Doubled Radials: Easy Solution or More Trouble than They're Worth?

Just Leo said:
Let me be the first to introduce the problem that plagues the bigger engines, vibration. Roy Fedden did not use dynamic balancers on the Mercury, because it didn't need them. The Hercules and Centaurus didn't either, although the Centaurus had some harmonics problems that delayed launch of a satisfactory production example, and led to adoption of metalastic motor mount bushings. Wright did some important work with the R-2600 in dynamic balancers, which aided P&W in resolving torsional vibration problems with the R-2800. If you consider that the R-2800 is a close analogy to the twinned Mercury or Pegasus, what are the chances of such twinned engines avoiding the shakes?
An excellent question. (One for which I have no answer.:p)

Given the design for a twin is likely to encounter vibration in testing before acceptance, where a "twinned" single may not, this could inhibit "twinned" single-rows.

OTOH, might a "twinned" single see a cancelling of vibration problems? That is, the second row, operating "out of phase" to the first, means the vibration disappears? (It does seem more likely to amplify than cancel...)

Adding a second row also offers opportunities to manipulate vibration issues by changing the firing order, doesn't it? (Or does it?:eek:)
 
Yes, twinning a proven single-row radial does give an opportunity to re-time power strokes, but you also open up a whole series of sympathetic vibration problems. Vibration problems increase with the square of the number of components.
During the first two revolutions, a single-row radial fires: 1, 3, 5, 7, 2, 4 and 6.
Meanwhile, a two-row radial fires: 1, 8, 3, 10, 5, 12, 7, 14, 2, 9, 4, 11, 6 and 13, quadrupling the opportunities for sympathetic vibration.
 

Driftless

Donor
How does that sympathetic vibration problem compare to multicylinder in-line or V-engines, as you increase the number of cylinders?
 

Driftless

Donor
I cannot comment here, as I have not enough knowledge to participate in the discussion, but I would like to ask a question. Would it be good if we had a thread on aircraft eingines et al...?

Something like Saphroneth's logistics thread, but for all the various aspects of in-line vs radial, as well as all the other points that might come up? I am not offering to write such a thread, mind you, but just asking if such a thread would be welcomed and useful for the community.

Yes please. Somebody with a good working knowledge of the different engine types would need to curate the thread, I think.
 
TeaDaemon got it mostly correct when he described the difficulties in converting an airplane from a single-row radial engine to a two-row engine.

May I expand on one point: balance?

Balance is critical when designing, loading and modifying airplanes.
Since most twin-row radial engines are heavier than single-row, they get shorter engine mounts to try to keep the airplane balanced about its aerodynamic centre. If the airplane is still too nose-heavy, then they start shifting heavy components (e.g. Batteries) to the tail cone.

Enlarging vertical tail fins and rudders is more a matter of correcting for increased torque, yaw and P-factor. P-factor is the extra thrust created by descending propeller blades. P-factor is at its worst at low airspeeds, high angles of attack and high power settings (e.g. going-around after a missed landing approach). P-factor kills far more twin pilots than the loss of thrust. In the worst case, a pilot lets the airplane get too slow, he runs out of rudder authority (to keep the plane flying straight) and the airplane yaws so badly that one wing stalls, flipping the airplane into a spin.

Another reason for increasing vertical tail area is to compensate for yaw problems created by floats. Consider that stock, piston-powered Cessna 207 never had enough power to take-off on floats, but when Soloy installs a more powerful Allison/Rolls-Royce turboprop engine, that lowly 207 finally has enough horsepower oft off from a lake. To compensate for the (yaw) destabilizing effect of the added lateral area of floats, Soloy installs an extra-large rudder.

Soloy always installs (larger) seaplane rudders on turbine 206 and 207 conversions for similar reasons that TeaDaemon stated. Since the Allison 250 engine weights less than the pistons it replaces, it requires longer engine mounts to re-balance the airplane. Since the longer engine cowling adds lateral area to the front, Soloy conversions need larger vertical tails to restore yaw stability.
 
Astrodragon said:
It will probably have vibration issues AND catch fire...:p:D

Which will make it perfect for this thread.:p (Even better if you have two of them.:p)

On the increased weight issue, agreed. It is (of course) a matter of trying to stay within the Cg range, which is also why turbines are mounted further forward on conversions. Expendables, like fuel, ideally should be placed on the Cg. (Doubtless one reason the F4U got the longer nose--tho IMO, a "seat tank" like the P-51 would have been a better call.)
 
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On the increased weight issue, agreed. It is (of course) a matter of trying to stay within the Cg range, which is also why turbines are mounted further forward on conversions. Expendables, like fuel, ideally should be placed on the Cg. (Doubtless one reason the F4U got the longer nose--tho IMO, a "seat tank" like the P-51 would have been a better call.)

The F4U didn't get a longer nose. It got the cockpit farther back. The Mustang didn't have a seat tank. The main tanks were located in the wings, between the spars.

The CoG question is not the number of rows, but the weight of the engines. A number of US aircraft could use either single-row R-1820 or 2-row R-1830 engines, as P-36, and F4F. The Spitfire's engine weight grew considerably over its development, from 1,375 lb to 1,980, not including the weight of the heavier props. The FW-190D9 series used a rear fuselage plug to counteract the weight and extra length of the Jumo 213 engine. If the aim is to fit a Gloster F5/34 with twice the power and more, the Mercury could have been replaced by a Hercules, but it would not be just a matter of stronger bolts.
 
Just Leo said:
The F4U didn't get a longer nose. It got the cockpit farther back. The Mustang didn't have a seat tank.
*sigh* Serves me right being imprecise.:eek: From where I'm sitting, moving the seat of the F4U back effectively lengthens the nose. And a fuel tank behind (& under?) the seat of the P-51 is a "seat tank" (wherever the mains might be). However, I acknowledge that isn't clear.:eek:
Just Leo said:
The CoG question is not the number of rows, but the weight of the engines.
And that I do understand.:)
Just Leo said:
If the aim is to fit a Gloster F5/34 with twice the power and more, the Mercury could have been replaced by a Hercules, but it would not be just a matter of stronger bolts.
No, indeed. The thing is, if the idea is to add power without a total redesign (which I'll wager a swap to the Herc would take:eek:), "doubling" the Mercury (& adding a tail plug or something) is the way to go--which is where I started.
 
On a vague tangent, Armstrong Whitworth engines of combat power were relatively Whitworthless during the war, but a car company, Alvis took an interest in aero engines and produced a two-row called the Pelides, which passed a type test in 1937 at 1,065 hp, of 52 in diameter, and 1190 lb dry wt. They seemed to have hopes of improvement, but the AM said it didn't have any use for it, and they sub-contracted for Rolls during the war. Post-war, the Alvis Leonides was a particularly good engine, which might indicate that some talent was squandered.

It reminds me of the ancient Greek tailor, whose motto was "Euripides, I mendides.

Armstrong - with mixed success with the Tiger, went 'triple' with the Deerhound' - which had cooling problems initially, but then I think development was affected by bombing.

The Alvis Pelides was ONTH a missed opportunity - a development of the Gnome-Rhone Mistral Major. With the Pelides Major - same size better supercharger. IMO it would have made a good replacement engine for the
Mercury and perhaps Taurus - a better fit for Gloster f.5/34
 
How does that sympathetic vibration problem compare to multicylinder in-line or V-engines, as you increase the number of cylinders?
As I understand it, such engines are inherently better balanced since there are much fewer angles between cylinders to consider. One bank, all cylinders are aligned. V engine, all cylinders are either aligned or at e.g. 60 degrees. In a two-row radial you have 14 or 18 cylinders no two of which are aligned.
The piston rod arrangements in a one or two bank straight engine are also much simpler than a radial with a master and six or eight slaves.

Mind you, plenty of failed V-motors have been attempted too...
 
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