single value of VRmust be used to show compliance with both the one-engine-inoperative takeoff and all-engines-operating takeoff requirements.

(4) The takeoff safety speed, V2, in terms of calibrated airspeed, must be selected by the applicant so as to allow the gradient of climb required in §23.67 (c)(1) and (c)(2) but mut not be less than 1.10 VMCor less than 1.20 VS1.

(5) The one-engine-inoperative takeoff distance, using a normal rotation rate at a speed 5 knots less than VR, established in accordance with paragraph (c)(2) of this section, must be shown not to exceed the corresponding one-engine-inoperative takeoff distance, determined in accordance with §23.57 and §23.59(a)(1), using the established VR. The takeoff, otherwise performed in accordance with §23.57, must be continued safely from the point at which the airplane is 35 feet above the takeoff surface and at a speed not less than the established V2minus 5 knots.

(6) The applicant must show, with all engines operating, that marked increases in the scheduled takeoff distances, determined in accordance with §23.59(a)(2), do not result from over-rotation of the airplane or out-of-trim conditions.

Could only find reference to VMU in FAR 25 aircraft.

FAR 25.107 Takeoff speeds.

(a) V1must be established in relation to VEFas follows:

(1) VEFis the calibrated airspeed at which the critical engine is assumed to fail. VEFmust be selected by the applicant, but may not be less than VMCGdetermined under §25.149(e).

(2) V1, in terms of calibrated airspeed, is selected by the applicant; however, V1may not be less than VEFplus the speed gained with critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognizes and reacts to the engine failure, as indicated by the pilot's initiation of the first action (e.g., applying brakes, reducing thrust, deploying speed brakes) to stop the airplane during accelerate-stop tests.

(b) V 2MIN,in terms of calibrated airspeed, may not be less than—

(1) 1.13 V SRfor—

(i) Two-engine and three-engine turbopropeller and reciprocating engine powered airplanes; and

(ii) Turbojet powered airplanes without provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed;

(2) 1.08 V SRfor—

(i) Turbopropeller and reciprocating engine powered airplanes with more than three engines; and

(ii) Turbojet powered airplanes with provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed; and

(3) 1.10 times V MCestablished under §25.149.

(c) V 2, in terms of calibrated airspeed, must be selected by the applicant to provide at least the gradient of climb required by §25.121(b) but may not be less than—

(1) V2MIN;

(2) V Rplus the speed increment attained (in accordance with §25.111(c)(2)) before reaching a height of 35 feet above the takeoff surface; and

(3) A speed that provides the maneuvering capability specified in §25.143(h).

(d) VMUis the calibrated airspeed at and above which the airplane can safely lift off the ground, and con- tinue the takeoff. VMUspeeds must be selected by the applicant throughout the range of thrust-to-weight ratios to be certificated. These speeds may be established from free air data if these data are verified by ground takeoff tests.

(e) V R,in terms of calibrated airspeed, must be selected in accordance with the conditions of paragraphs (e)(1) through (4) of this section:

(1) V Rmay not be less than—

(i) V 1;

(ii) 105 percent of V MC;

(iii) The speed (determined in accordance with §25.111(c)(2)) that allows reaching V 2before reaching a height of 35 feet above the takeoff surface; or

(iv) A speed that, if the airplane is rotated at its maximum practicable rate, will result in a VLOFof not less than 110 percent of VMUin the all-engines-operating condition and not less than 105 percent of VMUdetermined at the thrust-to-weight ratio corresponding to the one-engine-inoperative condition.

(2) For any given set of conditions (such as weight, configuration, and temperature), a single value of V R,obtained in accordance with this paragraph, must be used to show compliance with both the one-engine-inoperative and the all-engines-operating takeoff provisions.

(3) It must be shown that the one-engine-inoperative takeoff distance, using a rotation speed of 5 knots less than V Restablished in accordance with paragraphs (e)(1) and (2) of this section, does not exceed the corresponding one-engine-inoperative takeoff distance using the established V R.The takeoff distances must be determined in accordance with §25.113(a)(1).

(4) Reasonably expected variations in service from the established takeoff procedures for the operation of the airplane (such as over-rotation of the airplane and out-of-trim conditions) may not result in unsafe flight characteristics or in marked increases in the scheduled takeoff distances established in accordance with §25.113(a).

(f) V LOFis the calibrated airspeed at which the airplane first becomes airborne.

(g) V FTO, in terms of calibrated airspeed, must be selected by the applicant to provide at least the gradient of climb required by §25.121(c), but may not be less than—

(1) 1.18 V SR; and

(2) A speed that provides the maneuvering capability specified in §25.143(h).

(h) In determining the takeoff speeds V1, VR, and V2for flight in icing conditions, the values of VMCG, VMC, and VMUdetermined for non-icing conditions may be used.

[john_tullamarine]

.. and, as always .. if such data is to be considered for a specific Type .. then the TCDS needs to be checked to make sure that the appropriate historical reg data is being considered ..

[Brian Abraham]

John, how PC can you get? Correct.....as always. I hadn't thought of that. Or I could use the excuse I didn't know what type was under consideration.

[john_tullamarine]

PC ? ... I'm getting more and more boring the older I get, mate ... even the dogs roll their eyes up and yawn ... probably will eventually give up drinking as well as the rot sets well and truly in .. that reminds me .. we still haven't had that beer .. have to find somewhere else now that Jack's has closed.

As HarleyD observed .. civil certification doesn't recognise STOL in a manner similar to military due to the (much) higher risks associated with the low speed margins adopted for takeoff and landing .. all fine and beaut while the noise stays high ... low noise = potentially no fun at all.

[Double Zero]

Obidiah,

I am not a pilot but have a fair few hours in as either a 'sitting there' photographer or autopilot when the real pilot gets bored, or indeed conks out on one memorable occasion.

I think your basic message of ' be kind to the airframe ' got lost in techno-speak - I've studied a tiny bit of aerodynamics myself and you obviously have to be careful of lift / drag with the nose up - just a little - but I am surprised no-one as far as I can see has mentioned nose-wheel shimmy.

I was in a C 172 which suffered this to a cringe-making degree; it had been recently serviced on a grass strip where the effect did not make itself apparent, but on tarmac it was reasonably alarming !

Seems to me you have the right idea, and I've flown with the best T.P's going; one for instance would always give a gentle 'feel & pull back' on the stick of a Hawk to judge when it'd unstick - he never went onto the grass, though he did have to stump up for a lot of broken windows after his last ( Mach 1 +, low level ) Lightning flight before finishing with ETPS !

( Hello Tiger, and thanks ! ).

As I say, I think you're on the right lines, treat an aircraft badly and one day...

[Brian Abraham]

Obidiah, I'm taking a shot in the dark and assuming you are talking Twin Otter. The TCDS (Type Certificate) that John refers to may be found here. Just thought it may be of interest.
http://www.airweb.faa.gov/Regulatory...$FILE/A9ea.pdf
Question for you John, or any one else. The TCDS says "NOTE 5. The landing weight is 11400 lb. if the airport temperature at which the landing is to be made is at or above -20°F (-29°C). If the airport temperature is below -20°F, then the landing weight is restricted to 11000 lb."
Question. Why would that be the case?

I am surprised no-one as far as I can see has mentioned nose-wheel shimmy.

Thats because shimmy indicates maintenance is required, its not an issue of technique.

[john_tullamarine]

Why would that be the case?

Haven't come across this one before .. a quick look through the -20 TCDS doesn't help me much ....

A stab in the dark would be to query whether the particular engine (or installation) has an operational limit below that temperature which would restrict thrust output a little .. ie a constraint on missed approach performance limited RLW ? The aircraft TCDS appears to be constraining the original 11000lb instead of the higher weight ...

No doubt, someone who is familiar with the AFM will come to the rescue shortly ?

Now you have me interested .. if there are no answers it might be a case of checking with Brisbane to see if the OEM can throw some light onto it ?

[Double Zero]

I agree completely that nose wheel shimmy is a maintenance issue, but back in the real world you might well encounter it !

My thoughts stand, ' be kind to the airframe ' - it's only alloy and stringers.

Though there's the odd occasion when one has to plonk it down rather deliberatley;

I've been in that situation with an experienced ( though I was later to find a total berk ) fast jet pilot in a light twin, with the aircraft - PA44 Semilone - performing the mother of all Dutch Rolls - when we finally hit the ground - rather firmly - he remarked " at least we're still walking!" which did not do a lot to cheer up self or my Kenyan supposed fast jet companions...

On the other extreme, when in a D.H. Dove being flown by a rather famous Harrier Test Pilot, we didn't even know we'd touched the ground except for looking out of the windows, until we were on the flight line...

[Obidiah]

Thanks fellas,

The cigar goes to Brian for unearthing this one:

(a) For normal, utility, and acrobatic category airplanes, rotation speed, VR, is the speed at which the pilot makes a control input, with the intention of lifting the airplane out of contact with the runway or water surface.

That was what I was looking for.

It appears the process of raising the nosewheel for the purpose of load reduction and not lift off is sanctioned.

The aircraft I was referring to was the Shorts Skyvan.

The question raised by Brian relating to MLW reduction and the very low temperatures is a good one,
I don't know the answer but I'll put forward that it may be to do with the tyres, perhaps issues of flexibility or pressure, or perhaps the oleo's, be interested to hear the correct answer.