Wheel Drag vs. Drag Due to Lift

[MachNix]

A WWII training film on the B17 said that you would get to speed sooner if you don't lift the tail.  The reasoning was the drag caused by the wheels was greater than any lift induced drag caused by the wings.  Once airborne you would accelerate in ground-effect to climb speed.

On nose wheeled aircraft – B24s, B26s, and even P38s – I have seen film of them rolling down the runway with their nose wheels off the ground.  The B24 was plagued with nose wheel problems but I assumed they did it to get the aircraft weight off the tires and onto the wings to reduce the drag.

In AH, either there is no wheel drag or it is set a too low a value.  Testing with a fully loaded B24 showed that a takeoff with no flaps, and letting the aircraft lift off on its own with the default trim setting, the speed crossing the end of the runway was about 145 mph.  Using one notch of flaps, getting airborne as quickly as possible, retracting the gear, accelerating in ground effect, and letting the flaps auto-retract, the speed at the end of the runway was only 135 mph.

In the real world with prop planes, I like to raise the nose gear an inch or two as quickly as possible.  Does anyone have a real-world opinion about tire drag vs. lift-induced drag?

MachNix

[rshubert]

doesn't become a factor until you're actually producing lift.  If you leave the tail on the ground, the angle of attack is too high to create lift and the wing is stalled, and therefore the only drag is parasitic (form) and tire/bearing friction drag.  

IMO, the rolling friction would be very very low, not requiring much thrust to overcome.  Test this out by pushing your C-172 around--once it gets moving, there's not a lot of effort required to keep it moving on a flat surface.

[MachNix]

rshubert,

Stalled wings produce drag.  Maybe it is more like form drag than induced but it is still drag.

A B17 rolling along the runway has an angle of attack of about 10 deg.  It stalls at 15 degs.  As soon as you get any wind over the wings they are producing lift.  Sticking with the C-172, my question is whether it takes more effort to push a C-172 while rolling at 70 mph across a flat surface or when flying in ground effect? Cessna recommends raising the nose wheel at 60 mph.  They also do not recommend using any flaps during takeoff and no more than 10 deg of flaps for short or rough fields.  They say using 10 deg of flaps will get you airborne sooner but the flaps will hurt the climb to a 50-foot obstacle.  So it looks like Cessna agrees with you that rolling friction (drag) is less than induced drag – at least when flaps are used.

Maybe the training film was actually concerned about the bombers braking through the runway and wanted the weight off the mains as quickly as possible.  And they were only using "wheel drag" to sell the idea.   I bought it.  But not much chance of braking through the runway in a C-172.

MachNix

[bunch]

AFAIK, alll nosegear planes lift the nose wheel off at a speed significantly lower than liftoff speed.  The C-172 in rshubert's example has a Vr (rotation speed) listed in it's POH or on it's checklist & that speed  (Usually about 50kts in a 172 IIRC) is only  a few kts above the clean stall speed, so it certainly isnt a good idea to liftoff if you're at Vr.  In general if you use flaps to liftoff early, you'll still be slower when you cross the runway threshold than if you didnt because of drag from the flaps (not sure if it is parasitic or induced).  In theory if you liftoff early by using flaps, build speed in gound effect, until just safely - but not by too much - over the clean stall speed then haul up the flaps & gear fast, maybe that would give you the fastest speed over the runway threshold.   To do this IRL you would need cajones of steel, 3 hands & good insurance, in AH maybe you need only the 2nd on the list  (if wheel drag is a significant factor)

[HoHun]

Hi Machnix,

>A WWII training film on the B17 said that you would get to speed sooner if you don't lift the tail.  The reasoning was the drag caused by the wheels was greater than any lift induced drag caused by the wings.  

Hm, I can't picture that I'm afraid. What was the actual procedure to be used? Just accelerate down the runway tail-down until the aircraft flew itself off the ground in three-point attitude?

That would not be unusual for a tail sitter, I believe. The FM-2 works that way, but of course that's a fighter.

From the B-29 manual:

"At 90 mph, relieve pressure on the nosewheel by easing the control column back. The airplane then flies itself off the ground at 115 to 130 mph, depending on the gross weight."

Interestingly, no real rotation is specified.

Regards,

Henning (HoHun)

[Angus]

HoHun, just for curiosity, do you fly?

[MachNix]

HoHun,

The procedure for the B17 was as you described – leave it in a three-point attitude until it flew itself off the runway.  The question is why?  By leaving it in a three-point the aircraft weight would transfer from the wheels to the wings as speed increased.  Less weight on the wheels meant reduced flatting of the tires which means less tread squirm which means less drag.  At least that was my take on the situation.

But was wheel drag more than induced drag?  From what I'm hearing the answer is "No".  Maybe there was something unique about the B17 or the runway surfaces that made them do this.  Maybe this procedure was just easier to teach.  You definitely get better performance leaving the flaps up and getting to speed on the ground.

Thanks all.

MachNix

[SunTracker]

Zenos Warbird Videos has the video mentioned.  The video said to get off the runway quickly to clear either a runway obstacle or hill, use 1/3rd flaps.  Keep back pressure on the control column, but not too much.  Just enough to keep the tail-wheel on the ground.  The plane will lift off in a 3-point fashion.

[HoHun]

Hi Machnix,

>But was wheel drag more than induced drag?  From what I'm hearing the answer is "No".  Maybe there was something unique about the B17 or the runway surfaces that made them do this.  Maybe this procedure was just easier to teach.  

I think "easy to teach" along with "easy to perform reliably" must have been the resons. With the three-point method, the pilot could make no wrong decisions, and he'd automatically lift off the aircraft at a point where he hasn't wasted much runway, but is at a good flying speed. (This point was defined in the B-17s design stage by setting the ground attitude.)

>You definitely get better performance leaving the flaps up and getting to speed on the ground.

Hm, but a no-flap take-off will require a higher speed, consuming more runway during acceleration (even if acceleration might have been quicker). In a test on a runway of generous (and constant :-) length, that might not matter much, but in real life, the pilot would fly from different runways, and the safe assumption probably was that he'd always prefer the shortest ground run. That's where the three-point take-off procedure had its strength: kind of "One size fits all" :-)

By the way, does "no flaps" work better even for a fully loaded B-17?

Regards,

Henning (HoHun)

[MachNix]

Henning,

From what I'm learning from here is that flaps will help get you off the ground sooner.  As you alluded to, the decision to use flaps is based on runway conditions such as length and surface.  If the runway is short and you are going to be close to the clean stall speed by the time you reach the end of the runway, then you're going to need some flaps.  With a rough field you would want to get up quickly before you break something and would use flaps.  For a fully loaded B-17 I would say "no flaps" would work best.  As SunTracker mentioned you would still use flaps to get off quickly and clear an obstacle.

I have been talking about the B-17 because it was probably from that Zenos Warbird Video on the 17 that I leaned my "technique" for flying the heavies here in AH.   But my concern was with the B-24.  If you use a notch of flaps and try and climb out after takeoff in the B-24 you will lose the #2 drone.  I was frustrated because I thought the "technique" was getting the best performance out of the aircraft.  Turns out if you leave the flaps up and let the B-24 accelerate on the ground until it lifts itself off, you will capture the drones every time.   Guess I was doing way too much thinking.

Thanks again.

MachNix

[RedBeard]

Originally posted by SunTracker
Zenos Warbird Videos has the video mentioned.  The video said to get off the runway quickly to clear either a runway obstacle or hill, use 1/3rd flaps.  Keep back pressure on the control column, but not too much.  Just enough to keep the tail-wheel on the ground.  The plane will lift off in a 3-point fashion.

Short/grass field takeoff procedures do tend to use flaps and getting off the ground as quickly as possible and using ground effect to accelerate.  That's standard for most aircraft (at least for the ones I've flown - mostly Piper).

[Straiga]

I had a chance to fly a G model B-17 into Nellis air force base for the 50th anniversary air show, it was only about $350 for 1 hour of PIC time.

 To start with tire friction depends largely on tire inflation, low tire pressure more friction.
 Key to the B-17 and most airplanes, whats the runway surface like ?.  Pavement, grass, dirt, concrete, SPF( metal grate) I will talk later about this.
 Now with a tail dragger some planes need  stick forward on takeoff (ME-109) some need tail down on takeoff (P-51) some planes you let the tail fly buy itself (B-17).
 In the case of the P-51 if you didnt hold the tail down, ground loops would be happening.
 If you were to push forward on the stick in the B-17 at low speeds you would put more weight on the mains (more Friction) and you would have less directional control when the locked tail wheel is flying.
 The tail flys with very little air speed in the B-17 you just need to keep the tail wheel touching the runway until you transfer the weight from the mains to the wings (produce some lift and get the weight off the wheels) and at this speed you have real effective rudder, then let the tail fly, airlerons will be neutral.
 Easy translation, get the tires lite, let the wing produce lift, it gets you to a faster acceleration and less drag from the wheels for takeoff.
 When on uninproved runways this technic is used. Flaps are used to get the wing creating lift sooner to get the weight off the wheels sooner less drag due to grass or dirt or pots holes. Accelerating in ground effect, bring up the flaps to help accelerate sooner to Vy (best rate of climb) or Vx (Best Angle of climb).

Oh buy the way Vr or V2 is your rotation speed. Abort the takeoff pryor to V1 (decision speed )or after V1 get it flying.

In the B-767 as we accelerate to about 50kts we get the nose lite setting a slightly higher angle of attack letting the main wings carry some lift to get the weight off the main wheels. If no abort pryor to V1 accelerate to Vr-V2 rotate and accelerate to V2+10 (around 140kts) positive rate of climb gear up for climb out to a safe single engine altitude (about 1000 AGL) then lower the nose to accelerate to 250 kts to cruise altitude. Take off flaps are always used.

 Now in a C-172 for a short field takeoff and to clear a 50 ft tree at the end of the runway. Full flaps down control yoke full aft apply full power and hold brakes. Release brakes and accelerate, lift off as soon as possible, keep on lowering the nose to accelerate in ground effect at about 3 ft off the ground and bring up flaps slowly accelerate to VX or Vy pitch up to clear the tree then set best rate (Vy) airspeed.
 
Induced Drag- is anytime you induce something into the airstream, angle of attack, rudder, aileron, elavator, landing gear, flaps, cowl flaps.

Parisite Drag or Form Drag- angle of incidense, rivets, pitot tubes, door handles, body lines, antenas, struts, fixed landing gear, navigation lights.

Drag happens when lift is produced, friction drag from the tires is always there but reduces when a tire rotation increase or when you get them airbourne. Induced drag from the landing gear is reduced as gear is retracted. Parisite drag increase as airspeed increases and so does induced drag.

Tire friction would be less than induced drag.

Straiga

[Angus]

Lovely.
Often comes to soft field take-off and short field take-off anyway...

[Schutt]

I guess the point was to keep the (locked) tailwheel on the ground untill the aircraft is stable, having a b17 sway off the runway would result in some damage on the airfield.
Also i think tire wear is a concern, during ww2 i guess the tires were worse than now and having less wight on the tires will give longer lifetime and less risk of a blown tire.

[bunch]

If you're flying B-767s I'm sure it has been  several thousand hours since you've been regularly operating a C-172, but soft field T/O is 10 degrees (25% - 33%  on some later models) of flaps.  C-172 flaps dont add any lift over 20 degrees (50% - 66% on some later models).  I wouldn't try to take a C-172 off from any field considered small with flaps full down without a very good headwind & even in that case, if it were at max. weight i'm not sure it would get out of ground effect (except for maybe a 180hp version)

some later model C-172s have only 30degress of total flap travel