jhugz
Well-Known Member
It was a two blade thing, the three blade did not have it.
Can you damage a prop in-flight?
- Thread starter beasly
- Start date
It was a two blade thing, the three blade did not have it.
Douglas
Old School KSUX
Engine problems too, but the thread is addressing props.
I don't have any fancy equations to back up what I said, I'm just regurgitating what John Eckalbar said in the Propeller chapter of his book, "Flying high performance singles and twins". He has plenty of those equations. Me, not so much.
It fly.
Make loud noise.
Chucco
Well-Known Member
Yes, I agree. Please understand that I'm not trying to teach something I don't know about, but trying to participate in a discussion with people who do. Which would, I believe, help me to gain more knowledge on these subjects so that I can properly teach my students.
Hehe. I don't recall ever learning about galleries and bearing clearances. But that makes a hell of a lot of sense if I think about it with that in mind. The oil pressure wouldn't increase from the rpm decreasing.
Right, that I understand but maybe I'm not making an obvious connection to the scenario that was proposed. I understood the scenario to be where you were already cruising at speeds in excess of 100 knots with a lower RPM setting (say 2100).
Before I continue I'd like to try and clarify what I meant by "torque resistance". I was referring to the resistance to rotation (drag) the propeller airfoil induces by creating lift as it rotates at different blade angles, but I couldn't think of another phrase to use. Let me take another stab at it and maybe you can enlighten me a little:
An engine running at 25" MP and 2500 RPM is producing a specific amount of power (as is indicated by the MP even though its actually measuring a vacuum) to spin the propellers at a low angle of attack (relatively less drag on the propeller airfoil). If you stay straight and level at 25" MP and change the RPM to 2100, the propellers go to a higher angle of attack to allow the RPM to decrease (more drag for the propellers to rotate against). So now the same 25" of MP is spinning the propeller at a lower RPM.
Isn't it the same as during an engine run-up when you pull the prop lever back to check the governor and the engine bogs down for a second and the MP increases? So basically, doesn't reducing RPM without reducing the MP make it harder for the engine to spin the propeller?
Now please understand I'm not saying that this means I think the oversquared myth is true. I've seen the cruise settings in a POH and can see that it won't destroy your engine. However, the way I currently understand it, there should be some effect on how hard the engine has to work. Then using the same line of thought, I supposed that the inverse could be true.
Without adjusting the throttle from 25" MP cruise and changing the RPM from 2100 to 2500+, wouldn't the engine find itself momentarily having an easier time spinning the prop (less resistance to rotation)? Of course the high airspeed would also be driving the propeller and causing negative thrust until the forces balanced out. Am I completely misunderstanding something?
BTW, I'm really enjoying the chance to talk some of this stuff out. Most of my friends and family just want to change the subject.
msmspilot
Well-Known Member
Right, that I understand but maybe I'm not making an obvious connection to the scenario that was proposed. I understood the scenario to be where you were already cruising at speeds in excess of 100 knots with a lower RPM setting (say 2100).
Before I continue I'd like to try and clarify what I meant by "torque resistance". I was referring to the resistance to rotation (drag) the propeller airfoil induces by creating lift as it rotates at different blade angles, but I couldn't think of another phrase to use. Let me take another stab at it and maybe you can enlighten me a little:
An engine running at 25" MP and 2500 RPM is producing a specific amount of power (as is indicated by the MP even though its actually measuring a vacuum) to spin the propellers at a low angle of attack (relatively less drag on the propeller airfoil). If you stay straight and level at 25" MP and change the RPM to 2100, the propellers go to a higher angle of attack to allow the RPM to decrease (more drag for the propellers to rotate against). So now the same 25" of MP is spinning the propeller at a lower RPM.
Isn't it the same as during an engine run-up when you pull the prop lever back to check the governor and the engine bogs down for a second and the MP increases? So basically, doesn't reducing RPM without reducing the MP make it harder for the engine to spin the propeller?
Now please understand I'm not saying that this means I think the oversquared myth is true. I've seen the cruise settings in a POH and can see that it won't destroy your engine. However, the way I currently understand it, there should be some effect on how hard the engine has to work. Then using the same line of thought, I supposed that the inverse could be true.
Without adjusting the throttle from 25" MP cruise and changing the RPM from 2100 to 2500+, wouldn't the engine find itself momentarily having an easier time spinning the prop (less resistance to rotation)? Of course the high airspeed would also be driving the propeller and causing negative thrust until the forces balanced out. Am I completely misunderstanding something?
BTW, I'm really enjoying the chance to talk some of this stuff out. Most of my friends and family just want to change the subject.
To answer your question, you're not going to hurt the prop. It is possible to hurt the engine with certain MP/RPM combinations, and those vary based on the airframe/engine/prop combination.
The scenarios you describe can't hurt the prop (unless as someone mentioned you manage to overspeed it).
spbrian
Well-Known Member
I be getting old... argh
tgrayson
New Member
Just a place to hang this:
The Safety Board concluded that failure of the blade from NlllGC was caused by high cycle fatigue stresses induced by a resonant vibration of the propeller. This conclusion is further supported by the results of previous testing performed on another M-74 Sensenich propeller cut to 68 inches in diameter. That testing consisted of a comprehensive, in-flight vibration survey conducted on a homebuilt Thorp T-18 airplane powered by a 0-320 series Lycoming engine. The experiments showed that when the propeller operated above 2,500 rpm, the actual vibratory stresses at a point located 17 inches from the tip of the blade exceeded the allowable level by more than 2,000 psi.
Due to the complexity of a propeller design and the susceptibility of a propeller to failure when operated at speeds that excite resonance, propeller manufacturers ordinarily determine the vibration characteristics for each of their propeller designs. When the propeller diameter is changed, the propeller's vibration characteristics are also changed. Type Certificate (TC) P-886, issued for the original M-74DM Sensenich propeller, specifies a minimum propeller diameter of 72 inches for both the Lycoming 0-320 series and Lycoming 0-290-0, -D2, and -D2B. Furthermore, the TC states that from a vibration standpoint "no reduction below the minimum diameter listed is permissible."
The failure of the propeller was 'catastrophic"; that is, the service stresses were high and the fracture occurred in a relatively short time. Because the diameter of the propeller was changed, the propeller vibrated during normal engine operation at resonant frequencies that induced stresses far exceeding its design fatigue limit. With such a mode of failure, it is not feasible to predict when failure will occur: thus, it is not possible to determine, with confidence, an interval for inspection that could detect fatigue cracking before the complete failure would occur.
Full report:Due to the complexity of a propeller design and the susceptibility of a propeller to failure when operated at speeds that excite resonance, propeller manufacturers ordinarily determine the vibration characteristics for each of their propeller designs. When the propeller diameter is changed, the propeller's vibration characteristics are also changed. Type Certificate (TC) P-886, issued for the original M-74DM Sensenich propeller, specifies a minimum propeller diameter of 72 inches for both the Lycoming 0-320 series and Lycoming 0-290-0, -D2, and -D2B. Furthermore, the TC states that from a vibration standpoint "no reduction below the minimum diameter listed is permissible."
The failure of the propeller was 'catastrophic"; that is, the service stresses were high and the fracture occurred in a relatively short time. Because the diameter of the propeller was changed, the propeller vibrated during normal engine operation at resonant frequencies that induced stresses far exceeding its design fatigue limit. With such a mode of failure, it is not feasible to predict when failure will occur: thus, it is not possible to determine, with confidence, an interval for inspection that could detect fatigue cracking before the complete failure would occur.
msmspilot
Well-Known Member
Ok, allow me to rephrase. While operating within the limits the manufacturer prescribes, hurting the prop is EXTREMELY unlikely. The case here, the prop was operated outside its design parameters, and it failed. No surprises there.
tgrayson
New Member
You'd have to go to the engine manufacturer's engine operating guide to know how much of a split there can be; in the Lycoming IO/O-360 lines, the RPM can be quite a bit higher than the MP. If you exceed that, I suspect the problem is potential detonation, although resonance is a possibility, too. The only Continental operating guide that I looked at had a much smaller permissible gap between MP and RPM.
I have read of nothing that indicates there is a problem advancing the throttle before the prop, as long as the setting remains within the permissible operating range; even so, I would suspect that a brief excursion beyond that range wouldn't hurt anything. I have seen full throttle applied with the prop retarded all the way back for best glide range and the engine didn't blow up, although it didn't sound nice.
I have seen a lot of confusion about this before in all of the flight schools I have worked at. If you think of the MP/RPM relationship as "gearing" it usually makes more sense to the somewhat mechanically inclined. Imagine riding your bicycle up a steep hill. You would choose a low gear gear that allows your legs to pedal quickly with less resistance. This is the same thought process you should use while climbing an airplane. RPM is the gearing and MP is how hard the engine is working. During a climb, the best performance is going to come from a the highest RPM setting for a given MP setting.
beasly
Well-Known Member
Excellent. Believe it or not, last night while falling asleep, I wondered what would happen if you shortened its diameter below published limits. While I am not saying that this happens to a typical McCaulkey on a 172, it is a neat point of reference to have going further.