Can you hear out-of-synch props from the ground?

[Stephan Wilkinson]

B-26K Nimrod pilots in Vietnam were said to unsynch the props on a nighttime attack run-in, because the resulting "beat" would make it hard to tell exactly where the sound was coming from.

I have plenty of multi-engine time and have been in the cockpit listening to that beat while I synched the engine rpms, but I can't remember ever hearing out-of-synch props while standing on the ground as a twin flew overhead.

Question: is it possible to hear out-of-synch props from the ground?

[seagull61785]

I think you can hear them from the ground as I believe the Luftwaffe used to intentionally de-synchronize their props on their night raids over England in 1940/41 (and possibly throughout the war).

My parents (and other folk) used to say that they could always tell the difference between the 'Jerries' and 'one of ours', due to the regular 'thrum - thrum' sound they made.

Why did they do it? - I wouldn't have a clue.

Barry

[CAPFlyer]

I've heard them from the ground on a couple of occasions. Once I know it was intentional (post maintenance test flight where they were doing stuff) and once I don't think it was (random GA flying over my house).

It's definitely a very unique sound, so it catches your ear just as it does in the cockpit or when you're doing ground runs.

[m50a1ontos]

We heard it all the time when the Russian Bears flew over. They're a loud airplane, but they get louder when they screw with the sync. Instead of a constant WAAAAAAAAHHH it's a WAAAWAAAAWAAAWAAAH.

[seagull61785]

My theory is that the Luftwaffe possibly de-synchronised their props in order to try and fool the ground ack-ack gun laying equipment which I think was largely based upon sound location in the 1930's and early 40's. As the technical side of the equipment developed during the war and radar-assisted target acquisition became a reality then they probably would have dispensed with the de-synch of their props as tampering with their noise wouldn't necessarily fool radar.

That's my ten cents FWIW.

Barry

[JohnTerrell]

I know I've clearly heard the out-of-sync "beat" before coming when two Mustangs were both conducting their run-ups at the same time, right next to each other.

[JimH]

Out of sync props in the B-24 is probably the most annoying sound on the planet...the sound wave travels across the cockpit and vibrates the everything as it goes by...aside from taxiing, syncing the props is probably the hardest thing to teach. The tachs mean very little, its an "ear/feel/visual art. Your co-pilot ideally slides his seat back and looks across each engine to sync the shadows...left moving shadow goose, right moving shadow reduce on the governors. I have had guys spend an entire flight fooling with the governors. You can hear it from the ground as well...I like the analogy waaaaaaahwaaaaaaaah versus waahwahwaaah...that sums it up. The B-25 is just as touchy but it is so ungodly loud you just sync up the vibration.

Jim

[helinut]

I remember hear that the Canadair Argus with 4 turbo compound R3350 where run out of sync,when doing searches over land looking for lost people or survivors of crashes in the north.i can remember them flying over our house when growing up and hearing the dishes in the kitchen rattle,we where 20 miles from the airport....

[marine air]

ATP Flight school practices over my farm in the PA-44 Seminoles and yes you can hear them. If you go to Oshkosh, you'll hear them all day long as the Ford trimotor hops rides.

[Lynn Allen]

ATP Flight school practices over my farm in the PA-44 Seminoles and yes you can hear them. If you go to Oshkosh, you'll hear them all day long as the Ford trimotor hops rides.

+1 as its about 3 times a day here...

[Airlift48]

Had a worn prop governor pulley slip and send the #3 engine to 2100RPM, while the other 3 were sitting pretty at 1800. Woke everyone up in the back and we had no choice but to ride it out until landing.

One pulley replacement and shaft inspection later and we were back in business.

But it is a God awful sound.

KK