Would a stationary WWII fighter fall on its nose with full throttle and brakes on?

Question

It's a general question, but I am interested in the Republic P-47.

When you say "full throttle", I assume you're gunning it from a dead stop? Otherwise, a stationary object just sits there... because it's stationary. — Omegacron, Apr 07 '17 at 12:10
The aircraft is stationary with engine running and brakes on and tail resting on ground. I would then throttle up to full power. — Invariant, Apr 07 '17 at 12:54

score 25 · Accepted Answer · edited Apr 13 '17 at 12:59

25

(Wikimedia; arrows and CG icon are my additions) P-47 side drawing.

The thrust has a bigger moment arm around the tires contact points compared to the weight. It won't nose over below a certain RPM (it will be specified in the training). Below that RPM:

The weight will counter the thrust
The propeller downwash will produce downward force on the horizontal stabilizer
Applied brakes will resist the rotation of the wheels (airframe rotating around wheels)

From a training video for the comparable big-engined F6F Hellcat, the narrator says:

You can't gun this engine to full takeoff RPM and manifold pressure while holding the plane with the brakes, if you exceed 2,000 RPM the tail will lift and you risk nosing over.

Answer is yes it can happen.

Note: for comparison, the max RPM is 2,700 for the Hellcat.

edited Apr 13 '17 at 12:59

Community

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answered Apr 06 '17 at 23:07

Okay, does anyone know any introductory text available on the Internet that describes the pitch moment effect from propeller airflow, maybe with some example pitch coefficient curves? – Invariant Apr 06 '17 at 23:49
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The problem is that the movement arm would be based on the wheels, not the CG, so there is definitely a meaningful movement arm. Up elevator would probably be enough to counteract that, but (maybe) neutral or (especially) down elevator would probably get you in trouble. – Eugene Styer Apr 07 '17 at 00:39
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"Applied brakes will resist the rotation of the wheels (airframe rotating around wheels)" - But they are wheels. The whole structure is free to rotate around this 'locked' axle, so I don't see how this has any stabilising effect. Nor does this change when you throttle up (unless tire friction is low enough that you start to slide, which would actually help prevent a dive) – Baldrickk Apr 07 '17 at 08:49
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Now I'm watching an F6F instructional video at work. I'm.. pretty sure that doesn't relate to my job... – enderland Apr 07 '17 at 18:32
Pity the poor sod who forgot to take his brakes off and worked out where the biting point is. – Richard Apr 08 '17 at 18:57

score 15 · Answer 2 · edited Apr 07 '17 at 13:27

15

This was also an issue when doing maintenance. For engine ground running at full power, the tail of the aircraft was weighted down to stop the aircraft tipping over the front wheels.

You can see the "saddle bag" and weights hung over the fuselage in front of the tailplane in this still from a video of a WWII Spitfire ground run:

(YouTube)

edited Apr 07 '17 at 13:27

answered Apr 07 '17 at 11:31

alephzero

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Where exactly are the weights? It looks like the plane is tied down to the ground in those pictures. – nobody Apr 07 '17 at 12:09
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@AndrewMedico, eh? The ground isn't heavy enough? – Wossname Apr 07 '17 at 13:03
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@Wossname clearly, you're underestimating the power of an angry Merlin engine being restrained for emergency surgery. – Jules Apr 07 '17 at 18:30
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@AndrewMedico the weights are the two lumps of lead hanging either side of the fuselage. Lead is pretty dense stuff! The plane certainly isn't "tied down to the ground" - I've seen this done "live" several times. – alephzero Apr 07 '17 at 19:52
Note the full elevator deflection. The pilot has the stick buried in his gut to make sure that the horizontal stab/elevator are generating significant down-force. – Bob Jarvis - Слава Україні Apr 09 '17 at 02:04

DigitalRoss · Answer 3 · 2017-04-12T22:13:15.237

Depends on the elevator input

As long as the stick is held back, as it should be for any kind of runup, then the elevator would be applying plenty of downward force.

Now, without the elevator input to cancel it, the pitchover torque might nose over an airplane, and it might not take one of those big WWII engines to do it.

There is a force directed aft at the interface where the (brake-locked) wheels meet the ground that is equal to the force forward along the propellor shaft. This force is multiplied by the distance between the extension of the prop shaft and the ground between the wheels to create the torque.

It's the same torque, but with opposite magnitude, as that which produces the hotrod wheelie maneuver. (And the torque is there with or without the wheelie, which is why FWD cars don't make good drag racers.)

It will be worse for an airplane with long gear legs that sits well off the ground.

If the static thrust or the elevator moment is high enough, it could happen. I've always wondered what would happen if I didn't hold the stick back on runup.

Update: Addressing comments ... well, it might take a little unfavorable wind too, certainly airplanes have nosed over, see below, on (perhaps downwind) runups, but note that on the takeoff roll in a tailwheel airplane, the tail comes up quite early on, and that's with neutral stick inputs, so one can only imagine that it could happen pretty darn fast with any reckless amount of forward stick.

Apparently, even nose wheel airplanes can do it, I believe this is a Cessna on the runup pad at KSQL:

Could you estimate (or measure?) the airspeed at the tail when stopped but with the throttle at taxiing speed and at full throttle ? — Criggie, Apr 07 '17 at 21:28
Is really the jet produced by the propellor so strong for this to be true? — Vladimir F Героям слава, Apr 08 '17 at 11:48

score -1 · Answer 4 · answered Sep 28 '20 at 15:02

It depends on what position the elevator is in! As long as there's enough back pressure, there's not going to be an issue. If you were to push FORWARD, then the plane would go over onto its nose. I've got some experience with this in X-Plane, and it's definitely possible. If I was involved in a similar engine run-up, I'd want some sort of tie-down on the tail if the plane was to remain stationary, to avoid the possibility of the pilot bumping the stick and shoving the plane over.

Just because it behaves like that in X-Plane does not mean it did that in reality. Its not like X-Planes ground handling is the most accurate simulation. — Invariant, Sep 29 '20 at 00:41

Would a stationary WWII fighter fall on its nose with full throttle and brakes on?

4 Answers4

Depends on the elevator input

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