F-84F Top Speed Question

[mrparty]

Hello everybody. I'm new to the forums and have kind of a strange question regarding the top speed of the F-84F Thunderstreak (with both the W-3 and W-7 engines).

The SACs for both list the top speed as 595kt, or ~685mph, but there are two other sources that say different. The Encyclopedia of US Air Force and Missile Systems (1978) lists the top speed as 690mph, and Aerofax Minigraph 15 (1987) lists it as 691mph (I don't trust Wikipedia and other sources that say 695mph because I believe it is probably a mistranslation of the 595kt in the SACs).

The reason why I don't know whether the SAC might be the true top speed is that it lists the 595kt as after wing tanks are dropped, so the two inner pylons could still be attached to the plane and cause a little bit of extra drag. The pylons could be jettisoned but the SAC doesn't specify whether they're jettisoned or not, just that the tanks are dropped.

Pretty much all the photos and footage I've seen of operational Thunderstreaks had at the very least those two inner pylons attached to the wing in flight, and even the Thunderbirds flew with them on.

So my question would be, do you think it was standard procedure to jettison the pylons after stores are jettisoned (if so, then the top speed is certainly 595kt), or does the regular "cleanest" flying condition usually include the two inboard pylons? Also, how much do you think the pylons would actually affect top speed in general?

Here's the SAC for the W-7 engine F-84F-50 and subsequent models (It does say 595kt clean, but I just want to make sure clean is definitely without the inboard pylons):
http://alternatewars.com/SAC/F-84F_Thunderstreak_(W-7_Engine)_SAC_-_2_September_1958.pdf

 

[Graeme]

Also, how much do you think the pylons would actually affect top speed in general?

Welcome to the site Mr Party!

Somewhere around 300 lbs, so I'm thinking it would help. Based on the definition of "clean" given here and the fact that Republic had an option to jettison the pylons I'm of the opinion the fast speeds at sea level are in the pylon-less condition?
Incidentally I have paper sources prior (1955) to your 1958 SAC sheet that give a maximum sea level speed in a clean condition for the Thunderstreak of 720 mph.

Republic F-84 Thunderjet Pilot's Flight Operating Manual - United States Air Force - Google Books

 

[PStickney]

Hello everybody. I'm new to the forums and have kind of a strange question regarding the top speed of the F-84F Thunderstreak (with both the W-3 and W-7 engines).

The SACs for both list the top speed as 595kt, or ~685mph, but there are two other sources that say different. The Encyclopedia of US Air Force and Missile Systems (1978) lists the top speed as 690mph, and Aerofax Minigraph 15 (1987) lists it as 691mph (I don't trust Wikipedia and other sources that say 695mph because I believe it is probably a mistranslation of the 595kt in the SACs).

The reason why I don't know whether the SAC might be the true top speed is that it lists the 595kt as after wing tanks are dropped, so the two inner pylons could still be attached to the plane and cause a little bit of extra drag. The pylons could be jettisoned but the SAC doesn't specify whether they're jettisoned or not, just that the tanks are dropped.

Pretty much all the photos and footage I've seen of operational Thunderstreaks had at the very least those two inner pylons attached to the wing in flight, and even the Thunderbirds flew with them on.

So my question would be, do you think it was standard procedure to jettison the pylons after stores are jettisoned (if so, then the top speed is certainly 595kt), or does the regular "cleanest" flying condition usually include the two inboard pylons? Also, how much do you think the pylons would actually affect top speed in general?

Here's the SAC for the W-7 engine F-84F-50 and subsequent models (It does say 595kt clean, but I just want to make sure clean is definitely without the inboard pylons):
http://alternatewars.com/SAC/F-84F_Thunderstreak_(W-7_Engine)_SAC_-_2_September_1958.pdf

Well, I've pulled my F-84F manuals out, (Never know, I could find one in a barn somewhere), and here's what I've got.
First remember that with any airplane, context is important, and individual airplanes vary - don't get too hung up on a knot or 5 or 10 here or there. Very rarely is the airplane going to run out at full throttle to maximum speed. Also - jet performance, especially non-afterburning transonic jet performance is very dependent on context, particulalrly altitude. Your performance is diven more by Mach Number than Knots Indicated or Equivalent Airspeed.

That being said - Pylons were generally retained. Mounting them was a pain, they jettisoned by explosive bolts, (Which can get messy) and the weight penalty was minimal, and the drag penalty was small. (Weight doesn't mean that much at high speeds - it affects Induced Drag (Drag from how hard the wing is working), which decreases the faster you go). You might gain a knot or 2.
Both airplanes have airframe limits of 610 KIAS and Mach 1.175.
So - down to numbers - With the -3 engine, at Sea Level, No tanks, bare pylons, The Book gives Vmax at Military Power to be between 595 KTAS (Mach 0.9),
At 35,000', you're looking at 530-534 KTAS (Mach 0.915 - 0.925)

With the -7 engine, the corresponding numbers are: 600 KTAS (Mach 0.91) at Sea Level, and 535 - 540 KTAS (Mach 0.925 - 0.935) at 35,000'.

 

[Zipper730]

P PStickney ,

I appreciate the fact that you mentioned the airspeed and mach-number: Some of the differences in numbers are simply the altitude at which the aircraft's performance was measured. At lower altitudes, the limit is usually airspeed, and at altitude it's mach number as a general rule.

Though I'm not sure it's necessary, but so much of WWII aviation is in miles per hour, I'll compute the airspeed to miles an hour just for ease of reading

Airframe Limit: 701.5 mph
Sea-Level @ Military Power: 684.3 mph (-3 engine); 690 mph (-7 engine)
35000 feet @ Military Power: 609.5 mph (-3 enigne); 615.3 - 621 mph (-7 engine)

 

[mrparty]

Thank you everyone for your answers, they are really helpful. You guys are correct that Mach number is generally more important for top speed and that a few knots difference at sea level realistically wouldn't be that big of a deal.

Mostly I was just curious of the Thunderstreak's top speed at sea level since I've seen quite a few different numbers from 595kt from the SACs up to 601kt from a few books (that's why I thought that the performance tests for the SACs could have been with Pylons attached).

P PStickney The info you provided is really interesting to me, what book do you have that gives the top speed with Pylons off as 600kt for the W-7 engine? Most of the sources I've seen have said that despite the more powerful engine, the W-7 Thunderstreak wasn't any faster than the W-3 engine variant.

 

[PStickney]

Thank you everyone for your answers, they are really helpful. You guys are correct that Mach number is generally more important for top speed and that a few knots difference at sea level realistically wouldn't be that big of a deal.

Mostly I was just curious of the Thunderstreak's top speed at sea level since I've seen quite a few different numbers from 595kt from the SACs up to 601kt from a few books (that's why I thought that the performance tests for the SACs could have been with Pylons attached).

P PStickney The info you provided is really interesting to me, what book do you have that gives the top speed with Pylons off as 600kt for the W-7 engine? Most of the sources I've seen have said that despite the more powerful engine, the W-7 Thunderstreak wasn't any faster than the W-3 engine variant.

The numbers come from the Dash-1 for the F-84F, specifically
T.O. 1F84(25)F-1, Flight Manual for USAF Series F-84F-25 and Later Aircraft,
Issued 15 February 1963, and updated on 23 September 1963.
The specific data I posted is from the Range Cruise Chart curves for the airplane in that specific configuration at 100% power.

I'm assuming that you're referencing the SAC Charts posted by Ryan Crierie at alternatewars
(A great resource, he's done a lot of hard work to collect that up)
Those charts were released in Sept, 1958 in the case of the -7 SAC, and March, 1957 for the -3 SAC. SAC Chart Data and Flight Handbook Data come from Flight Test Data (if available), and the provenance is noted in the charts.
Note that Flight Test Data is always corrected to Standard Conditions. so that comparisons are valid.
In the case of the J65-W-3 powered '84s, the data source for the SAC Chart are the Contractor's Flight Tests, and the USAF Phase IV Flight Tests,
In the case of the J85-W-7 Chart, the basis is the same Flight Tests as the -3 to determine the aircraft's drag data, and estimated engine performance.

Performance Basis for the Manuals are (except in the case of a Preliminary Manual, issued during Initial Flight Test) the reduced results of the flight tests themselves. (Albeit that fuel consumption numbers are "pessimized", increased beyond the test figures) These numbers are checked periodically by follow-on flight tests, and the manuals updated accordingly.

Note that for Transonic aircraft, like the Super Hog/Thud's Mother/Thunderstreak the top speed is far enough into the drag rise region that increases in powe can produce little, if any, change in Vmax. (Climb performance, which peaks at lower speeds, will benefit proportionally more.)

So - it looks like what we have here is a case of the SAC Charts using the 1954-55 Test Data, and, in the case of the -7 engine, estimated thrust data.
The 1963 manual is using measured test data.

There's a danger in cutting things too fine - the differences in individual aircraft of the same version will vary by more than the differences we're seeing here.
In Aviation, valid answers will always start with "That Depends..." - context is everything, and conditions, both environmental and mechanical count for a lot.

When evaluating information, it's important to know the Who, What, When, Where, Why/How of the sources of the data being used to derive that information.
Then, if you're of an Engineering Bent (Like Me) you take the data back to first principals, use it to derive the basic characteristics, then see if things like Drag Coefficients or Powerplant Performance are plausible.